quantum mechanics light experiment

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1 Quantum Behavior

1–1 Atomic mechanics

“Quantum mechanics” is the description of the behavior of matter and light in all its details and, in particular, of the happenings on an atomic scale. Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.

Newton thought that light was made up of particles, but then it was discovered that it behaves like a wave. Later, however (in the beginning of the twentieth century), it was found that light did indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So it really behaves like neither. Now we have given up. We say: “It is like neither .”

There is one lucky break, however—electrons behave just like light. The quantum behavior of atomic objects (electrons, protons, neutrons, photons, and so on) is the same for all, they are all “particle waves,” or whatever you want to call them. So what we learn about the properties of electrons (which we shall use for our examples) will apply also to all “particles,” including photons of light.

The gradual accumulation of information about atomic and small-scale behavior during the first quarter of the 20th century, which gave some indications about how small things do behave, produced an increasing confusion which was finally resolved in 1926 and 1927 by Schrödinger, Heisenberg, and Born. They finally obtained a consistent description of the behavior of matter on a small scale. We take up the main features of that description in this chapter.

Because atomic behavior is so unlike ordinary experience, it is very difficult to get used to, and it appears peculiar and mysterious to everyone—both to the novice and to the experienced physicist. Even the experts do not understand it the way they would like to, and it is perfectly reasonable that they should not, because all of direct, human experience and of human intuition applies to large objects. We know how large objects will act, but things on a small scale just do not act that way. So we have to learn about them in a sort of abstract or imaginative fashion and not by connection with our direct experience.

In this chapter we shall tackle immediately the basic element of the mysterious behavior in its most strange form. We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by “explaining” how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.

1–2 An experiment with bullets

To try to understand the quantum behavior of electrons, we shall compare and contrast their behavior, in a particular experimental setup, with the more familiar behavior of particles like bullets, and with the behavior of waves like water waves. We consider first the behavior of bullets in the experimental setup shown diagrammatically in Fig.  1–1 . We have a machine gun that shoots a stream of bullets. It is not a very good gun, in that it sprays the bullets (randomly) over a fairly large angular spread, as indicated in the figure. In front of the gun we have a wall (made of armor plate) that has in it two holes just about big enough to let a bullet through. Beyond the wall is a backstop (say a thick wall of wood) which will “absorb” the bullets when they hit it. In front of the backstop we have an object which we shall call a “detector” of bullets. It might be a box containing sand. Any bullet that enters the detector will be stopped and accumulated. When we wish, we can empty the box and count the number of bullets that have been caught. The detector can be moved back and forth (in what we will call the $x$-direction). With this apparatus, we can find out experimentally the answer to the question: “What is the probability that a bullet which passes through the holes in the wall will arrive at the backstop at the distance $x$ from the center?” First, you should realize that we should talk about probability, because we cannot say definitely where any particular bullet will go. A bullet which happens to hit one of the holes may bounce off the edges of the hole, and may end up anywhere at all. By “probability” we mean the chance that the bullet will arrive at the detector, which we can measure by counting the number which arrive at the detector in a certain time and then taking the ratio of this number to the total number that hit the backstop during that time. Or, if we assume that the gun always shoots at the same rate during the measurements, the probability we want is just proportional to the number that reach the detector in some standard time interval.

For our present purposes we would like to imagine a somewhat idealized experiment in which the bullets are not real bullets, but are indestructible bullets—they cannot break in half. In our experiment we find that bullets always arrive in lumps, and when we find something in the detector, it is always one whole bullet. If the rate at which the machine gun fires is made very low, we find that at any given moment either nothing arrives, or one and only one—exactly one—bullet arrives at the backstop. Also, the size of the lump certainly does not depend on the rate of firing of the gun. We shall say: “Bullets always arrive in identical lumps.” What we measure with our detector is the probability of arrival of a lump. And we measure the probability as a function of $x$. The result of such measurements with this apparatus (we have not yet done the experiment, so we are really imagining the result) are plotted in the graph drawn in part (c) of Fig.  1–1 . In the graph we plot the probability to the right and $x$ vertically, so that the $x$-scale fits the diagram of the apparatus. We call the probability $P_{12}$ because the bullets may have come either through hole $1$ or through hole $2$. You will not be surprised that $P_{12}$ is large near the middle of the graph but gets small if $x$ is very large. You may wonder, however, why $P_{12}$ has its maximum value at $x=0$. We can understand this fact if we do our experiment again after covering up hole $2$, and once more while covering up hole $1$. When hole $2$ is covered, bullets can pass only through hole $1$, and we get the curve marked $P_1$ in part (b) of the figure. As you would expect, the maximum of $P_1$ occurs at the value of $x$ which is on a straight line with the gun and hole $1$. When hole $1$ is closed, we get the symmetric curve $P_2$ drawn in the figure. $P_2$ is the probability distribution for bullets that pass through hole $2$. Comparing parts (b) and (c) of Fig.  1–1 , we find the important result that \begin{equation} \label{Eq:III:1:1} P_{12}=P_1+P_2. \end{equation} The probabilities just add together. The effect with both holes open is the sum of the effects with each hole open alone. We shall call this result an observation of “ no interference ,” for a reason that you will see later. So much for bullets. They come in lumps, and their probability of arrival shows no interference.

1–3 An experiment with waves

Now we wish to consider an experiment with water waves. The apparatus is shown diagrammatically in Fig.  1–2 . We have a shallow trough of water. A small object labeled the “wave source” is jiggled up and down by a motor and makes circular waves. To the right of the source we have again a wall with two holes, and beyond that is a second wall, which, to keep things simple, is an “absorber,” so that there is no reflection of the waves that arrive there. This can be done by building a gradual sand “beach.” In front of the beach we place a detector which can be moved back and forth in the $x$-direction, as before. The detector is now a device which measures the “intensity” of the wave motion. You can imagine a gadget which measures the height of the wave motion, but whose scale is calibrated in proportion to the square of the actual height, so that the reading is proportional to the intensity of the wave. Our detector reads, then, in proportion to the energy being carried by the wave—or rather, the rate at which energy is carried to the detector.

With our wave apparatus, the first thing to notice is that the intensity can have any size. If the source just moves a very small amount, then there is just a little bit of wave motion at the detector. When there is more motion at the source, there is more intensity at the detector. The intensity of the wave can have any value at all. We would not say that there was any “lumpiness” in the wave intensity.

Now let us measure the wave intensity for various values of $x$ (keeping the wave source operating always in the same way). We get the interesting-looking curve marked $I_{12}$ in part (c) of the figure.

We have already worked out how such patterns can come about when we studied the interference of electric waves in Volume I. In this case we would observe that the original wave is diffracted at the holes, and new circular waves spread out from each hole. If we cover one hole at a time and measure the intensity distribution at the absorber we find the rather simple intensity curves shown in part (b) of the figure. $I_1$ is the intensity of the wave from hole $1$ (which we find by measuring when hole $2$ is blocked off) and $I_2$ is the intensity of the wave from hole $2$ (seen when hole $1$ is blocked).

The intensity $I_{12}$ observed when both holes are open is certainly not the sum of $I_1$ and $I_2$. We say that there is “interference” of the two waves. At some places (where the curve $I_{12}$ has its maxima) the waves are “in phase” and the wave peaks add together to give a large amplitude and, therefore, a large intensity. We say that the two waves are “interfering constructively” at such places. There will be such constructive interference wherever the distance from the detector to one hole is a whole number of wavelengths larger (or shorter) than the distance from the detector to the other hole.

At those places where the two waves arrive at the detector with a phase difference of $\pi$ (where they are “out of phase”) the resulting wave motion at the detector will be the difference of the two amplitudes. The waves “interfere destructively,” and we get a low value for the wave intensity. We expect such low values wherever the distance between hole $1$ and the detector is different from the distance between hole $2$ and the detector by an odd number of half-wavelengths. The low values of $I_{12}$ in Fig.  1–2 correspond to the places where the two waves interfere destructively.

You will remember that the quantitative relationship between $I_1$, $I_2$, and $I_{12}$ can be expressed in the following way: The instantaneous height of the water wave at the detector for the wave from hole $1$ can be written as (the real part of) $h_1e^{i\omega t}$, where the “amplitude” $h_1$ is, in general, a complex number. The intensity is proportional to the mean squared height or, when we use the complex numbers, to the absolute value squared $\abs{h_1}^2$. Similarly, for hole $2$ the height is $h_2e^{i\omega t}$ and the intensity is proportional to $\abs{h_2}^2$. When both holes are open, the wave heights add to give the height $(h_1+h_2)e^{i\omega t}$ and the intensity $\abs{h_1+h_2}^2$. Omitting the constant of proportionality for our present purposes, the proper relations for interfering waves are \begin{equation} \label{Eq:III:1:2} I_1=\abs{h_1}^2,\quad I_2=\abs{h_2}^2,\quad I_{12}=\abs{h_1+h_2}^2. \end{equation}

You will notice that the result is quite different from that obtained with bullets (Eq.  1.1 ). If we expand $\abs{h_1+h_2}^2$ we see that \begin{equation} \label{Eq:III:1:3} \abs{h_1+h_2}^2=\abs{h_1}^2+\abs{h_2}^2+2\abs{h_1}\abs{h_2}\cos\delta, \end{equation} where $\delta$ is the phase difference between $h_1$ and $h_2$. In terms of the intensities, we could write \begin{equation} \label{Eq:III:1:4} I_{12}=I_1+I_2+2\sqrt{I_1I_2}\cos\delta. \end{equation} The last term in ( 1.4 ) is the “interference term.” So much for water waves. The intensity can have any value, and it shows interference.

1–4 An experiment with electrons

Now we imagine a similar experiment with electrons. It is shown diagrammatically in Fig.  1–3 . We make an electron gun which consists of a tungsten wire heated by an electric current and surrounded by a metal box with a hole in it. If the wire is at a negative voltage with respect to the box, electrons emitted by the wire will be accelerated toward the walls and some will pass through the hole. All the electrons which come out of the gun will have (nearly) the same energy. In front of the gun is again a wall (just a thin metal plate) with two holes in it. Beyond the wall is another plate which will serve as a “backstop.” In front of the backstop we place a movable detector. The detector might be a geiger counter or, perhaps better, an electron multiplier, which is connected to a loudspeaker.

We should say right away that you should not try to set up this experiment (as you could have done with the two we have already described). This experiment has never been done in just this way. The trouble is that the apparatus would have to be made on an impossibly small scale to show the effects we are interested in. We are doing a “thought experiment,” which we have chosen because it is easy to think about. We know the results that would be obtained because there are many experiments that have been done, in which the scale and the proportions have been chosen to show the effects we shall describe.

The first thing we notice with our electron experiment is that we hear sharp “clicks” from the detector (that is, from the loudspeaker). And all “clicks” are the same. There are no “half-clicks.”

We would also notice that the “clicks” come very erratically. Something like: click ….. click-click … click …….. click …. click-click …… click …, etc., just as you have, no doubt, heard a geiger counter operating. If we count the clicks which arrive in a sufficiently long time—say for many minutes—and then count again for another equal period, we find that the two numbers are very nearly the same. So we can speak of the average rate at which the clicks are heard (so-and-so-many clicks per minute on the average).

As we move the detector around, the rate at which the clicks appear is faster or slower, but the size (loudness) of each click is always the same. If we lower the temperature of the wire in the gun, the rate of clicking slows down, but still each click sounds the same. We would notice also that if we put two separate detectors at the backstop, one or the other would click, but never both at once. (Except that once in a while, if there were two clicks very close together in time, our ear might not sense the separation.) We conclude, therefore, that whatever arrives at the backstop arrives in “lumps.” All the “lumps” are the same size: only whole “lumps” arrive, and they arrive one at a time at the backstop. We shall say: “Electrons always arrive in identical lumps.”

Just as for our experiment with bullets, we can now proceed to find experimentally the answer to the question: “What is the relative probability that an electron ‘lump’ will arrive at the backstop at various distances $x$ from the center?” As before, we obtain the relative probability by observing the rate of clicks, holding the operation of the gun constant. The probability that lumps will arrive at a particular $x$ is proportional to the average rate of clicks at that $x$.

The result of our experiment is the interesting curve marked $P_{12}$ in part (c) of Fig.  1–3 . Yes! That is the way electrons go.

1–5 The interference of electron waves

Now let us try to analyze the curve of Fig.  1–3 to see whether we can understand the behavior of the electrons. The first thing we would say is that since they come in lumps, each lump, which we may as well call an electron, has come either through hole $1$ or through hole $2$. Let us write this in the form of a “Proposition”:

Proposition A: Each electron either goes through hole $1$ or it goes through hole $2$.

Assuming Proposition A, all electrons that arrive at the backstop can be divided into two classes: (1) those that come through hole $1$, and (2) those that come through hole $2$. So our observed curve must be the sum of the effects of the electrons which come through hole $1$ and the electrons which come through hole $2$. Let us check this idea by experiment. First, we will make a measurement for those electrons that come through hole $1$. We block off hole $2$ and make our counts of the clicks from the detector. From the clicking rate, we get $P_1$. The result of the measurement is shown by the curve marked $P_1$ in part (b) of Fig.  1–3 . The result seems quite reasonable. In a similar way, we measure $P_2$, the probability distribution for the electrons that come through hole $2$. The result of this measurement is also drawn in the figure.

The result $P_{12}$ obtained with both holes open is clearly not the sum of $P_1$ and $P_2$, the probabilities for each hole alone. In analogy with our water-wave experiment, we say: “There is interference.” \begin{equation} \label{Eq:III:1:5} \text{For electrons:}\quad P_{12}\neq P_1+P_2. \end{equation}

How can such an interference come about? Perhaps we should say: “Well, that means, presumably, that it is not true that the lumps go either through hole $1$ or hole $2$, because if they did, the probabilities should add. Perhaps they go in a more complicated way. They split in half and …” But no! They cannot, they always arrive in lumps … “Well, perhaps some of them go through $1$, and then they go around through $2$, and then around a few more times, or by some other complicated path … then by closing hole $2$, we changed the chance that an electron that started out through hole $1$ would finally get to the backstop …” But notice! There are some points at which very few electrons arrive when both holes are open, but which receive many electrons if we close one hole, so closing one hole increased the number from the other. Notice, however, that at the center of the pattern, $P_{12}$ is more than twice as large as $P_1+P_2$. It is as though closing one hole decreased the number of electrons which come through the other hole. It seems hard to explain both effects by proposing that the electrons travel in complicated paths.

It is all quite mysterious. And the more you look at it the more mysterious it seems. Many ideas have been concocted to try to explain the curve for $P_{12}$ in terms of individual electrons going around in complicated ways through the holes. None of them has succeeded. None of them can get the right curve for $P_{12}$ in terms of $P_1$ and $P_2$.

Yet, surprisingly enough, the mathematics for relating $P_1$ and $P_2$ to $P_{12}$ is extremely simple. For $P_{12}$ is just like the curve $I_{12}$ of Fig.  1–2 , and that was simple. What is going on at the backstop can be described by two complex numbers that we can call $\phi_1$ and $\phi_2$ (they are functions of $x$, of course). The absolute square of $\phi_1$ gives the effect with only hole $1$ open. That is, $P_1=\abs{\phi_1}^2$. The effect with only hole $2$ open is given by $\phi_2$ in the same way. That is, $P_2=\abs{\phi_2}^2$. And the combined effect of the two holes is just $P_{12}=\abs{\phi_1+\phi_2}^2$. The mathematics is the same as that we had for the water waves! (It is hard to see how one could get such a simple result from a complicated game of electrons going back and forth through the plate on some strange trajectory.)

We conclude the following: The electrons arrive in lumps, like particles, and the probability of arrival of these lumps is distributed like the distribution of intensity of a wave. It is in this sense that an electron behaves “sometimes like a particle and sometimes like a wave.”

Incidentally, when we were dealing with classical waves we defined the intensity as the mean over time of the square of the wave amplitude, and we used complex numbers as a mathematical trick to simplify the analysis. But in quantum mechanics it turns out that the amplitudes must be represented by complex numbers. The real parts alone will not do. That is a technical point, for the moment, because the formulas look just the same.

Since the probability of arrival through both holes is given so simply, although it is not equal to $(P_1+P_2)$, that is really all there is to say. But there are a large number of subtleties involved in the fact that nature does work this way. We would like to illustrate some of these subtleties for you now. First, since the number that arrives at a particular point is not equal to the number that arrives through $1$ plus the number that arrives through $2$, as we would have concluded from Proposition A, undoubtedly we should conclude that Proposition A is false . It is not true that the electrons go either through hole $1$ or hole $2$. But that conclusion can be tested by another experiment.

1–6 Watching the electrons

We shall now try the following experiment. To our electron apparatus we add a very strong light source, placed behind the wall and between the two holes, as shown in Fig.  1–4 . We know that electric charges scatter light. So when an electron passes, however it does pass, on its way to the detector, it will scatter some light to our eye, and we can see where the electron goes. If, for instance, an electron were to take the path via hole $2$ that is sketched in Fig.  1–4 , we should see a flash of light coming from the vicinity of the place marked $A$ in the figure. If an electron passes through hole $1$, we would expect to see a flash from the vicinity of the upper hole. If it should happen that we get light from both places at the same time, because the electron divides in half … Let us just do the experiment!

Here is what we see: every time that we hear a “click” from our electron detector (at the backstop), we also see a flash of light either near hole $1$ or near hole $2$, but never both at once! And we observe the same result no matter where we put the detector. From this observation we conclude that when we look at the electrons we find that the electrons go either through one hole or the other. Experimentally, Proposition A is necessarily true.

What, then, is wrong with our argument against Proposition A? Why isn’t  $P_{12}$ just equal to $P_1+P_2$? Back to experiment! Let us keep track of the electrons and find out what they are doing. For each position ($x$-location) of the detector we will count the electrons that arrive and also keep track of which hole they went through, by watching for the flashes. We can keep track of things this way: whenever we hear a “click” we will put a count in Column $1$ if we see the flash near hole $1$, and if we see the flash near hole $2$, we will record a count in Column $2$. Every electron which arrives is recorded in one of two classes: those which come through $1$ and those which come through $2$. From the number recorded in Column $1$ we get the probability $P_1'$ that an electron will arrive at the detector via hole $1$; and from the number recorded in Column $2$ we get $P_2'$, the probability that an electron will arrive at the detector via hole $2$. If we now repeat such a measurement for many values of $x$, we get the curves for $P_1'$ and $P_2'$ shown in part (b) of Fig.  1–4 .

Well, that is not too surprising! We get for $P_1'$ something quite similar to what we got before for $P_1$ by blocking off hole $2$; and $P_2'$ is similar to what we got by blocking hole $1$. So there is not any complicated business like going through both holes. When we watch them, the electrons come through just as we would expect them to come through. Whether the holes are closed or open, those which we see come through hole $1$ are distributed in the same way whether hole $2$ is open or closed.

But wait! What do we have now for the total probability, the probability that an electron will arrive at the detector by any route? We already have that information. We just pretend that we never looked at the light flashes, and we lump together the detector clicks which we have separated into the two columns. We must just add the numbers. For the probability that an electron will arrive at the backstop by passing through either hole, we do find $P_{12}'=P_1'+P_2'$. That is, although we succeeded in watching which hole our electrons come through, we no longer get the old interference curve $P_{12}$, but a new one, $P_{12}'$, showing no interference! If we turn out the light $P_{12}$ is restored.

We must conclude that when we look at the electrons the distribution of them on the screen is different than when we do not look. Perhaps it is turning on our light source that disturbs things? It must be that the electrons are very delicate, and the light, when it scatters off the electrons, gives them a jolt that changes their motion. We know that the electric field of the light acting on a charge will exert a force on it. So perhaps we should expect the motion to be changed. Anyway, the light exerts a big influence on the electrons. By trying to “watch” the electrons we have changed their motions. That is, the jolt given to the electron when the photon is scattered by it is such as to change the electron’s motion enough so that if it might have gone to where $P_{12}$ was at a maximum it will instead land where $P_{12}$ was a minimum; that is why we no longer see the wavy interference effects.

You may be thinking: “Don’t use such a bright source! Turn the brightness down! The light waves will then be weaker and will not disturb the electrons so much. Surely, by making the light dimmer and dimmer, eventually the wave will be weak enough that it will have a negligible effect.” O.K. Let’s try it. The first thing we observe is that the flashes of light scattered from the electrons as they pass by does not get weaker. It is always the same-sized flash . The only thing that happens as the light is made dimmer is that sometimes we hear a “click” from the detector but see no flash at all . The electron has gone by without being “seen.” What we are observing is that light also acts like electrons, we knew that it was “wavy,” but now we find that it is also “lumpy.” It always arrives—or is scattered—in lumps that we call “photons.” As we turn down the intensity of the light source we do not change the size of the photons, only the rate at which they are emitted. That explains why, when our source is dim, some electrons get by without being seen. There did not happen to be a photon around at the time the electron went through.

This is all a little discouraging. If it is true that whenever we “see” the electron we see the same-sized flash, then those electrons we see are always the disturbed ones. Let us try the experiment with a dim light anyway. Now whenever we hear a click in the detector we will keep a count in three columns: in Column (1) those electrons seen by hole $1$, in Column (2) those electrons seen by hole $2$, and in Column (3) those electrons not seen at all. When we work up our data (computing the probabilities) we find these results: Those “seen by hole $1$” have a distribution like $P_1'$; those “seen by hole $2$” have a distribution like $P_2'$ (so that those “seen by either hole $1$ or $2$” have a distribution like $P_{12}'$); and those “not seen at all” have a “wavy” distribution just like $P_{12}$ of Fig.  1–3 ! If the electrons are not seen, we have interference!

That is understandable. When we do not see the electron, no photon disturbs it, and when we do see it, a photon has disturbed it. There is always the same amount of disturbance because the light photons all produce the same-sized effects and the effect of the photons being scattered is enough to smear out any interference effect.

Is there not some way we can see the electrons without disturbing them? We learned in an earlier chapter that the momentum carried by a “photon” is inversely proportional to its wavelength ($p=h/\lambda$). Certainly the jolt given to the electron when the photon is scattered toward our eye depends on the momentum that photon carries. Aha! If we want to disturb the electrons only slightly we should not have lowered the intensity of the light, we should have lowered its frequency (the same as increasing its wavelength). Let us use light of a redder color. We could even use infrared light, or radiowaves (like radar), and “see” where the electron went with the help of some equipment that can “see” light of these longer wavelengths. If we use “gentler” light perhaps we can avoid disturbing the electrons so much.

Let us try the experiment with longer waves. We shall keep repeating our experiment, each time with light of a longer wavelength. At first, nothing seems to change. The results are the same. Then a terrible thing happens. You remember that when we discussed the microscope we pointed out that, due to the wave nature of the light, there is a limitation on how close two spots can be and still be seen as two separate spots. This distance is of the order of the wavelength of light. So now, when we make the wavelength longer than the distance between our holes, we see a big fuzzy flash when the light is scattered by the electrons. We can no longer tell which hole the electron went through! We just know it went somewhere! And it is just with light of this color that we find that the jolts given to the electron are small enough so that $P_{12}'$ begins to look like $P_{12}$—that we begin to get some interference effect. And it is only for wavelengths much longer than the separation of the two holes (when we have no chance at all of telling where the electron went) that the disturbance due to the light gets sufficiently small that we again get the curve $P_{12}$ shown in Fig.  1–3 .

In our experiment we find that it is impossible to arrange the light in such a way that one can tell which hole the electron went through, and at the same time not disturb the pattern. It was suggested by Heisenberg that the then new laws of nature could only be consistent if there were some basic limitation on our experimental capabilities not previously recognized. He proposed, as a general principle, his uncertainty principle , which we can state in terms of our experiment as follows: “It is impossible to design an apparatus to determine which hole the electron passes through, that will not at the same time disturb the electrons enough to destroy the interference pattern.” If an apparatus is capable of determining which hole the electron goes through, it cannot be so delicate that it does not disturb the pattern in an essential way. No one has ever found (or even thought of) a way around the uncertainty principle. So we must assume that it describes a basic characteristic of nature.

The complete theory of quantum mechanics which we now use to describe atoms and, in fact, all matter, depends on the correctness of the uncertainty principle. Since quantum mechanics is such a successful theory, our belief in the uncertainty principle is reinforced. But if a way to “beat” the uncertainty principle were ever discovered, quantum mechanics would give inconsistent results and would have to be discarded as a valid theory of nature.

“Well,” you say, “what about Proposition A? Is it true, or is it not true, that the electron either goes through hole $1$ or it goes through hole $2$?” The only answer that can be given is that we have found from experiment that there is a certain special way that we have to think in order that we do not get into inconsistencies. What we must say (to avoid making wrong predictions) is the following. If one looks at the holes or, more accurately, if one has a piece of apparatus which is capable of determining whether the electrons go through hole $1$ or hole $2$, then one can say that it goes either through hole $1$ or hole $2$. But , when one does not try to tell which way the electron goes, when there is nothing in the experiment to disturb the electrons, then one may not say that an electron goes either through hole $1$ or hole $2$. If one does say that, and starts to make any deductions from the statement, he will make errors in the analysis. This is the logical tightrope on which we must walk if we wish to describe nature successfully.

If the motion of all matter—as well as electrons—must be described in terms of waves, what about the bullets in our first experiment? Why didn’t we see an interference pattern there? It turns out that for the bullets the wavelengths were so tiny that the interference patterns became very fine. So fine, in fact, that with any detector of finite size one could not distinguish the separate maxima and minima. What we saw was only a kind of average, which is the classical curve. In Fig.  1–5 we have tried to indicate schematically what happens with large-scale objects. Part (a) of the figure shows the probability distribution one might predict for bullets, using quantum mechanics. The rapid wiggles are supposed to represent the interference pattern one gets for waves of very short wavelength. Any physical detector, however, straddles several wiggles of the probability curve, so that the measurements show the smooth curve drawn in part (b) of the figure.

1–7 First principles of quantum mechanics

We will now write a summary of the main conclusions of our experiments. We will, however, put the results in a form which makes them true for a general class of such experiments. We can write our summary more simply if we first define an “ideal experiment” as one in which there are no uncertain external influences, i.e., no jiggling or other things going on that we cannot take into account. We would be quite precise if we said: “An ideal experiment is one in which all of the initial and final conditions of the experiment are completely specified.” What we will call “an event” is, in general, just a specific set of initial and final conditions. (For example: “an electron leaves the gun, arrives at the detector, and nothing else happens.”) Now for our summary.

• The probability of an event in an ideal experiment is given by the square of the absolute value of a complex number $\phi$ which is called the probability amplitude: \begin{equation} \begin{aligned} P&=\text{probability},\\ \phi&=\text{probability amplitude},\\ P&=\abs{\phi}^2. \end{aligned} \label{Eq:III:1:6} \end{equation}
• When an event can occur in several alternative ways, the probability amplitude for the event is the sum of the probability amplitudes for each way considered separately. There is interference: \begin{equation} \begin{aligned} \phi&=\phi_1+\phi_2,\\ P&=\abs{\phi_1+\phi_2}^2. \end{aligned} \label{Eq:III:1:7} \end{equation}
• If an experiment is performed which is capable of determining whether one or another alternative is actually taken, the probability of the event is the sum of the probabilities for each alternative. The interference is lost: \begin{equation} \label{Eq:III:1:8} P=P_1+P_2. \end{equation}

One might still like to ask: “How does it work? What is the machinery behind the law?” No one has found any machinery behind the law. No one can “explain” any more than we have just “explained.” No one will give you any deeper representation of the situation. We have no ideas about a more basic mechanism from which these results can be deduced.

We would like to emphasize a very important difference between classical and quantum mechanics . We have been talking about the probability that an electron will arrive in a given circumstance. We have implied that in our experimental arrangement (or even in the best possible one) it would be impossible to predict exactly what would happen. We can only predict the odds! This would mean, if it were true, that physics has given up on the problem of trying to predict exactly what will happen in a definite circumstance. Yes! physics has given up. We do not know how to predict what would happen in a given circumstance , and we believe now that it is impossible—that the only thing that can be predicted is the probability of different events. It must be recognized that this is a retrenchment in our earlier ideal of understanding nature. It may be a backward step, but no one has seen a way to avoid it.

We make now a few remarks on a suggestion that has sometimes been made to try to avoid the description we have given: “Perhaps the electron has some kind of internal works—some inner variables—that we do not yet know about. Perhaps that is why we cannot predict what will happen. If we could look more closely at the electron, we could be able to tell where it would end up.” So far as we know, that is impossible. We would still be in difficulty. Suppose we were to assume that inside the electron there is some kind of machinery that determines where it is going to end up. That machine must also determine which hole it is going to go through on its way. But we must not forget that what is inside the electron should not be dependent on what we do, and in particular upon whether we open or close one of the holes. So if an electron, before it starts, has already made up its mind (a) which hole it is going to use, and (b) where it is going to land, we should find $P_1$ for those electrons that have chosen hole $1$, $P_2$ for those that have chosen hole $2$, and necessarily the sum $P_1+P_2$ for those that arrive through the two holes. There seems to be no way around this. But we have verified experimentally that that is not the case. And no one has figured a way out of this puzzle. So at the present time we must limit ourselves to computing probabilities. We say “at the present time,” but we suspect very strongly that it is something that will be with us forever—that it is impossible to beat that puzzle—that this is the way nature really is .

1–8 The uncertainty principle

This is the way Heisenberg stated the uncertainty principle originally: If you make the measurement on any object, and you can determine the $x$-component of its momentum with an uncertainty $\Delta p$, you cannot, at the same time, know its $x$-position more accurately than $\Delta x\geq\hbar/2\Delta p$, where $\hbar$ is a definite fixed number given by nature. It is called the “reduced Planck constant,” and is approximately $1.05\times10^{-34}$ joule-seconds. The uncertainties in the position and momentum of a particle at any instant must have their product greater than or equal to half the reduced Planck constant. This is a special case of the uncertainty principle that was stated above more generally. The more general statement was that one cannot design equipment in any way to determine which of two alternatives is taken, without, at the same time, destroying the pattern of interference.

Let us show for one particular case that the kind of relation given by Heisenberg must be true in order to keep from getting into trouble. We imagine a modification of the experiment of Fig.  1–3 , in which the wall with the holes consists of a plate mounted on rollers so that it can move freely up and down (in the $x$-direction), as shown in Fig.  1–6 . By watching the motion of the plate carefully we can try to tell which hole an electron goes through. Imagine what happens when the detector is placed at $x=0$. We would expect that an electron which passes through hole $1$ must be deflected downward by the plate to reach the detector. Since the vertical component of the electron momentum is changed, the plate must recoil with an equal momentum in the opposite direction. The plate will get an upward kick. If the electron goes through the lower hole, the plate should feel a downward kick. It is clear that for every position of the detector, the momentum received by the plate will have a different value for a traversal via hole $1$ than for a traversal via hole $2$. So! Without disturbing the electrons at all , but just by watching the plate , we can tell which path the electron used.

Now in order to do this it is necessary to know what the momentum of the screen is, before the electron goes through. So when we measure the momentum after the electron goes by, we can figure out how much the plate’s momentum has changed. But remember, according to the uncertainty principle we cannot at the same time know the position of the plate with an arbitrary accuracy. But if we do not know exactly where the plate is, we cannot say precisely where the two holes are. They will be in a different place for every electron that goes through. This means that the center of our interference pattern will have a different location for each electron. The wiggles of the interference pattern will be smeared out. We shall show quantitatively in the next chapter that if we determine the momentum of the plate sufficiently accurately to determine from the recoil measurement which hole was used, then the uncertainty in the $x$-position of the plate will, according to the uncertainty principle, be enough to shift the pattern observed at the detector up and down in the $x$-direction about the distance from a maximum to its nearest minimum. Such a random shift is just enough to smear out the pattern so that no interference is observed.

The uncertainty principle “protects” quantum mechanics. Heisenberg recognized that if it were possible to measure the momentum and the position simultaneously with a greater accuracy, the quantum mechanics would collapse. So he proposed that it must be impossible. Then people sat down and tried to figure out ways of doing it, and nobody could figure out a way to measure the position and the momentum of anything—a screen, an electron, a billiard ball, anything—with any greater accuracy. Quantum mechanics maintains its perilous but still correct existence.

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Double-slit Experiment

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Plane wave representing a particle passing through two slits, resulting in an interference pattern on a screen some distance away from the slits. [1] .

The double-slit experiment is an experiment in quantum mechanics and optics demonstrating the wave-particle duality of electrons , photons , and other fundamental objects in physics. When streams of particles such as electrons or photons pass through two narrow adjacent slits to hit a detector screen on the other side, they don't form clusters based on whether they passed through one slit or the other. Instead, they interfere: simultaneously passing through both slits, and producing a pattern of interference bands on the screen. This phenomenon occurs even if the particles are fired one at a time, showing that the particles demonstrate some wave behavior by interfering with themselves as if they were a wave passing through both slits.

Niels Bohr proposed the idea of wave-particle duality to explain the results of the double-slit experiment. The idea is that all fundamental particles behave in some ways like waves and in other ways like particles, depending on what properties are being observed. These insights led to the development of quantum mechanics and quantum field theory , the current basis behind the Standard Model of particle physics , which is our most accurate understanding of how particles work.

The original double-slit experiment was performed using light/photons around the turn of the nineteenth century by Thomas Young, so the original experiment is often called Young's double-slit experiment. The idea of using particles other than photons in the experiment did not come until after the ideas of de Broglie and the advent of quantum mechanics, when it was proposed that fundamental particles might also behave as waves with characteristic wavelengths depending on their momenta. The single-electron version of the experiment was in fact not performed until 1974. A more recent version of the experiment successfully demonstrating wave-particle duality used buckminsterfullerene or buckyballs , the \(C_{60}\) allotrope of carbon.

Waves vs. Particles

Double-slit experiment with electrons, modeling the double-slit experiment.

To understand why the double-slit experiment is important, it is useful to understand the strong distinctions between wave and particles that make wave-particle duality so intriguing.

Waves describe oscillating values of a physical quantity that obey the wave equation . They are usually described by sums of sine and cosine functions, since any periodic (oscillating) function may be decomposed into a Fourier series . When two waves pass through each other, the resulting wave is the sum of the two original waves. This is called a superposition since the waves are placed ("-position") on top of each other ("super-"). Superposition is one of the most fundamental principles of quantum mechanics. A general quantum system need not be in one state or another but can reside in a superposition of two where there is some probability of measuring the quantum wavefunction in one state or another.

Left: example of superposed waves constructively interfering. Right: superposed waves destructively interfering. [2]

If one wave is \(A(x) = \sin (2x)\) and the other is \(B(x) = \sin (2x)\), then they add together to make \(A + B = 2 \sin (2x)\). The addition of two waves to form a wave of larger amplitude is in general known as constructive interference since the interference results in a larger wave.

If one wave is \(A(x) = \sin (2x)\) and the other is \(B(x) = \sin (2x + \pi)\), then they add together to make \(A + B = 0\) \(\big(\)since \(\sin (2x + \pi) = - \sin (2x)\big).\) This is known as destructive interference in general, when adding two waves results in a wave of smaller amplitude. See the figure above for examples of both constructive and destructive interference.

Two speakers are generating sounds with the same phase, amplitude, and wavelength. The two sound waves can make constructive interference, as above left. Or they can make destructive interference, as above right. If we want to find out the exact position where the two sounds make destructive interference, which of the following do we need to know?

a) the wavelength of the sound waves b) the distances from the two speakers c) the speed of sound generated by the two speakers

This wave behavior is quite unlike the behavior of particles. Classically, particles are objects with a single definite position and a single definite momentum. Particles do not make interference patterns with other particles in detectors whether or not they pass through slits. They only interact by colliding elastically , i.e., via electromagnetic forces at short distances. Before the discovery of quantum mechanics, it was assumed that waves and particles were two distinct models for objects, and that any real physical thing could only be described as a particle or as a wave, but not both.

In the more modern version of the double slit experiment using electrons, electrons with the same momentum are shot from an "electron gun" like the ones inside CRT televisions towards a screen with two slits in it. After each electron goes through one of the slits, it is observed hitting a single point on a detecting screen at an apparently random location. As more and more electrons pass through, one at a time, they form an overall pattern of light and dark interference bands. If each electron was truly just a point particle, then there would only be two clusters of observations: one for the electrons passing through the left slit, and one for the right. However, if electrons are made of waves, they interfere with themselves and pass through both slits simultaneously. Indeed, this is what is observed when the double-slit experiment is performed using electrons. It must therefore be true that the electron is interfering with itself since each electron was only sent through one at a time—there were no other electrons to interfere with it!

When the double-slit experiment is performed using electrons instead of photons, the relevant wavelength is the de Broglie wavelength \(\lambda:\)

\[\lambda = \frac{h}{p},\]

where \(h\) is Planck's constant and \(p\) is the electron's momentum.

Calculate the de Broglie wavelength of an electron moving with velocity \(1.0 \times 10^{7} \text{ m/s}.\)

Usain Bolt, the world champion sprinter, hit a top speed of 27.79 miles per hour at the Olympics. If he has a mass of 94 kg, what was his de Broglie wavelength?

Express your answer as an order of magnitude in units of the Bohr radius \(r_{B} = 5.29 \times 10^{-11} \text{m}\). For instance, if your answer was \(4 \times 10^{-5} r_{B}\), your should give \(-5.\)

Image Credit: Flickr drcliffordchoi.

While the de Broglie relation was postulated for massive matter, the equation applies equally well to light. Given light of a certain wavelength, the momentum and energy of that light can be found using de Broglie's formula. This generalizes the naive formula \(p = m v\), which can't be applied to light since light has no mass and always moves at a constant velocity of \(c\) regardless of wavelength.

The below is reproduced from the Amplitude, Frequency, Wave Number, Phase Shift wiki.

In Young's double-slit experiment, photons corresponding to light of wavelength \(\lambda\) are fired at a barrier with two thin slits separated by a distance \(d,\) as shown in the diagram below. After passing through the slits, they hit a screen at a distance of \(D\) away with \(D \gg d,\) and the point of impact is measured. Remarkably, both the experiment and theory of quantum mechanics predict that the number of photons measured at each point along the screen follows a complicated series of peaks and troughs called an interference pattern as below. The photons must exhibit the wave behavior of a relative phase shift somehow to be responsible for this phenomenon. Below, the condition for which maxima of the interference pattern occur on the screen is derived.

Left: actual experimental two-slit interference pattern of photons, exhibiting many small peaks and troughs. Right: schematic diagram of the experiment as described above. [3]

Since \(D \gg d\), the angle from each of the slits is approximately the same and equal to \(\theta\). If \(y\) is the vertical displacement to an interference peak from the midpoint between the slits, it is therefore true that

\[D\tan \theta \approx D\sin \theta \approx D\theta = y.\]

Furthermore, there is a path difference \(\Delta L\) between the two slits and the interference peak. Light from the lower slit must travel \(\Delta L\) further to reach any particular spot on the screen, as in the diagram below:

Light from the lower slit must travel further to reach the screen at any given point above the midpoint, causing the interference pattern.

The condition for constructive interference is that the path difference \(\Delta L\) is exactly equal to an integer number of wavelengths. The phase shift of light traveling over an integer \(n\) number of wavelengths is exactly \(2\pi n\), which is the same as no phase shift and therefore constructive interference. From the above diagram and basic trigonometry, one can write

\[\Delta L = d\sin \theta \approx d\theta = n\lambda.\]

The first equality is always true; the second is the condition for constructive interference.

Now using \(\theta = \frac{y}{D}\), one can see that the condition for maxima of the interference pattern, corresponding to constructive interference, is

\[n\lambda = \frac{dy}{D},\]

i.e. the maxima occur at the vertical displacements of

\[y = \frac{n\lambda D}{d}.\]

The analogous experimental setup and mathematical modeling using electrons instead of photons is identical except that the de Broglie wavelength of the electrons \(\lambda = \frac{h}{p}\) is used instead of the literal wavelength of light.

• Lookang, . CC-3.0 Licensing . Retrieved from https://commons.wikimedia.org/w/index.php?curid=17014507
• Haade, . CC-3.0 Licensing . Retrieved from https://commons.wikimedia.org/w/index.php?curid=10073387
• Jordgette, . CC-3.0 Licensing . Retrieved from https://commons.wikimedia.org/w/index.php?curid=9529698

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• 07 August 2018

Two slits and one hell of a quantum conundrum

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Bands of light in a double-slit experiment. Credit: Timm Weitkamp/CC BY 3.0

Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality Anil Ananthaswamy Dutton (2018)

According to the eminent physicist Richard Feynman, the quantum double-slit experiment puts us “up against the paradoxes and mysteries and peculiarities of nature”. By Feynman’s logic, if we could understand what is going on in this deceptively simple experiment, we would penetrate to the heart of quantum theory — and perhaps all its puzzles would dissolve.

That’s the premise of Through Two Doors at Once . Science writer Anil Ananthaswamy focuses on this single experiment, which has taken many forms since quantum mechanics debuted in the early twentieth century with the work of Max Planck, Albert Einstein, Niels Bohr and others. In some versions, nature seems magically to discern our intentions before we enact them — or perhaps retroactively to alter the past. In others, the outcome seems dependent on what we know, not what we do. In yet others, we can deduce something about a system without looking at it. All in all, the double-slit experiment seems, to borrow from Feynman again, “screwy”.

The original experiment, as Ananthaswamy notes, was classical, conducted by British polymath Thomas Young in the early 1800s to show that light is a wave. He passed light through two closely spaced parallel slits in a screen, and on the far side saw several bright bands. This, he realized, was an ‘interference’ pattern. Caused by the interaction of waves emanating from the openings, it’s not unlike the pattern that appears when two pebbles are dropped into water and the ripples they create add to or dampen each other’s peaks and troughs. With ordinary particles, the slits would act more like stencils for sprayed paint, creating two defined bands.

We now know that quantum particles create such an interference pattern, too — evidence that they have a wave-like nature. Postulated in 1924 by French physicist Louis de Broglie, this idea was verified for electrons a few years later by US physicists Clinton Davisson and Lester Germer. Even large molecules such as buckminsterfullerene — made of 60 carbon atoms — will behave in this way.

You can get used to that. What’s odd is that the interference pattern remains — accumulating over many particle impacts — even if particles go through the slits one at a time. The particles seem to interfere with themselves. Odder, the pattern vanishes if we use a detector to measure which slit the particle goes through: it’s truly particle-like, with no more waviness. Oddest of all, that remains true if we delay the measurement until after the particle has traversed the slits (but before it hits the screen). And if we make the measurement but then delete the result without looking at it, interference returns.

It’s not the physical act of measurement that seems to make the difference, but the “act of noticing”, as physicist Carl von Weizsäcker (who worked closely with quantum pioneer Werner Heisenberg) put it in 1941. Ananthaswamy explains that this is what is so strange about quantum mechanics: it can seem impossible to eliminate a decisive role for our conscious intervention in the outcome of experiments. That fact drove physicist Eugene Wigner to suppose at one point that the mind itself causes the ‘collapse’ that turns a wave into a particle.

Ananthaswamy offers some of the most lucid explanations I’ve seen of other interpretations. Bohr’s answer was that quantum mechanics doesn’t let us say anything about the particle’s ‘path’ — one slit or two — before it is measured. The role of the theory, said Bohr, is to furnish predictions of measurement outcomes; in that regard, it has never been found to fail. (However, he did not, as is often implied, deny that there is any physical reality beyond measurement.) Yet this does feel rather unsatisfactory. Ananthaswamy seems tempted by the alternative idea offered by David Bohm in the 1950s. Here, quantum objects are both particle and wave, the wave somehow ‘piloting’ the particle through space while being sensitive to influences beyond the particle’s location. But Ananthaswamy concludes that “physics has yet to complete its passage through the double-slit experiment. The case remains unsolved.”

With apologies to researchers convinced that they have the answer, this is true: there is no consensus. At any rate, Bohr was right to advise caution in how we use language. There is nothing in quantum mechanics as it stands, shorn of interpretation, that lets us speak of particles becoming waves or taking two paths at once. And there is no reason to regard the wavefunction as more or less than an abstraction. This mathematical function, which embodies all we can know about a quantum object (and features in the iconic equation devised by Erwin Schrödinger to describe the object’s wave-like behaviour) was characterized rather nicely by physicist Roland Omnès. He called it “the fuel of a machine that manufactures probabilities” — that is, probabilities of measurement outcomes.

Refracting all of quantum mechanics through the double slits is both a strength and a weakness of Through Two Doors at Once . It brings unity to a knotty subject, but downplays some important strands. Those include John Bell’s 1964 thought experiment on the nature of quantum entanglement (conducted for real many times since the 1970s); the role of decoherence in the emergence of classical physics from quantum phenomena (adduced in the 1970s and 1980s); and the emphasis on information and causality in the past two decades. Still, given that popularization of quantum mechanics seems to be the flavour of the month — summoning Adam Becker’s 2018 book What is Real? , Jean Bricmont’s 2017 Quantum Sense and Nonsense , a forthcoming book by physicist Sean Carroll, and my own 2018 Beyond Weird — there’s no lack of a wider perspective.

And we need that. Ananthaswamy’s conclusion — that perhaps all the major interpretations are “touching the truth in their own way” — is not a shrugging capitulation. It’s a well-advised commitment to pluralism, shared with Becker’s book and mine. For now, uncertainty seems the wisest position in the quantum world.

Nature 560 , 165 (2018)

doi: https://doi.org/10.1038/d41586-018-05892-6

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You are here, phys 201: fundamentals of physics ii,  - quantum mechanics i: key experiments and wave-particle duality.

The double slit experiment, which implies the end of Newtonian Mechanics, is described. The de Broglie relation between wavelength and momentum is deduced from experiment for photons and electrons. The photoelectric effect and Compton scattering, which provided experimental support for Einstein’s photon theory of light, are reviewed. The wave function is introduced along with the probability interpretation. The uncertainty principle is shown to arise from the fact that the particle’s location is determined by a wave and that waves diffract when passing a narrow opening.

Lecture Chapters

• Recap of Young's Double Slit Experiment
• The Particulate Nature of Light
• The Photoelectric Effect
• Compton's Scattering
• Particle-Wave Duality of Matter
• The Uncertainty Principle

The double-slit experiment: Is light a wave or a particle?

The double-slit experiment is universally weird.

How does the double-slit experiment work?

Interference patterns from waves, particle patterns, double-slit experiment: quantum mechanics, history of the double-slit experiment, additional resources.

The double-slit experiment is one of the most famous experiments in physics and definitely one of the weirdest. It demonstrates that matter and energy (such as light) can exhibit both wave and particle characteristics — known as the particle-wave duality of matter — depending on the scenario, according to the scientific communication site Interesting Engineering .

According to the University of Sussex , American physicist Richard Feynman referred to this paradox as the central mystery of quantum mechanics. 

We know the quantum world is strange, but the two-slit experiment takes things to a whole new level. The experiment has perplexed scientists for over 200 years, ever since the first version was first performed by British scientist Thomas Young in 1801.

Related: 10 mind-boggling things you should know about quantum physics  

Christian Huygens was the first to describe light as traveling in waves whilst Isaac Newton thought light was composed of tiny particles according to Las Cumbres Observatory . But who is right? British polymath Thomas Young designed the double-slit experiment to put these theories to the test. 

To appreciate the truly bizarre nature of the double-split experiment we first need to understand how waves and particles act when passing through two slits. 

When Young first carried out the double-split experiment in 1801 he found that light behaved like a wave. 

Firstly, if we were to shine a light on a wall with two parallel slits — and for the sake of simplicity, let's say this light has only one wavelength. 

As the light passes through the slits, each, in turn, becomes almost like a new source of light. On the far side of the divider, the light from each slit diffracts and overlaps with the light from the other slit, interfering with each other. 

According to Stony Brook University , any wave can create an interference pattern, whether it be a sound wave, light wave or waves across a body of water. When a wave crest hits a wave trough they cancel each other out — known as destructive interference — and appear as a dark band. When a crest hits a crest they amplify each other — known as constructive interference — and appear as a bright band. The combination of dark and bright bands is known as an interference pattern and can be seen on the sensor screen opposite the slits. 

This interference pattern was the evidence Young needed to determine that light was a wave and not a particle as Newton had suggested. 

But that is not the whole story. Light is a little more complicated than that, and to see how strange it really is we also need to understand what pattern a particle would make on a sensor field. 

If you were to carry out the same experiment and fire grains of sand or other particles through the slits, you would end up with a different pattern on the sensor screen. Each particle would go through a slit end up in a line in roughly the same place (with a little bit of spread depending on the angle the particle passed through the slit).  

Clearly, waves and particles produce a very different pattern, so it should be easy to distinguish between the two right? Well, this is where the double-slit experiment gets a little strange when we try and carry out the same experiment but with tiny particles of light called photons. Enter the realm of quantum mechanics. 

The smallest constituent of light is subatomic particles called photons. By using photons instead of grains of sand we can carry out the double-slit experiment on an atomic scale. 

If you block off one of the slits, so it is just a single-slit experiment, and fire photons through to the sensor screen, the photons will appear as pinprick points on the sensor screen, mimicking the particle patterns produced by sand in the previous example. From this evidence, we could suggest that photons are particles. 

Now, this is where things start to get weird. 

If you unblock the slit and fire photons through both slits, you start to see something very similar to the interference pattern produced by waves in the light example. The photons appear to have gone through the pair of slits acting like waves. 

But what if you launch photons one by one, leaving enough time between them that they don't have a chance of interfering with each other, will they behave like particles or waves? 

At first, the photons appear on the sensor screen in a random scattered manner, but as you fire more and more of them, an interference pattern begins to emerge. Each photon by itself appears to be contributing to the overall wave-like behavior that manifests as an interference pattern on the screen — even though they were launched one at a time so that no interference between them was possible.

It's almost as though each photon is "aware" that there are two slits available. How? Does it split into two and then rejoin after the slit and then hit the sensor? To investigate this, scientists set up a detector that can tell which slit the photon passes through. 

Again, we fire photons one at a time at the slits, as we did in the previous example. The detector finds that about 50% of the photons have passed through the top slit and about 50% through the bottom, and confirms that each photon goes through one slit or the other. Nothing too unusual there. 

But when we look at the sensor screen on this experiment, a different pattern emerges. 

This pattern matches the one we saw when we fired particles through the slits. It appears that monitoring the photons triggers them to switch from the interference pattern produced by waves to that produced by particles. 

If the detection of photons through the slits is apparently affecting the pattern on the sensor screen, what happens if we leave the detector in place but switch it off? (Shh, don't tell the photons we're no longer spying on them!) 

This is where things get really, really weird. 

Same slits, same photons, same detector, just turned off. Will we see the same particle-like pattern? 

No. The particles again make a wave-like interference pattern on the sensor screen. 

The atoms appear to act like waves when you're not watching them, but as particles when you are. How? Well, if you can answer that, a Nobel Prize is waiting for you. 

In the 1930s, scientists proposed that human consciousness might affect quantum mechanics. Mathematician John Von Neumann first postulated this in 1932 in his book " The Mathematical Foundations of Quantum Mechanics ." In the 1960s, theoretical physicist, Eugene Wigner conceived a thought experiment called Wigner's friend — a paradox in quantum physics that describes the states of two people, one conducting the experiment and the observer of the first person, according to science magazine Popular Mechanics . The idea that the consciousness of a person carrying out the experiment can affect the result is knowns as the Von Neumann–Wigner interpretation.

Though a spiritual explanation for quantum mechanic behavior is still believed by a few individuals, including author and alternative medicine advocate Deepak Chopra , a majority of the science community has long disregarded it. 

As for a more plausible theory, scientists are stumped. 

Furthermore —and perhaps even more astonishingly — if you set up the double-slit experiment to detect which slit the photon went through after the photon has already hit the sensor screen, you still end up with a particle-type pattern on the sensor screen, even though the photon hadn't yet been detected when it hit the screen. This result suggests that detecting a photon in the future affects the pattern produced by the photon on the sensor screen in the past. This experiment is known as the quantum eraser experiment and is explained in more detail in this informative video from Fermilab . 

We still don't fully understand how exactly the particle-wave duality of matter works, which is why it is regarded as one of the greatest mysteries of quantum mechanics. 

The first version of the double-slit experiment was carried out in 1801 by British polymath Thomas Young, according to the American Physical Society (APS). His experiment demonstrated the interference of light waves and provided evidence that light was a wave, not a particle. 

Young also used data from his experiments to calculate the wavelengths of different colors of light and came very close to modern values.

Despite his convincing experiment that light was a wave, those who did not want to accept that Isaac Newton could have been wrong about something criticized Young. (Newton had proposed the corpuscular theory, which posited that light was composed of a stream of tiny particles he called corpuscles.) 

According to APS, Young wrote in response to one of the critics, "Much as I venerate the name of Newton, I am not therefore obliged to believe that he was infallible."

Since the development of quantum mechanics, physicists now acknowledge light to be both a particle and a wave. 

Explore the double-slit experiment in more detail with this article from the University of Cambridge, which includes images of electron patterns in a double-slit experiment. Discover the true nature of light with Canon Science Lab . Read about fragments of energy that are not waves or particles — but could be the fundamental building blocks of the universe — in this article from The Conversation . Dive deeper into the two-slit experiment in this article published in the journal Nature . 

Bibliography

Grangier, Philippe, Gerard Roger, and Alain Aspect. " Experimental evidence for a photon anticorrelation effect on a beam splitter: a new light on single-photon interferences. " EPL (Europhysics Letters) 1.4 (1986): 173.

Thorn, J. J., et al. "Observing the quantum behavior of light in an undergraduate laboratory. " American Journal of Physics 72.9 (2004): 1210-1219.

Ghose, Partha. " The central mystery of quantum mechanics. " arXiv preprint arXiv:0906.0898 (2009).

Aharonov, Yakir, et al. " Finally making sense of the double-slit experiment. " Proceedings of the National Academy of Sciences 114.25 (2017): 6480-6485.

Peng, Hui. " Observations of Cross-Double-Slit Experiments. " International Journal of Physics 8.2 (2020): 39-41. 

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The Double-Slit Experiment That Blew Open Quantum Mechanics

Is light a wave or a particle? Yes.

The physics equations you learned in school don't work on the atomic scale. We have Newtonian physics to explain the world we can see and feel, and we have Einsteinian physics to explain the behavior of matter and light in the universe, but we observe a bunch of bizarre phenomena on the atomic scale that we can't explain fully yet with equations and mathematical laws.

The experiment that started physicists down the path to discovering the wonderfully spooky behaviors of atomic particles is called the double-slit experiment. We know that light travels in waves, and when those waves pass through two parallel slits, a single wave gets separated into two waves that run into each other. PBS's Space Time series has a great new video explaining the double-slit experiment.

When we shoot two waves of light through a double slit, they form a pattern based on the way their peaks and troughs match up or clash. When we shoot a single photon through, we'd expect it to just go through unchanged. But it won't. When you shoot enough single photons through—one at a time, alternating slits—they form the same interference pattern as the waves of light. Basically, that means that all the possible paths of these particles can interfere with each other, even though only one of the possible paths actually happens. Mind blown?

We can't fully explain these phenomena yet, but we can observe them. (The video above runs through some of the leading theories for these odd quantum behaviors. It's trippy stuff.) It is only a matter of time before someone comes up with the correct mathematical equations to fully predict and model these events, and when they do, the third major set of physical laws will be born.

Jay Bennett is the associate editor of PopularMechanics.com. He has also written for Smithsonian, Popular Science and Outside Magazine. 

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Quantum Mechanics and the Famous Double-slit Experiment

Table of Contents

• Quantum mechanics is known for its strangeness, including phenomena like wave-particle duality, which allows particles to behave like waves.
• The double-slit experiment is a key demonstration of this duality, showing that even single particles, like photons, exhibit wave-like behavior.
• When the experiment measures which slit a particle goes through, it behaves like a particle. When this measurement is not made, it exhibits interference patterns typical of waves.
• Heisenberg’s uncertainty principle plays a crucial role, connecting the accuracy of position measurement to the momentum of particles.
• Matter waves, proposed by de Broglie, suggest that all matter, not just light, exhibits wave-like properties.
• Wheeler’s delayed-choice experiment demonstrates that making a decision about what to measure after a particle has passed through the slits can affect its behavior, effectively rewriting the past.
• This temporal editing could theoretically reach back to the beginning of the universe, the big bang.
• The interpretation of quantum phenomena like this varies, and different physicists may have differing views on the implications of these experiments.

Introduction

Quantum mechanics is famously strange. But Schroedinger’s dead and alive cat, Einstein’s spooky action at a distance, and de Broglie’s matter waves are mere trifles compared to the greatest strangeness: quantum mechanics can be used to edit events that should have already happened; in other words, to re-write history. However, before we can understand how this is possible, we first need to know how Heisenberg’s uncertainty principle accounts for wave-particle duality in the double-slit experiment.

The Double-Slit Experiment

The double-slit experiment is famous because it provides an unequivocal demonstration that light behaves like a wave. But the importance of the double-slit experiment extends far beyond that demonstration because, as Richard Feynman said in 1966:

In reality, it contains the only mystery…In telling you how it works, we will have to tell you about the basic peculiarities of all quantum mechanics .

Figure 1: The double-slit experiment. Waves travel from the source (top) until they reach the first barrier, which contains a slit. A semi-circular wave emanates from the slit until it reaches the second barrier, which contains two slits. The two semi-circular waves emanating from these slits interfere with each other, producing peaks and troughs along radial lines that form an interference pattern on a screen (bottom).

Based on wor k first described by Young in 1802, the experimental apparatus consists of two vertical slits and a screen, as shown in Figure 1. The light emanating from each slit interferes with light from the other slit to produce an  interference pattern  on the screen. This seems to prove conclusively that light consists of waves, even though the only entities detected at the screen are individual particles, which we now call  photons . The bright regions in the interference pattern correspond to areas where photons land with high probability and the dark regions to areas where photons land with low probability.

Figure 2: Emergence (a→d) of an interference pattern in a double-slit experiment8, where each dot represents a photon. Reproduced with permission from Tanamura (Wikimedia).

Remarkably, the same type of interference pattern is obtained even when the light is made so dim that only one photon at a time reaches the screen, as shown in Figure 2. With such a low photon rate, it can take several weeks for the interference pattern to emerge. However, the very fact that an interference pattern emerges at all implies that even a single photon behaves like a wave. This, in turn, seems to imply that each photon passes through both slits at the same time, which is clearly nonsense. Only a wave can pass through both slits, but only a photon can hit a single point on the screen. However, this single-photon interference behavior is not the most remarkable feature of the double-slit experiment.

If a light detector is used to measure which slit each photon passes through then the interference pattern (shown in Figures 2d and 3b) is replaced by a  diffraction envelope  (shown in Figure 3a), as if the light consists of two streams of particles which spread out and merge over time.

Figure 3: The double-slit experiment can produce either (a) a broad diffraction pattern, or, (b) an interference pattern. Here, the height of the curve indicates light intensity. This also shows how probability density evolves over distance after photons pass through two slits19. (a) If slit identity (photon position) is measured then the waves from the two slits do not interfere with each other. (b) If slit identity is not measured then the waves from the two slits produce an interference pattern as they travel further from the slits. For display purposes, the distribution at each distance has unit height.

Crucially, a diffraction envelope is exactly what is observed if the waves from different slits are prevented from interfering with each other. This can be achieved by using a photographic plate to record photons as the first slit is opened on its own, and then the other slit is opened on its own. In this case, the image captured by the photographic plate is the simple sum of photons from each slit (i.e. a diffraction envelope), with the guarantee that photons from the two slits could not possibly have interfered with each other. Thus, we can choose to use a light detector to find out which slit each photon passed through, but as soon as we do this, we see a diffraction envelope instead of an interference pattern.

Despite many ingenious experiments, any attempt to ascertain which slit each photon passed through forces the light to stop behaving like a wave (which yields an interference pattern) and to start behaving like a stream of billiard balls (which yields a diffraction envelope). The transformation from wave-like behavior to particle-like behavior is intimately related to Heisenberg’s uncertainty principle , as explained below. However, even this wave-particle duality is not the most remarkable feature of the double-slit experiment.

Matter Waves

If the light is replaced by a beam of electrons then an interference pattern is obtained, as if the electrons were waves (which looks like Figure 2d). This is especially surprising because only a few years before this result was obtained in 1927, electrons were thought to behave exactly like miniature billiard balls. So, just as light can be forced to behave like billiard balls, electrons can be forced to behave like waves.

The radical idea that electrons could behave like waves was proposed by de Broglie (pronounced de Broil) in 1923. In fact, de Broglie made the far more radical proposal that all matter has wave-like properties, now referred to as  matter waves . Since that time, the double-slit experiment has been used to demonstrate the existence of matter waves using whole atoms and even whole molecules of  buckminsterfullerene  (each buckminsterfullerene molecule is about 1 nanometre in diameter and contains 60 carbon atoms).

But I digress, so let’s return to the subject at hand.

Did the Photon Pass-Through One Slit or Two?

The problem is that a photon (or a molecule of buckminsterfullerene), unlike a wave, cannot pass through both slits, which raises the question: did the photon really pass through only one slit, and if so, which one?

To attempt to answer this question, consider what happens if we replace the screen with an array of long tubes, each of which points at just one slit, as shown in Figure 4a. At the end of each tube is a photodetector, such that any photon it detects could have come from only one slit. Note that there should be a pair of detectors at every screen position, with each member of the pair pointing at a different slit. Thus, irrespective of where a photon lands on the screen, the slit from which it originated is measured.

If we were to use this imaginary apparatus then we would find that the distribution of photons is a diffraction envelope, as in Figures 3a and 4a. Crucially, this envelope is identical to the pattern that would be obtained if the slits were opened one at a time, as described above.

Figure 4: An imaginary experiment for demonstrating wave-particle duality. a) If slit identity (photon position at the barrier) is measured using an array of oriented detectors at the screen then momentum (direction) precision is reduced, and a diffraction envelope is observed on the screen, as here and in Figure 5.9a. b) If slit identity is not measured then the screen is allowed to measure direction (photon momentum) with high precision, so an interference pattern is observed, as here. In Wheeler’s delayed-choice experiment, we decide to measure either a) slit identity, or b) photon momentum, but the decision is made after the photon has passed through the slit(s).

Thus, using detectors to measure each photon’s slit identity (i.e. which slit the photon passed through) prevents any wave-like behavior, just as if each photon had traveled in complete isolation as a single particle. If both slots are left open (and no photodetectors are used) then the original interference pattern is restored, as if the individual photons behave like waves (as in Figures 3b and 4b). This is the famous wave-particle duality .

Heisenberg’s Uncertainty Principle

As counter-intuitive as the result above seems, it is consistent with  Heisenberg’s uncertainty principle .  This ensures that when the slit identity is  not  measured, uncertainty in the particle’s screen position is roughly equal to the distance between successive maxima in the interference pattern (i.e. it is of the same order of magnitude as the width of each bright region in Figure 1).

More generally, Heisenberg’s uncertainty principle guarantees   that  any  reduction in the uncertainty in slit identity (position at the barrier) must increase uncertainty in the momentum of photons as they exit the slits; because momentum includes direction, uncertainty in momentum translates to uncertainty in screen position. This is the famous  position-momentum trade-off  traditionally associated with Heisenberg’s uncertainty principle, where position indicates slit identity and momentum indicates screen position here (i.e. position-momentum uncertainty translates to slit identity-screen position uncertainty in the double-slit experiment).

Heisenberg’s uncertainty principle means that, in practice, it is possible to adjust the amount of information gained regarding position by varying the accuracy of the measurement devices. As more information on position (slit identity) is gained, less information on the fine structure of the interference pattern is available. Consequently, as more information about the position is gained, the interference pattern is gradually replaced by a broad diffraction envelope.

As an aside, Heisenberg explained his uncertainty principle by showing that shining a light on an electron must alter the position and momentum of that electron, which therefore introduces uncertainty in the electron’s position and momentum. But Heisenberg’s uncertainty principle does not depend on the uncertainty of measurements (in fact, it is the other way round). Because both light and matter behave as if they are waves, they can be analyzed using Fourier analysis . And if position and momentum are waves then they must obey a key result from Fourier analysis:  Heisenberg’s inequality.  Speaking very loosely, this states that if position wavelengths are long then momentum wavelengths are short, and  vice versa . More precisely, Heisenberg’s inequality implies that reducing position uncertainty increases momentum uncertainty (and  vice versa ) so that the values of both cannot be known exactly, independently of whether or not they are measured.

Wheeler’s Delayed-Choice Experiment

So far, we have chosen to measure two different aspects of each photon: first, by measuring each photon’s position on the screen (which translates into momentum at a slit; see the previous section); and second, by using tube detectors to measure which slit each photon came from (which translates into the photon’s position at the barrier).

Clearly, when measuring photon position, the experimental setup does not change over time, so it seems plausible (or at least acceptable) that each photon could have passed through both slits. Similarly, when measuring slit identity, it seems plausible that each photon passes through only one slit.

But there is an alternative experiment, which involves changing the experimental setup while each photon is in transit between the slits and the screen.  If what we choose to measure alters how each photon behaves then it seems reasonable that we must make this decision before each photon reaches the slits.  However, what if the decision on whether to measure screen position or slit identity is made after each photon has passed through the slit(s) but before it has reached the screen or tube detectors? This is  Wheeler’s delayed-choice experiment , depicted in Figure 4.

Such an experiment is conceptually easy to set up. Once a photon is in transit between the slits and the screen, we can decide whether to measure the photon’s screen position (by leaving the screen in place, Figure 4b) or slit identity (by removing the screen so that the tube detectors can function, Figure 4a).

The results of an experiment that is conceptually no different from this were published by Jacques et al, 2007 (who used interferometers). Translating from the interferometer experiment performed by Jacques, when the screen is left in place, the photons’ screen positions are effectively measured, which yields an interference pattern on the screen. In contrast, when the screen is removed, an array of detectors is revealed that detects photons, and the pattern of detected photons is consistent with a broad diffraction envelope (which is the sum of two such envelopes; one per slit), as if no interference had occurred.

Crucially, the decision about whether to measure screen position (by leaving the screen in place) or slit identity (by removing the screen) was made (at random) after each photon had passed through the slit(s); so the behavior of each photon as it passed through the slit(s) depended on a decision made after that photon had passed through the slit(s). In essence, it is as if a decision made now about whether to measure slit identity or screen position of a photon (that is already in transit from the slits to the screen) retrospectively affects whether that photon passed through just one slit or both slits.

Incidentally, we can be certain that the decision was made after a photon passed through the slit(s), as follows. If the distance between the slits and the screen is S then the time taken for a photon to travel from the slit(s) to the screen is T = cS seconds, where c is the speed of light . Suppose it was decided at time t to leave the screen in place, and a photon arrived at the screen dt seconds later. Clearly, if dt < T then the photon must have been in transit between the slit(s) and the screen when the decision was made. Armed with this knowledge, we can select the subset P of photons for which dt<T , and record where they landed to find the pattern they collectively made on the screen. Using the same logic, if the decision was made to remove the screen then the tube detectors measure which slit each photon (in the subset P ) came from, with the guarantee that all these photons were in transit when the decision was made. Given a sufficiently large number of tube detectors, we could find the pattern collectively made by photons at the array of tube detectors.

In principle, the slit–screen distance can be made so large that it takes billions of years for each photon to travel from the slit(s) to the screen. In this case, a decision made now about whether to measure the slit identity or screen position of a photon seems to retrospectively affect whether that photon passed through just one slit or both slits billions of years ago.

As we should expect, these results are consistent with Heisenberg’s uncertainty principle. Regardless of when the decision is made, if the detectors measure slit identity (position) then this must increase uncertainty regarding the particle’s screen position (momentum), which leads to the disappearance of the interference pattern. Even though it is far from obvious how  any physical mechanism could produce this result, the fact remains that if such a mechanism did not exist then Heisenberg’s uncertainty principle would be violated.

Re-Writing Quantum History

So, suppose we wanted to re-write a little history. First, how could we do so, and second, how far back in time could that edit be?

Well, as we have seen, a decision made now about whether or not to leave the screen in place effectively determines how photons behaved in the past. Specifically, if we wish to ensure that each photon passed through just one slit then we should remove the screen (allowing the detectors to measure which slit each photon exited). Conversely, if we wish to ensure that photons passed through both slits then we should leave the screen in place. In both cases, this decision can be made after the photons have passed through the slit(s).

Second, the temporal range of our edit depends on how long the photons have been in transit from the double slits to the screen/detectors. This looks as if we need to have a double-slit experiment that has been set up in the distant past. However, there are natural phenomena, like gravitational lensing , which can be used to effectively mimic the double-slit experiment; as if slits are so far away that the photons we measure have been in transit for many years. Thus, in principle, temporal editing can reach back to the big bang , some 14 billion years ago.

Finally, we should acknowledge that not everyone (including Wheeler) agrees that Wheeler’s delayed-choice experiment edits the past. Like most quantum mechanical equations, the equations that define the results of the delayed choice experiment do not have a single unambiguous physical interpretation. However, in order to appreciate the nature of this ambiguity, it is necessary to understand the equations that govern quantum mechanics . For this, there is no shortcut, but there are many detailed maps and guide books.

James V Stone is an Honorary Associate Professor at the University of Sheffield, UK.

V Jacques, et al. Experimental realization of Wheeler’s delayed-choice gedanken experiment. Science, 315(5814):966–968, 2007.

Note :  This is an edited extract from  The Quantum Menagerie by James V Stone (published December 2020). Chapter 1, the table of contents, and book reviews can be seen here  The Quantum Menagerie .

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A Quantum Leap Through Time: Famous Double-Slit Experiment Reimagined

Physicists have recreated the double-slit experiment in time rather than space, using materials that change their optical properties in femtoseconds. This research could lead to ultrafast optical switches and advancements in time crystals and metamaterials .

A team of international physicists has recreated the famous double-slit experiment, which showed light behaving as particles and a wave, in time rather than space.

The experiment relies on materials that can change their optical properties in fractions of a second, which could be used in new technologies or to explore fundamental questions in physics.

The original double-slit experiment, performed in 1801 by Thomas Young at the Royal Institution, showed that light acts as a wave. Further experiments, however, showed that light actually behaves as both a wave and as particles – revealing its quantum nature.

These experiments had a profound impact on quantum physics, revealing the dual particle and wave nature of not just light, but other ‘particles’ including electrons, neutrons, and whole atoms.

The research team led by Imperial College London physicists performed the experiment using ‘slits’ in time rather than space. They achieved this by firing light through a material that changes its properties in femtoseconds (quadrillionths of a second), only allowing light to pass through at specific times in quick succession.

Monash University Head of the School of Physics and Astronomy, Professor Stefan Maier, was part of the team involved with this exciting experiment and a co-author on the study published in the scientific journal Nature Physics .

“The concept of time crystals has the potential to lead to ultrafast, parallelized optical switches,” Professor Maier said.

“It is additionally a beautiful demonstration of wave physics and how we can transfer concepts such as interference from the domain of space to the domain of time.”

Lead researcher Professor Riccardo Sapienza, from the Department of Physics at Imperial College London , said: “Our experiment reveals more about the fundamental nature of light while serving as a stepping-stone to creating the ultimate materials that can minutely control light in both space and time.”

The original double slit setup involved directing light at an opaque screen with two thin parallel slits in it. Behind the screen was a detector for the light that passed through.

To travel through the slits as a wave, light splits into two waves that go through each slit. When these waves cross over again on the other side, they ‘interfere’ with each other. Where peaks of the wave meet, they enhance each other, but where a peak and a trough meet, they cancel each other out. This creates a striped pattern on the detector of regions of more light and less light.

Light can also be parcelled up into ‘particles’ called photons, which can be recorded hitting the detector one at a time, gradually building up the striped interference pattern. Even when researchers fired just one photon at a time, the interference pattern still emerged, as if the photon split in two and travelled through both slits.

In the classic version of the experiment, light emerging from the physical slits changes its direction, so the interference pattern is written in the angular profile of the light. Instead, the time slits in the new experiment change the frequency of the light, which alters its colour. This created colours of light that interfere with each other, enhancing and cancelling out certain colours to produce an interference-type pattern.

The material the team used was a thin film of indium-tin-oxide, which forms most mobile phone screens. The material had its reflectance changed by lasers on ultrafast timescales, creating the ‘slits’ for light. The material responded much quicker than the team expected to the laser control, varying its reflectivity in a few femtoseconds.

The material is a metamaterial – one that is engineered to have properties not found in nature. Such fine control of light is one of the promises of metamaterials, and when coupled with spatial control, could create new technologies and even analogues for studying fundamental physics phenomena like black holes .

Co-author Professor Sir John Pendry from Imperial College said: “The double time slits experiment opens the door to a whole new spectroscopy capable of resolving the temporal structure of a light pulse on the scale of one period of the radiation.”

The team next want to explore the phenomenon in a ‘time crystal’, which is analogous to an atomic crystal, but where the optical properties vary in time.

For more on this experiment, see Physicists Reveal Quantum Nature of Light in a New Dimension .

Reference: “Double-slit time diffraction at optical frequencies” by Romain Tirole, Stefano Vezzoli, Emanuele Galiffi, Iain Robertson, Dries Maurice, Benjamin Tilmann, Stefan A. Maier, John B. Pendry and Riccardo Sapienza, 3 April 2023, Nature Physics . DOI: 10.1038/s41567-023-01993-w

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A netizen once left a statement in the comment. Science is not concerned with where an idea comes from. The sole test of the validity of an idea is experiment. The man who wrote that on the blackboard was Richard Feynman. Experiments can indeed promote the progress of theory. However, the limitations of experiments make it inevitable that many theories cannot be tested through experiments. The reason why humans are great is because they have powerful theories based on logic and mathematics. Any experiment cannot do without correct and scientific theories. Just like the famous Double-Slit Experiment was reimagined, what is the foundation and basis for designing a scientific experiment? Objective conditions? Subjective thinking? What can humans do?

a bit too overexcited about an on/off switch that yielded the expected results.

First, the general result was predicted but not yet tested. The test and its method was a first, and the material response time was unexpected.

Second, this – experiment, rapid response – yielded a method that “could create new technologies and even analogues for studying fundamental physics phenomena like black holes.”

Where is the overexcitement!?

This is very exciting stuff! So easy to have it slip by. Things are happening super fast!

Light is a wave, not a particle. One error is in thinking they fired one “photon” at a time. You can’t fire a single photon when light is a wave, which is why the interference pattern still builds up. This is another monster error in physics (along with Red Shift vs Light Dimming) leading to the inability to describe gravity, which is a form of space deformation that can be simulated using EM wave generators in a standing wave-like pattern (the same way we can build images with lasers). Just because a laser is a narrow directed wave, that doesn’t mean it’s a particle stream either, but rather a single directional wave (instead of two dimensional). Until science begins to recognize such basic fundamental thinking errors, we will never move into the age of warping space to get around the speed of light, which itself is the CPU clock speed of the computer simulation we call the “universe” (Sorry, but we’re actually inside a computer simulation).

Wave-particle duality?

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What is the *DETECTOR* in the double slit experiment and how does it work?

Is the detector a passive device or is it just a fictional mathematical probe?

I think the detector is somehow consuming the energy responsible for the wave nature of the photons, electrons or atoms, but I can't find any information about the detector and how it works.

Any help is appreciated since all videos and articles are suspiciously skipping the detector or simplifying it as a 3d cat or fictional cartoon eye. I know about the quantum eraser experiment but before moving to it, I need to know about the detector and how it exactly measures.

I'm a software programmer trying to understand how quantum computers work.

• quantum-mechanics
• experimental-physics
• double-slit-experiment
• particle-detectors

• 4 $\begingroup$ You can use a wall as a detector. $\endgroup$ –  rob ♦ Commented Mar 23, 2019 at 1:11
• 5 $\begingroup$ @rob I mean the detector that is supposedly causing the photones to change their mind and act as particles withot wave characteristics and not the surface $\endgroup$ –  USER249 Commented Mar 23, 2019 at 1:21
• $\begingroup$ @rob Its somthing scientists said or imply they placed behind the slits to determine which slit a particular photon or atom went through before hitting the surface. Th surface hier is the wall you are talking about. $\endgroup$ –  USER249 Commented Mar 23, 2019 at 1:59
• $\begingroup$ @AmrBerag I’m glad to see you’re still interested in this considering you never did get an actual physical answer. In experiments involving photons there’s only one way to detect them. Place matter in the path of the photon and the photon will be absorbed. From this you will know what photons are missing or where they landed. Like spectral lines you can have emission lines or absorption lines. It’s all physical and no waves involved. $\endgroup$ –  Bill Alsept Commented Aug 21, 2020 at 15:40
• 1 $\begingroup$ @USER249 did you ever get a good answer or explanation to this? I feel as though it would be impossible to build such a detector withour perturbing the whole experiment in the first place. I'm not a physicist though so I don't know what I don't know. $\endgroup$ –  rollsch Commented Aug 7, 2023 at 0:11

3 Answers 3

I misunderstood your question at first. I thought you were asking about the detector downstream of the double slit, where the interference pattern is visible; every practical double-slit experiment includes such a detector. But instead, you are asking about a hypothetical detector which could "tag" a particle as having gone through one slit or the other. Most interference experiments do not have such a detector.

The idea of "tagging" a particle as having gone through one slit or the other, and the realization that such tagging would destroy the double-slit interference pattern, was hashed out in a long series of debates between Bohr and Einstein . Most introductory quantum mechanics textbooks will have at least some summary of the history of these discussions, which include many possible "detectors" with varying degrees of fancifulness.

A practical way to tag photons as having gone through one slit or another is to cover both slits with polarizing films. If the light polarizations are parallel, it's not possible to use this technique to tell which slit a given photon came through, and the interference pattern survives. If the light polarizations are perpendicular, it would be possible in principle to detect whether a given photon went through one slit or the other; in this case, the interference pattern is also absent. If the polarizers are at some other angle, it's a good homework problem to predict the intensity of the interference pattern.

• 5 $\begingroup$ Wouldn't perpendicular ploarizers also explain the lack of an interference pattern because perpendicular waves wouldn't interfere with each other? $\endgroup$ –  JohnFx Commented Jun 28, 2020 at 1:49
• 1 $\begingroup$ @JohnFx I thought that's what I wrote? $\endgroup$ –  rob ♦ Commented Jun 28, 2020 at 6:05
• $\begingroup$ When they go through the polarizers, are they in a superposition of both polarities, or just in one of either? $\endgroup$ –  Juan Perez Commented May 20, 2023 at 13:50
• $\begingroup$ @JuanPerez I'm not sure that question has a simple answer like you want it to. It might be possible to use the ambiguity to make a variation of the "quantum eraser" experiment, but I don't think I can explain that in a comment. $\endgroup$ –  rob ♦ Commented May 20, 2023 at 16:20
• $\begingroup$ I think that polarizers at the slits are a bad example because even classical electrodynamics predicts the disappearance of the interference pattern in that case, so it can't be said to provide evidence for quantum mechanics. $\endgroup$ –  benrg Commented May 20, 2023 at 19:22

I watched a couple of videos on the double slit experiment and I had exactly the same question as the original poster (Amr Berag) and stumbled upon this post and was just wondering how come everyone else isn't wondering the same thing.

There are so many videos showing the actual double slit experiment but none show the actual wave function collapse in reality when the particles are "observed".

It turns out that it was merely a thought experiment when it was first proposed and it's not super trivial to put an actual detector, but in 1987 an experiment was performed and subsequent experiments were performed but none shown on video.

This link explains that a bit

Please look at the "Which-way" section here

I'm just surprised that they don't mention this in any of the videos and how come no one else asks for proof of this. When I saw the original video I was just waiting till the end to see them put a "detector" and see the interference pattern disappear, but nope.

• 3 $\begingroup$ It seems nuts that this is taken as a given when it was just a thought experiment. I read the link but I don't see how it demonstrates the thought experiment practically... $\endgroup$ –  Cloudyman Commented Dec 3, 2021 at 23:13
• $\begingroup$ The reason why no serious physicists does these experiments is trivial: one can't learn anything from them. We know what "a detector" does to a system: it either removes energy from it or it adds energy to it. The consequences of that are trivial to calculate, both in quantum mechanics and in classical physics. So what's the point of wasting time and money on experiments that we can predicts at the introductory textbook level? In practice science is about what we don't know. It's not an endless repetition of stuff that we do know. We owe that to the taxpayer who funds us. $\endgroup$ –  FlatterMann Commented May 20, 2023 at 19:49
• 3 $\begingroup$ For such a big result, experimental verification seems like a good idea! The failure of the thought experiment to happen in practice can lead to new results in themselves, for example. $\endgroup$ –  apg Commented Dec 23, 2023 at 20:42
• 1 $\begingroup$ It almost seems like a conspiracy that noone ever filmed this famous experiment, while at the same time we have hundreds of thousands of the double-slit experiment adaptations. Good to see that I'm not the only one confused here. This would be one of the greatest resources for introducing newbies to the uncertainity in the world of quantum physics. The description of the experiment is so mind-boggling that it is one of not very few things that I remember from high school. It would be so live-changing to be able to see it then, even on film. $\endgroup$ –  jannis Commented Feb 10 at 11:56
• 1 $\begingroup$ And you know @FlatterMann "seeing is believing". When I first learned about the uncertainty and wave-particle dualism it was like "meh, this cannot be true, just some random theoretical stuff they are telling us so that they can later do an exam about it". Having this on film would have great educational consequences. IMO putting such content in the Internet would give the author eternal fame (not mentioning likes/views and, therefore, money). And when it comes to cost Mr Beast puts millions of dollars in his movies, maybe he'd get interested:) Anyway I would surely pay to see it. $\endgroup$ –  jannis Commented Feb 10 at 12:03

It's anything that gives you information about where the particle passed by. The problem in measurement in QM is that to measure anything you need to interact with the "thing" you want to measure. If you want to measure temperature in a drop of water with a large thermometer, the heat of the thermometer will affect the drop temperature. If you want to measure the distance to the moon you may shoot a laser (knowing c=the speed of light in vacuum) and wait till it returns. But if the smallest thing you have is a rock, you can throw it and do the same process knowing the rock's speed (ignoring gravity and air resistance), but if the smallest thing you have, to measure it, is another moon, you will affect the position of the "original" moon and so affecting the whole state of the thing you want to measure. Well, the quantum world is so small that to measure things you have to destroy the original state or perturb it. The device in the double slit is just to block or interact with the particle passing through that slit.

• 3 $\begingroup$ You just rephrased my question bro. Are you telling me that the detector in the double slit experiment is not passive enough to detect without afecting the outcome? Then why all these credited sientists jumping to the assumption that photones are conscious instead of saying that the very act of trying to detect is affecting these photones and particles in a way that they lose their wave characteristics by withdrawing their enrgy for example. Or do you mean the detector is nothing but another surface moving toward the slits but then it will be too close and prevent the interference! $\endgroup$ –  USER249 Commented Mar 23, 2019 at 1:47
• 1 $\begingroup$ I din't rephrase your question. There's no single question mark on my reply. The math is a model to represent a behavior that is very consistent. It is not passive enough $\endgroup$ –  Gndk Commented Mar 23, 2019 at 1:59
• 1 $\begingroup$ Do you mean there is no detector? $\endgroup$ –  USER249 Commented Mar 23, 2019 at 2:04
• 1 $\begingroup$ The behavior Im asking about is that these things behave like waves when it is unknown which path they take but behave like sand partjcles or bullets when sientists attempt to detect their pass. I would like to know what device they used to detect the path and how it works. This device is referred to as "The Detector" that is causing the mind puzzling behavior implying that such device exists and is good enough to make scientists and univesity professors believe that atoms not only conscious but can communicate and tell their friends that some curious humans are trying to spy on them. $\endgroup$ –  USER249 Commented Mar 23, 2019 at 2:37
• 1 $\begingroup$ @Esther You can start here if you are coming from that cartoon robotic eye detector video youtu.be/yotBpxXiivA $\endgroup$ –  USER249 Commented Dec 5, 2020 at 13:25

Not the answer you're looking for? Browse other questions tagged quantum-mechanics photons experimental-physics double-slit-experiment particle-detectors or ask your own question .

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• Quantum mechanics
• Research update

Do atoms going through a double slit ‘know’ if they are being observed?

Does a massive quantum particle – such as an atom – in a double-slit experiment behave differently depending on when it is observed? John Wheeler’s famous “delayed choice” Gedankenexperiment asked this question in 1978, and the answer has now been experimentally realized with massive particles for the first time. The result demonstrates that it does not make sense to decide whether a massive particle can be described by its wave or particle behaviour until a measurement has been made. The techniques used could have practical applications for future physics research, and perhaps for information theory.

In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. In 1978 American theoretical physicist John Wheeler proposed a series of thought experiments wherein he wondered whether a particle apparently going through a slit could be considered to have a well-defined trajectory, in which it passes through one slit or both. In the experiments, the decision to observe the photons is made only after they have been emitted, thereby testing the possible effects of the observer.

For example, what happens if the decision to open or close one of the slits is made after the particle has committed to pass through one slit or both? If an interference pattern is still seen when the second slit is opened, this would force us either to conclude that our decision to measure the particle’s path affects its past decision about which path to take, or to abandon the classical concept that a particle’s position is defined independent of our measurement.

Photon first

While Wheeler conceived of this purely as a thought experiment, experimental advances allowed Alain Aspect and colleagues at the Institut d’Optique, Ecole Normale Supérieure de Cachan and the National Centre for Scientific Research, all in France, to actually perform it in 2007 with single photons, using beamsplitters in place of the slits envisage by Wheeler. By inserting or removing a second beamsplitter randomly, the researchers could either recombine the two paths or leave them separate, making it impossible for an observer to know which path a photon had taken. They showed that if the second beamsplitter was inserted, even after the photon would have passed the first, an interference pattern was created.

The wave–particle duality of quantum mechanics dictates that all quantum objects, massive or otherwise, can behave as either waves or particles. Now, Andrew Truscott and colleagues at Australian National University carried out Wheeler’s experiment using atoms deflected by laser pulses in place of photons deflected by mirrors and beamsplitters. The helium atoms, released one by one from an optical dipole trap, fell under gravity until they were hit by a laser pulse, which deflected them into an equal superposition of two momentum states travelling in different directions with an adjustable phase difference. This was the first “beamsplitter”. The researchers then decide whether to apply a second laser pulse to recombine the two states and create mixed states – one formed by adding the two waves and one formed by subtracting them – by using a quantum random-number generator. When applied, this final laser pulse made it impossible to tell which of the two paths the photon had travelled along. The team ran the experiment repeatedly, varying the phase difference between the paths.

Double pulse

Truscott’s team found that when the second laser pulse was not applied, the probability of the atom being detected in each of the momentum states was 0.5, regardless of the phase lag between the two. However, application of the second pulse produced a distinct sine-wave interference pattern. When the waves were perfectly in phase on arrival at the beamsplitter, they interfered constructively, always entering the state formed by adding them. When the waves were in antiphase, however, they interfered destructively and were always found in the state formed by subtracting them. This means that accepting our classical intuition about particles travelling well-defined paths would indeed force us into accepting backward causation. “I can’t prove that isn’t what occurs,” says Truscott, “But 99.999% of physicists would say that the measurement – i.e. whether the beamsplitter is in or out – brings the observable into reality, and at that point the particle decides whether to be a wave or a particle.”

Indeed, the results of both Truscott and Aspect’s experiments shows that a particle’s wave or particle nature is most likely undefined until a measurement is made. The other less likely option would be that of backward causation – that the particle somehow has information from the future – but this involves sending a message faster than light, which is forbidden by the rules of relativity.

Aspect is impressed. “It’s very, very nice work,” he says, “Of course, in this kind of thing there is no more real surprise, but it’s a beautiful achievement.” He adds that, beyond curiosity, the technology developed may have practical applications. “The fact that you can master single atoms with this degree of accuracy may be useful in quantum information,” he says.

The research is published in Nature Physics .

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On the speed of light as a key element in the structure of quantum mechanics.

1. Introduction

2. constructing quantum mechanics through the speed of light, 3. the classical limit c → ∞, 4. effective curved geometry in quantum systems for position-dependent speed light, 5. experimental evidence and analogical realization, 6. discussion, data availability statement, conflicts of interest.

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Shushi, T. On the Speed of Light as a Key Element in the Structure of Quantum Mechanics. Foundations 2024 , 4 , 411-421. https://doi.org/10.3390/foundations4030026

Shushi T. On the Speed of Light as a Key Element in the Structure of Quantum Mechanics. Foundations . 2024; 4(3):411-421. https://doi.org/10.3390/foundations4030026

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August 9, 2024

Experiments Prepare to Test Whether Consciousness Arises from Quantum Weirdness

Researchers wish to probe whether consciousness has a basis in quantum mechanical phenomena

By Hartmut Neven & Christof Koch

nopparit/Getty Images

The brain is a mere piece of furniture in the vastness of the cosmos, subject to the same physical laws as asteroids, electrons or photons. On the surface, its three pounds of neural tissue seem to have little to do with quantum mechanics , the textbook theory that underlies all physical systems, since quantum effects are most pronounced on microscopic scales. Newly proposed experiments, however, promise to bridge this gap between microscopic and macroscopic systems, like the brain, and offer answers to the mystery of consciousness.

Quantum mechanics explains a range of phenomena that cannot be understood using the intuitions formed by everyday experience. Recall the Schrödinger’s cat thought experiment , in which a cat exists in a superposition of states, both dead and alive. In our daily lives there seems to be no such uncertainty—a cat is either dead or alive. But the equations of quantum mechanics tell us that at any moment the world is composed of many such coexisting states, a tension that has long troubled physicists.

Taking the bull by its horns, the cosmologist Roger Penrose in 1989 made the radical suggestion that a conscious moment occurs whenever a superimposed quantum state collapses. The idea that two fundamental scientific mysteries—the origin of consciousness and the collapse of what is called the wave function in quantum mechanics—are related, triggered enormous excitement.

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Penrose’s theory can be grounded in the intricacies of quantum computation . Consider a quantum bit, a qubit, the unit of information in quantum information theory that exists in a superposition of a logical 0 with a logical 1. According to Penrose, when this system collapses into either 0 or 1, a flicker of conscious experience is created, described by a single classical bit.

Penrose, together with anesthesiologist Stuart Hameroff, suggested that such collapse takes place in microtubules , tubelike, elongated structural proteins that form part of the cytoskeleton of cells, such as those making up the central nervous system.

These ideas have never been taken up by the scientific community as brains are wet and warm, inimical to the formation of superpositions, at least compared to existing quantum computers that operate at temperatures 10,000 times colder than room temperature to avoid destroying superposition states.

Penrose’s proposal suffers from a flaw when applied to two or more entangled qubits. Measuring one of these entangled qubits instantaneously reveals the state of the other one, no matter how far away. Their states are correlated, but correlation is not causation, and, according to standard quantum mechanics, entanglement cannot be employed to achieve faster-than-light communication. However, per Penrose’s proposal, qubits participating in an entangled state share a conscious experience. When one of them assumes a definite state, we could use this to establish a communication channel capable of transmitting information faster than the speed of light, a violation of special relativity.

In our view, the entanglement of hundreds of qubits, if not thousands or more, is essential to adequately describe the phenomenal richness of any one subjective experience: the colors, motions, textures, smells, sounds, bodily sensations, emotions, thoughts, shards of memories and so on that constitute the feeling of life itself.

In an article published in the open-access journal Entropy , we and our colleagues turned the Penrose hypothesis on its head, suggesting that an experience is created whenever a system goes into a quantum superposition rather than when it collapses. According to our proposal, any system entering a state with one or more entangled superimposed qubits will experience a moment of consciousness.

You, the astute reader, must by now be saying to yourself: But wait a minute here—I don’t ever consciously experience a superposition of states. Any one experience has a definitive quality; it is one thing and not the other. I see a particular shade of red, feel a toothache. I don’t simultaneously experience red and not-red, pain and not-pain.

The definitiveness of any conscious experience naturally arises within the many-worlds interpretation of quantum mechanics . A metaphysical position first put forward by physicist Hugh Everett in 1957, the many-worlds view, posits time’s evolution as an enormously branched tree, with every possible outcome of a quantum event splitting off its own universe. A single qubit entering a superposition gives birth to two universes, in one of which the qubit’s state is 0 while in a twin universe everything is identical except that the qubit’s state is 1.

Entanglement potentially offers something else for brain scientists by providing a natural solution to what is called the binding problem, the subjective unity of every experience that has long posed a key challenge to the study of consciousness. Consider seeing the Statue of Liberty: her face, the crown on her head, the torch in her raised right hand, and so on. All these distinctions and relationships are bound together into a single perception whose substrate might be numerous qubits, all entangled with each other.

To make these esoteric ideas concrete, we propose three experiments that would increasingly shape our thinking on these matters. The first experiment, progressing right now thanks to funding from the Santa Monica–based Tiny Blue Dot Foundation, seeks to provide evidence of the relevance of quantum mechanics to neuroscience in two very accessible test beds: tiny fruit flies and cerebral organoids, the latter lentil-sized assemblies of thousands of neurons grown from human-induced pluripotent stem cells. It is known that the inert noble gas xenon can act as anesthetic in animals and people. Remarkably, an earlier experiment claimed that its anesthetic potency, measured as the concentration of the gas that induces immobility, depends on the specific isotopes of xenon. Two isotopes of an element contain the same number of positively charged protons but different numbers of noncharged neutrons in their nuclei. The chemical properties of isotopes—that is, what they interact with—are similar, by and large, even though their masses and magnetic properties differ slightly.

If fruit flies and organoids can be used to detect different xenon isotopes, the hunt will be on for the exact mechanisms by which a gas that is inert and that remains aloof from binding to proteins or other molecules achieves this. Is it the tiny difference in the mass of these isotopes (131 versus 132 nucleons) that makes the difference? Or is it their nuclear spin, a quantum mechanical property of the nucleus? These xenon isotopes differ substantially in their nuclear spin; some have zero spin and others 1 / 2 or 3 / 2 .

These xenon experiments will inform a second follow-on experiment in which we will attempt to couple qubits to brain organoids in a way that allows entanglement to spread between biological and technical qubits. The final experiment, which at this stage is still a purely conceptual one, aims to enhance consciousness by coupling engineered quantum states to a human brain in an entangled manner. The person may then experience an expanded state of consciousness like those accessed under the influence of ayahuasca or psilocybin.

Both quantum engineering and the design of brain-machine interfaces are progressing rapidly. It may not be beyond human ingenuity to directly probe and expand our conscious mind by making use of quantum science and technology.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.

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An experiment with beams of photons has confirmed quantum weirdness exists

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The world is as weird as we feared. A new experiment confirms yet again the existence of correlations between distant entangled quantum particles – and this time we have measured the phenomenon so precisely, there is only a minuscule chance it is a fluke.

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‘World’s smallest disco’ spins diamonds 1.2 billion RPM for quantum gravity

Spinning the diamond at incredible speeds, the researchers observed how this rotation uniquely impacted the spin qubits, a phenomenon known as the berry phase..

Aman Tripathi

The team created nanodiamonds, each about 750 nanometers in size, using high pressure and temperature.

Purdue University

Scientists at Purdue University have achieved a breakthrough in quantum physics by levitating and spinning nanoscale diamonds at an incredible speed of 1.2 billion rotations per minute.

This “world’s smallest disco” is expected to play a crucial role in investigating the complex relationship between quantum mechanics and gravity.

“Imagine tiny diamonds floating in an empty space or vacuum. Inside these diamonds, there are spin qubits that scientists can use to make precise measurements and explore the mysterious relationship between quantum mechanics and gravity,” said Tongcang Li, who led the research team.

Crafting and trapping nanodiamonds

The team first created nanodiamonds, each approximately 750 nanometers in size, under conditions of high pressure and temperature. They then successfully trapped the nanodiamond in a high vacuum using a surface ion trap.

This trap consisted of a thin layer of gold, precisely etched into an omega shape, on a sapphire wafer. Passing an electrical current through this gold pattern generated an electromagnetic field, which levitated a nanodiamond in a vacuum chamber above the surface.

This allowed the researchers to observe and manipulate the spin qubits within the diamond.

“We can adjust the driving voltage to change the spinning direction,” explained Kunhong Shen, an author of the study.

By spinning the diamond at an incredibly high speed, “they were able to observe how the rotation affected the spin qubits in a unique way known as the Berry phase,” read the news release .

“The levitated diamond can rotate around the z-axis (which is perpendicular to the surface of the ion trap), shown in the schematic, either clockwise or counterclockwise, depending on our driving signal. If we don’t apply the driving signal, the diamond will spin omnidirectionally, like a ball of yarn,” Shen highlighted.

Illuminating the quantum world

The team’s achievement goes beyond just levitation and rotation . The nanodiamonds used in this experiment were specially prepared to host electron spin qubits.

When illuminated with a green laser, they emit red light, enabling researchers to read out their spin states. An additional infrared laser was used to monitor the rotation of the levitated nanodiamond, similar to how a disco ball reflects light as it spins.

“For the first time, we could observe and control the behavior of the spin qubits inside the levitated diamond in high vacuum,” emphasized Li.

By comparing these two measurements, the researchers could infer how the diamonds’ spin affects the quantum information stored in their defects.

This breakthrough addresses previous challenges in levitating diamonds in a high vacuum and reading out the spin qubits within them.

By achieving both, the team has opened up new avenues for investigating how gravity can be explained in quantum terms, a fundamental question that has long puzzled physicists.

Pushing boundaries of quantum exploration

Beyond fundamental research, this discovery could have a significant impact on industrial applications. Levitated micro and nano-scale particles in vacuum can serve as highly sensitive accelerometers and electric field sensors, with potential applications in navigation and communication.

While the nanodiamonds’ spinning speed is impressive, it falls short of the world record held by the same team, who previously spun a nanoscale “dumbbell” at 300 billion rpm.

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However, the current research’s primary focus lies in its potential to unlock deeper understandings of the quantum world and its interplay with gravity, rather than simply breaking speed records.

The “world’s smallest disco party” may just be the beginning of a revolution in quantum physics and its practical applications. As researchers continue to explore the possibilities of levitated nanodiamonds and their embedded spin qubits, we can expect even more exciting developments in the future.

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ABOUT THE EDITOR

Aman Tripathi An active and versatile journalist and news editor. He has covered regular and breaking news for several leading publications and news media, including The Hindu, Economic Times, Tomorrow Makers, and many more. Aman holds expertise in politics, travel, and tech news, especially in AI, advanced algorithms, and blockchain, with a strong curiosity about all things that fall under science and tech.

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Quantum sensing milestone draws closer to exquisitely accurate, GPS-free navigation

by Sandia National Laboratories

Peel apart a smartphone, fitness tracker or virtual reality headset, and inside you'll find a tiny motion sensor tracking its position and movement. Bigger, more expensive versions of the same technology, about the size of a grapefruit and a thousand times more accurate, help navigate ships, airplanes and other vehicles with GPS assistance.

Now, scientists are attempting to make a motion sensor so precise it could minimize the nation's reliance on global positioning satellites. Until recently, such a sensor—a thousand times more sensitive than today's navigation-grade devices—would have filled a moving truck. But advancements are dramatically shrinking the size and cost of this technology.

For the first time, researchers from Sandia National Laboratories have used silicon photonic microchip components to perform a quantum sensing technique called atom interferometry, an ultra-precise way of measuring acceleration. It is the latest milestone toward developing a kind of quantum compass for navigation when GPS signals are unavailable.

The team published its findings and introduced a new high-performance silicon photonic modulator—a device that controls light on a microchip—as the cover story in the journal Science Advances .

The research was supported by Sandia's Laboratory Directed Research and Development program. It took place, in part, at the National Security Photonics Center, a collaborative research center developing integrated photonics solutions for complex problems in the national security sector.

GPS-free navigation a matter of national security

"Accurate navigation becomes a challenge in real-world scenarios when GPS signals are unavailable," said Sandia scientist Jongmin Lee.

In a war zone, these challenges pose national security risks, as electronic warfare units can jam or spoof satellite signals to disrupt troop movements and operations.

Quantum sensing offers a solution.

"By harnessing the principles of quantum mechanics, these advanced sensors provide unparalleled accuracy in measuring acceleration and angular velocity, enabling precise navigation even in GPS-denied areas," Lee said.

Modulator the centerpiece of a chip-scale laser system

Typically, an atom interferometer is a sensor system that fills a small room. A complete quantum compass—more precisely called a quantum inertial measurement unit—would require six atom interferometers.

But Lee and his team have been finding ways to reduce its size, weight and power needs. They have already replaced a large, power-hungry vacuum pump with an avocado-sized vacuum chamber and consolidated several components usually delicately arranged across an optical table into a single, rigid apparatus.

The new modulator is the centerpiece of a laser system on a microchip. Rugged enough to handle heavy vibrations, it would replace a conventional laser system typically the size of a refrigerator.

Lasers perform several jobs in an atom interferometer, and the Sandia team uses four modulators to shift the frequency of a single laser to perform different functions.

However, modulators often create unwanted echoes called sidebands that need to be mitigated.

Sandia's suppressed-carrier, single-sideband modulator reduces these sidebands by an unprecedented 47.8 decibels—a measure often used to describe sound intensity but also applicable to light intensity—resulting in a nearly 100,000-fold drop.

"We have drastically improved the performance compared to what's out there," said Sandia scientist Ashok Kodigala.

Silicon device mass-producible and more affordable

Besides size, cost has been a major obstacle to deploying quantum navigation devices. Every atom interferometer needs a laser system, and laser systems need modulators.

"Just one full-size single-sideband modulator, a commercially available one, is more than $10,000," Lee said.

Miniaturizing bulky, expensive components into silicon photonic chips helps drive down these costs.

"We can make hundreds of modulators on a single 8-inch wafer and even more on a 12-inch wafer," Kodigala said.

And since they can be manufactured using the same process as virtually all computer chips, "This sophisticated four-channel component, including additional custom features, can be mass-produced at a much lower cost compared to today's commercial alternatives, enabling the production of quantum inertial measurement units at a reduced cost," Lee said.

As the technology gets closer to field deployment, the team is exploring other uses beyond navigation. Researchers are investigating whether it could help locate underground cavities and resources by detecting the tiny changes these make to Earth's gravitational force. They also see potential for the optical components they invented, including the modulator, in LIDAR, quantum computing and optical communications.

"I think it's really exciting," Kodigala said. "We're making a lot of progress in miniaturization for a lot of different applications."

Multidisciplinary team lifting quantum compass concept to reality

Lee and Kodigala represent two halves of a multidisciplinary team. One half, including Lee, consists of experts in quantum mechanics and atomic physics. The other half, like Kodigala, are specialists in silicon photonics—think of a microchip, but instead of electricity running through its circuits, there are beams of light.

These teams collaborate at Sandia's Microsystems Engineering, Science and Applications complex, where researchers design, produce and test chips for national security applications.

"We have colleagues that we can go down the hall and talk to about this and figure out how to solve these key problems for this technology to get it out into the field," said Peter Schwindt, a quantum sensing scientist at Sandia.

The team's grand plan—to turn atom interferometers into a compact quantum compass—bridges the gap between basic research at academic institutions and commercial development at tech companies. An atom interferometer is a proven technology that could be an excellent tool for GPS-denied navigation. Sandia's ongoing efforts aim to make it more stable, fieldable and commercially viable.

The National Security Photonics Center collaborates with industry, small businesses, academia and government agencies to develop new technologies and help launch new products. Sandia has hundreds of issued patents and dozens more in prosecution that support its mission.

"I have a passion around seeing these technologies move into real applications," Schwindt said.

Michael Gehl, a Sandia scientist who works with silicon photonics, shares the same passion. "It's great to see our photonics chips being used for real-world applications," he said.

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More From Forbes

Sas defines hybrid reality for quantum computing.

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Development engineer Louise Hoppe checks a crystal wafer with light channels milled by a laser at ... [+] German technology company Trumpf in Ditzingen near Stuttgart, southern Germany, on January 30, 2020. - The light channels are the heart in the construction of quantum sensors. (Photo by THOMAS KIENZLE / AFP) (Photo by THOMAS KIENZLE/AFP via Getty Images)

Quantum is huge. Because quantum computing allows us to step beyond the current limitations of digital systems, it paves the way for a new era of computing machines with previously unthinkable power. Without recounting another simplified explanation of how quantum gets its power at length, we can reference the double-slit experiment and perhaps the spinning coin explanation.

A coin sat on a desk is either heads or tails, rather like the 1s and 0s that express the on or off values in binary code . Quantum theorists would prefer we think of the coin above the desk, spinning in the air. In this state, the coin is both heads and tails at the same time. This is because, at the quantum level, both values exist until we make an observation of its state at any given point in time. We could further increase the number of positions possible (literally known as quantum superposition) by altering the angle of view we take on the coin, which is somewhat similar to how we work with qubits in quantum mechanics.

So then, Schrödinger’s cat is both alive and dead at the same time and the dummies guide to quantum entanglement is out there on the web if needed. What matters most now is how we will make practical use of quantum computing and where it will be applied for best advantage.

What Next For Quantum?

Across the universe of quantum entanglement , we’re at a point where quantum computing could redefine data analysis and model training in AI. This is the suggestion made by Bill Wisotsky , lead quantum architect and quantum computing researcher at SAS. In our near and immediate future, quantum computers could handle the complex calculations of AI algorithms much faster than classical computers and with less data, resulting in AI that can learn and adapt in ways we can’t currently imagine.

The main advancements Wisotsky and team are seeing inside SAS when speaking to customers includes work focused on fuelling the creation of a greater number of “good quality” qubits, which (in theory if not in practice) should get us to a point where we can tackle larger and more complex problems.

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“Qubits are the smallest functional unit of QPU [quantam processing unit], analogous to classical bits. A larger number of computation qubits results in larger problems being solved on the QPU. Better quality qubits result in more reliable results that are less prone to ‘noise’ and instability,” said Wisotsky, in a company analysis blog.

As a quantum architect, Wisotsky explains quantum’s transformative potential to handle intricate computations and address detailed “combinatorial” analysis problems i.e. a mathematical term describing the number of elements that exist in a larger sum of values and the complexity of their arrangement. This is the type of analysis needed in all industries, but it has particular relevance for healthcare and life sciences and across finance and insurance.

Hope Lies In Hybrid

SAS underlines the fact that today, what has been most promising is the use of quantum-classical hybrid approaches, which aim at splitting computing processes and sending the pieces to quantum that quantum does best and the pieces to classical computing that classical does best. It’s slightly reminiscent of when cloud computing arrived and we were told that “cloud is for everyone, but not for every thing” - so therefore, similarly, it’s important to realize that not all problems will benefit from quantum computing.

“Problems need to be complex enough that classical systems struggle to solve them, only then will quantum properties be valuable,” explained Wisotsky. “For example, traditional computing might take days or months to run trial and error scenarios to find the best drug for a clinical trial, which impacts a hospital’s budget and personnel. Quantum computing offers the ability to search all solutions instantaneously, finding optimal drugs for trial much faster and more efficiently. By using both traditional computing and quantum computing in concert with one another, organizations can start to realize the benefits of quantum now.”

It is perhaps refreshing to hear enterprise tech firms talk realistically about the still-embryonic nature of an emerging technology in this way. With all the generative AI hype we’ve been working through for the last year or two, we need to handle this great power with really great responsibility. Extending and dovetailing with Wisotsky’s insight on this subject is Bryan Harris in his core capacity as CTO of SAS.

SAS R&D Factor

Harris talks about the quantum research work currently being carried out inside SAS R&D and, although the company is not building a quantum computer (it’s a discipline and skillset best left to specialized manufacturers and those tech behemoths with enough muscle to underpin development investment in this space such as IBM), it is actively exploring real-world applications and running actual use cases with third-party quantum computers.

In terms of practical tangible developments SAS researchers are investigating four opportunities for quantum computing:

• Drug discovery: In the pharmaceutical industry, quantum computing is said to reduce the time and cost associated with discovering new drugs by simulating the behavior of molecules. This includes understanding the interactions between drugs and the complex biological systems they target.
• Financial modeling : As the world becomes more digitally connected, the complexity of modeling risk is beginning to overwhelm classical methods. In the banking industry, quantum algorithms have the ability to improve the modeling of financial markets, portfolio optimization and systemic risk associated with hyperconnectivity.
• Chemical simulations: Quantum computers can simulate the behavior of atoms and molecules at a quantum level with high accuracy. SAS researchers have noted that this is a challenge for classical computers because of the complex nature of quantum mechanics. In the science field, quantum computing can enable the discovery of new materials for improved sustainability to include much needed breakthroughs in batteries for electric vehicles.
• Optimization: Fourthly here, quantum computing could dramatically reduce computational time by finding near-optimal solutions through its ability to search an entire space instantaneously, leading to cost savings in cloud computing operations and tackling previously unsolvable problems.

For an example of the above-noted process of optimization, in traditional cloud computing, running trial and error scenarios to find the best solution could take days or months, which is expensive. Quantum computing offers the ability to search the space instantaneously, finding optimal solutions much faster and more efficiently.

Quantum In Data & Analytics

With these four cornerstones under development then, what is the future for of this technology in data and analytics, especially given new post-quantum cryptography standards guidance from NIST? To put the question another way, what are some possible pros and cons resulting from quantum’s introduction to modern enterprises?

“With every new technology, there are opportunities and risks,” advised SAS CTO Harris, speaking to press and analysts this week in London. “First, quantum presents many positive opportunities for businesses to solve challenging problems that were previously unsolvable. However, quantum computers have the potential to break many of the cryptographic systems we rely on today for secure communications and data protection. This reality has put it on national agendas with the recent announcement of NIST’s post-quantum cryptography standards.”

As a result he says, there are two important workstreams that must happen simultaneously.

“First, organizations must allocate research dollars into quantum computing to understand how it be leveraged to create a competitive advantage in products and services. Second, IT and product development organizations must plan for the integration of quantum-resilient encryption algorithms to maintain the security integrity of their infrastructure,” insisted Harris.

Which Verticals Will Adopt Quantum?

As we have noted, not every business or life problem can take advantage of quantum computing. We need to look for application scenarios that must be complex enough that classical computing systems struggle to solve them. The sectors most likely to benefit include life sciences, banking and materials science.

The big question is, no… the “first” and most important question we might ask here is just exactly how will quantum computing change the data and analytics industry, or indeed other industries?

“The first major impact that quantum computing will have on data and analytics is search space optimization for AI,” said SAS’ Harris. “In many AI problems, the training of a model or machine learning requires the exploration of a highly-dimensional data space for potential solutions. With classical computers, searching this space can be slow and expensive, especially in the cloud. With quantum computing, the entire space can be searched simultaneously to find the best solution that can be used as a starting point in classical computing.”

Our takeaways here are, like quantum theory itself, both simple and complex.

Put simply, quantum computing is as occasionally fragile as it is magnificently powerful and we’re still at a prototyping analysis stage with this technology, but we’re quickly coming out of that phase into real world applications. Put in slightly more complex terms, our quantum reality is likely to be a hybrid combination of traditional compute architectures made up of CPUs and GPU as also we now add QPUs into the mix.

As for Schrödinger’s cat, the lid isn’t off that box yet, thankfully.

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Chemical Society Reviews

Computer-aided nanodrug discovery: recent progress and future prospects.

* Corresponding authors

a Laboratory of Theoretical and Computational Nanoscience, National Center for Nanoscience and Technology of China, Beijing 100190, China E-mail: [email protected]

b University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China

Nanodrugs, which utilise nanomaterials in disease prevention and therapy, have attracted considerable interest since their initial conceptualisation in the 1990s. Substantial efforts have been made to develop nanodrugs for overcoming the limitations of conventional drugs, such as low targeting efficacy, high dosage and toxicity, and potential drug resistance. Despite the significant progress that has been made in nanodrug discovery, the precise design or screening of nanomaterials with desired biomedical functions prior to experimentation remains a significant challenge. This is particularly the case with regard to personalised precision nanodrugs, which require the simultaneous optimisation of the structures, compositions, and surface functionalities of nanodrugs. The development of powerful computer clusters and algorithms has made it possible to overcome this challenge through in silico methods, which provide a comprehensive understanding of the medical functions of nanodrugs in relation to their physicochemical properties. In addition, machine learning techniques have been widely employed in nanodrug research, significantly accelerating the understanding of bio–nano interactions and the development of nanodrugs. This review will present a summary of the computational advances in nanodrug discovery, focusing on the understanding of how the key interfacial interactions, namely, surface adsorption, supramolecular recognition, surface catalysis, and chemical conversion, affect the therapeutic efficacy of nanodrugs. Furthermore, this review will discuss the challenges and opportunities in computer-aided nanodrug discovery, with particular emphasis on the integrated “computation + machine learning + experimentation” strategy that can potentially accelerate the discovery of precision nanodrugs.

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