quantum laser experiment

Physicists ‘entangle’ individual molecules for the first time, bringing about a new platform for quantum science

Laser setup for cooling, controlling, and entangling individual molecules.

In a noteworthy first, a team of Princeton physicists has been able to link together individual molecules into special states that are quantum mechanically “entangled.” In these bizarre states, the molecules remain correlated with each other — and can interact simultaneously — even if they are miles apart, or indeed, even if they occupy opposite ends of the universe. This research was published in the current issue of the journal Science .

Members of the Princeton research team. From left to right, Assistant Professor of Physics Lawrence Cheuk, graduate student in electrical engineering Yukai Lu, and graduate student in physics Connor Holland.

“This is a breakthrough in the world of molecules because of the fundamental importance of quantum entanglement,” said Lawrence Cheuk , an assistant professor of physics at Princeton University, the senior author of the paper and a graduate of Princeton's Class of 2010. “But it is also a breakthrough for practical applications because entangled molecules can be the building blocks for many future applications.”

These include, for example, quantum computers that can solve certain problems much faster than conventional computers, quantum simulators that can model complex materials whose behaviors are difficult to model, and quantum sensors that can measure faster than their traditional counterparts.

“One of the motivations in doing quantum science is that in the practical world it turns out that if you harness the laws of quantum mechanics, you can do a lot better in many areas,” said Connor Holland, a graduate student in the physics department and a co-author on the work.

The ability of quantum devices to outperform classical ones is known as “quantum advantage.” And at the core of quantum advantage are the principles of superposition and quantum entanglement. While a classical computer bit can assume the value of either 0 or 1, quantum bits, called qubits, can simultaneously be in a superposition of 0 and 1.

The latter concept, entanglement, is a major cornerstone of quantum mechanics. It occurs when two particles become inextricably linked with each other so that this link persists, even if one particle is light-years away from the other particle. It is the phenomenon that Albert Einstein, who at first questioned its validity, described as “spooky action at a distance.” Since then, physicists have demonstrated that entanglement is, in fact, an accurate description of the physical world and how reality is structured.

“Quantum entanglement is a fundamental concept,” said Cheuk, “but it is also the key ingredient that bestows quantum advantage.”

But building quantum advantage and achieving controllable quantum entanglement remain a challenge, not least because engineers and scientists are still unclear about which physical platform is best for creating qubits. In the past decades, many different technologies — such as trapped ions, photons, superconducting circuits, to name only a few — have been explored as candidates for quantum computers and devices. The optimal quantum system or qubit platform could very well depend on the specific application.

Until this experiment, however, molecules had long defied controllable quantum entanglement. But Cheuk and his colleagues found a way, through careful manipulation in the laboratory, to control individual molecules and coax them into these interlocking quantum states. They also believed that molecules have certain advantages — over atoms, for example — that made them especially well-suited for certain applications in quantum information processing and quantum simulation of complex materials. Compared to atoms, for example, molecules have more quantum degrees of freedom and can interact in new ways.

“What this means, in practical terms, is that there are new ways of storing and processing quantum information,” said Yukai Lu, a graduate student in electrical and computer engineering and a co-author of the paper. “For example, a molecule can vibrate and rotate in multiple modes. So, you can use two of these modes to encode a qubit. If the molecular species is polar, two molecules can interact even when spatially separated.”

Nonetheless, molecules have proven notoriously difficult to control in the laboratory because of their complexity. The very degrees of freedom that make them attractive also make them hard to control, or corral, in laboratory settings. Cheuk and his team addressed many of these challenges through a carefully thought-out experiment involving a sophisticated experimental platform known as a “ tweezer array, ” in which individual molecules were picked up by a complex system of tightly focused laser beams, so-called “optical tweezers.”

“Using molecules for quantum science is a new frontier and our demonstration of on-demand entanglement is a key step in demonstrating that molecules can be used as a viable platform for quantum science,” said Cheuk.

In a separate article published in the same issue of Science , an independent research group led by John Doyle and Kang-Kuen Ni at Harvard University and Wolfgang Ketterle at the Massachusetts Institute of Technology achieved similar results.

“The fact that they got the same results verify the reliability of our results,” Cheuk said. “They also show that molecular tweezer arrays are becoming an exciting new platform for quantum science.”

“ On-Demand Entanglement of Molecules in a Reconfigurable Optical Tweezer Array ,” by Connor M. Holland, Yukai Lu, and Lawrence W. Cheuk was published in Science on December 8, 2023, (DOI: 10.1126/science.adf4272 ). The work was supported by Princeton University, the National Science Foundation (2207518) and the Sloan Foundation (FG-2022-19104).

Princeton scientists measure quantum correlations between molecules for the first time .

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New silicon structure opens the gate to quantum computers .

In a major step toward making a quantum computer using everyday materials, a team led by researchers at Princeton has constructed a key piece of silicon hardware capable of controlling quantum behavior between two electrons with extremely high precision.

Martonosi sketches a path for a new type of computing .

As new devices move quantum computing closer to practical use, the journal Nature recently asked Princeton computer scientist Margaret Martonosi and two colleagues to assess the state of software needed to exploit this powerful computational approach.

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Choreographing dance of electrons offers promise in pursuit of quantum computers .

Princeton University engineers Alexei Tyryshkin and Stephen Lyon have choreographed the dance of 100 billion electrons across a silicon crystal — an impressive achievement on its own — and also a stride toward developing the technology for powerful machines known as quantum computers.

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A Princeton University researcher and his international collaborators have used lasers to peek into the complex relationship between a single electron and its environment, a breakthrough that could aid the development of quantum computers.

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LHC Experiments at CERN Observe Quantum Entanglement at the Highest Energy Yet

The results open up a new perspective on the complex world of quantum physics.

September 18, 2024

Artist's impression of a quantum-entangled pair of top quarks. (Image: CERN)

Editor’s note: The following press release was issued today by CERN, the European Organization for Nuclear Research. The U.S. Department of Energy's Brookhaven National Laboratory serves as the U.S. host laboratory for the ATLAS experiment at CERN’s Large Hadron Collider and plays multiple roles in this international collaboration, from construction and project management to data storage, distribution, and analysis. For more details on Brookhaven’s contributions to the ATLAS experiment, visit the Lab’s ATLAS website . For more information on Brookhaven’s role in this research, contact Stephanie Kossman ( [email protected] , 631-344-8671).

Geneva, 18 September 2024. Quantum entanglement is a fascinating feature of quantum physics – the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, which has no analogue in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell and pioneered quantum information science.

Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published today in Nature , the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet. First reported by ATLAS in September 2023 and since confirmed by two observations made by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.

"While particle physics is deeply rooted in quantum mechanics, the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable,” says ATLAS spokesperson Andreas Hoecker. “It paves the way for new investigations into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow."

The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study entanglement.

The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum traits to its decay particles. Physicists observe and use these decay products to infer the top quark’s spin orientation.

To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018. In particular, they looked for pairs in which the two quarks are simultaneously produced with low particle momentum relative to each other. This is where the spins of the two quarks are expected to be strongly entangled.

The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted. By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations .

In its second study , the CMS collaboration also looked for pairs of top quarks in which the two quarks are simultaneously produced with high momentum relative to each other. In this domain, for a large fraction of top quark pairs, the relative positions and times of the two top quark decays are predicted to be such that classical exchange of information by particles traveling at no more than the speed of light is excluded, and CMS observed spin entanglement between top quarks also in this case.

“With measurements of entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible, we can test the Standard Model of particle physics in new ways and look for signs of new physics that may lie beyond it,” says CMS spokesperson Patricia McBride.

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CERN Accelerating science

LHC experiments at CERN observe quantum entanglement at the highest energy yet

The results open up a new perspective on the complex world of quantum physics

18 September, 2024

Artist’s impression of a quantum-entangled pair of top quarks. (Image: CERN)

Quantum entanglement is a fascinating feature of quantum physics – the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, which has no analogue in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell and pioneered quantum information science.

Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published today in Nature , the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet. First reported by ATLAS in September 2023 and since confirmed by a first and a second observation made by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.

ATLAS Nature paper
CMS first study
CMS second study

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February 27, 2024

Schrödinger’s Pendulum Experiment Will Search for the Quantum Limit

Physicists seek the dividing line between the quantum world and the classical one

By Tim Folger

An illustration representing Schrödinger’s pendulum.

Mark Ross Studios

There’s a rift in reality—an invisible border that separates two utterly different realms. On one side lies our everyday world, where things obey commonsense rules: objects never occupy more than one place at a time, and they exist even when we’re not looking at them. The other side is the dreamscape of quantum mechanics, where nothing is fixed, uncertainty reigns and a single atom or molecule can be in multiple places simultaneously , at least while no one is watching.

Does that mean reality has one set of laws for the macrocosm and another for the micro? Most physicists instinctively dislike the idea of a bifurcated universe . Sougato Bose, a theorist at University College London (UCL), certainly does. “My view is that quantum mechanics hasn’t been seen [at large scales] because we have not yet been able to isolate things well enough,” he says, meaning that researchers haven’t found a way to shield big objects from their environment, so their quantum properties are apparent. Like most physicists, Bose believes that quantum mechanics applies to all things great and small . He and three colleagues—two in the U.K. and one in India—hope to put that view to a stringent test within the next year or two with an intriguing experiment that ultimately aims to determine whether or not large objects obey the strange rules of quantum theory .

The experiment, described in a recent issue of Physical Review Letters , harks back to a conundrum vividly framed nearly a century ago by Erwin Schrödinger, one of the founders of quantum mechanics. What would happen, Schrödinger asked, to a cat trapped in a closed box with a vial of poison that has a 50–50 chance of shattering and killing the cat? According to quantum mechanics the cat is at once alive and dead, existing in both states until someone opens the box and looks inside. That’s because it’s only when an observer makes a measurement of the system—opens the box and checks—that the two possibilities have to collapse into one, according to quantum theory. The story is meant to illustrate how applying these quantum rules to big things—basically, anything visible to the naked eye—leads to absurdities.

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An experimental setup with optical levitation in Hendrik Ulbricht’s research lab at the University of Southampton in England.

Credit: Tiberius Georgescu

So if quantum mechanics is true—and it has been a phenomenally successful theory for predicting the behavior of particles—why do we never see cats that are both dead and alive ? Do the laws of quantum mechanics break down at a certain level? Some physicists see that as a possibility. But most would argue that the apparent absence of quantum effects in our own experience of the world arises because the countless interactions of atoms with the surrounding environment blur the true nature of things. As a result we perceive a kind of dumbed-down, nonquantum version of reality.

If that’s the case, then a carefully designed experiment that isolates an object from nearly everything in its environment should allow physicists to glimpse the actual quantum behavior of that object, even if it’s relatively large. That’s the goal of the experiment proposed by Bose, Debarshi Das, also of UCL, Hendrik Ulbricht of the University of Southampton in England and Dipankar Home of the Bose Institute in India. “There are two possible outcomes,” Home says. “One is that quantum mechanics is valid [at all scales. The other is that] there is a region where quantum mechanics does not hold.”

Most of the hardware needed for the experiment is already in place and fits on a tabletop in Ulbricht’s lab. (He’s the lone experimentalist of the group; Home, Das and Bose are theorists.) The experiment will use lasers to suspend a single nanocrystal of silica—a microscopic glass bead—as it oscillates around the focal point of a small parabolic mirror carved out of a block of aluminum housed in a vacuum chamber. Although the bead is only about 100 nanometers in diameter—roughly the size of a virus—it is nonetheless at least 1,000 times larger than the clumps of molecules that until now have set the experimental benchmark for “quantumness.”

An illuminated red silica nanoparticle in an optical parabola trap at Ulbricht’s lab.

For all its technical complexity, the experiment mimics a very simple phenomenon: the motion of a pendulum. An electromagnetic field drives the silica bead back and forth. Like a metronome, the bead regularly ticks from point A to point B and back again. As far as classical, nonquantum physics goes, that should be the end of the story. But a quantum pendulum should behave very differently. Its position will change depending on whether or not someone is watching: it might start at A but end up somewhere to the left or right of B. Call it Schrödinger’s pendulum.

The experiment will test the very nature of reality: Is it completely objective, or do our own observations play a role in creating what we see? To find out, the experiment will be run in two slightly different ways. In one version, a laser will be aimed at a spot where classical physics predicts the bead to be, say, at position B. If the bead is indeed there, it will reflect the laser light back to a detector. In the second case, the laser will be shined twice: first at an intermediate position and then a second time a little later in the bead’s path. According to classical physics, the intermediate measurement should not affect the subsequent position of the bead—it should always end up at B. After all, in daily life we can’t change the movement of a metronome simply by looking at it.

But in the quantum case, that intermediate measurement has a profound effect. As with Schrödinger’s cat, the bead doesn’t actually exist in any fixed state until it’s observed. Before that, the bead can’t be said to be anywhere at all; it’s just a cloud of possibilities and assumes a definite position only when measured. The mere act of observing the bead at one moment in time changes where it will be at a later moment when the laser shines the second time. If the rules of quantum mechanics hold, the bead may sometimes be found at B, but sometimes it won’t be.

“When you measure, you create that reality,” Bose says. “In quantum mechanics the thing does not exist in a particular place before that. There is no truth before you measure.”

Close up view of particle in magnetic trap

Particle in magnetic trap with field coils in Ulbricht’s lab.

Credit: Marion Cromb

For statistically robust results, Ulbricht will have to work quickly and make about 100,000 measurements of the bead over the course of an hour. (The longer the experiment runs, the greater the risk that slight temperature changes or other subtle effects might interfere with the quantum aspects of the setup.) At the same time, he’ll have to calibrate the position of his detectors so that they count only photons that interact with the bead and not any that might bounce off the small parabolic mirror in the aluminum block.

In principle, Bose, Ulbricht, Das and Home believe that their experimental approach could eventually be scaled up to work with much larger objects, perhaps even with something as massive as a few kilograms. But in that case the array of potential contaminating effects, or “noise,” would become far more difficult to control. “Noise scales very badly [with size],” says Vlatko Vedral, an experimental physicist at the University of Oxford, who also studies the classical-quantum divide. “It scales exponentially. Would I be surprised if this could be done? I don’t know—it’s not a trivial experiment.”

If the current experiment turns out to violate the predictions of classical physics, it will bring the quantum world almost palpably close to our own. “We believe that quantum mechanics is a universal theory,” Das says. “The theory itself does not have any limit. But in reality, whether that is true or not, we don’t know yet. Only experiment can resolve this dilemma.”

One of LIGO's quantum squeezers in operation. (Image credit: Georgia Mansell/LIGO Hanford Observatory)

LIGO Surpasses the Quantum Limit

Feature Story • October 23, 2023

Researchers achieve landmark in quantum squeezing

In 2015, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, made history when it made the first direct detection of gravitational waves, or ripples in space and time, produced by a pair of colliding black holes. Since then, the U.S. National Science Foundation (NSF)-funded LIGO and its sister detector in Europe, Virgo, have detected gravitational waves from dozens of mergers between black holes as well as from collisions between a related class of stellar remnants called neutron stars. At the heart of LIGO's success is its ability to measure the stretching and squeezing of the fabric of spacetime on scales 10 thousand trillion times smaller than a human hair.

As incomprehensibly small as these measurements are, LIGO's precision has continued to be limited by the laws of quantum physics. At very tiny, subatomic scales, empty space is filled with a faint crackling of quantum noise, which interferes with LIGO's measurements and restricts how sensitive the observatory can be. Now, writing in the journal Physical Review X , LIGO researchers report a significant advance in a quantum technology called "squeezing" that allows them to skirt around this limit and measure undulations in spacetime across the entire range of gravitational frequencies detected by LIGO.

This new "frequency-dependent squeezing" technology, in operation at LIGO since it turned back on in May of this year , means that the detectors can now probe a larger volume of the Universe and are expected to detect about 60 percent more mergers than before. This greatly boosts LIGO's ability to study the exotic events that shake space and time.

"We can't control nature, but we can control our detectors," says Lisa Barsotti, a senior research scientist at MIT who oversaw the development of the new LIGO technology, a project that originally involved research experiments at MIT led by Professor of Physics Matt Evans (PhD '02) and Professor of Astrophysics Nergis Mavalvala. The effort now includes dozens of scientists and engineers based at MIT, Caltech, and the twin LIGO observatories in Hanford, Washington, and Livingston, Louisiana.

"A project of this scale requires multiple people, from facilities to engineering and optics—basically the full extent of the LIGO Lab with important contributions from the LIGO Scientific Collaboration. It was a grand effort made even more challenging by the pandemic," Barsotti says.

"Now that we have surpassed this quantum limit, we can do a lot more astronomy," explains Lee McCuller, assistant professor of physics at Caltech and one of the leaders of the new study. "LIGO uses lasers and large mirrors to make its observations, but we are working at a level of sensitivity that means the device is affected by the quantum realm."

The results also have ramifications for future quantum technologies such as quantum computers and other microelectronics as well as for fundamental physics experiments. "We can take what we have learned from LIGO and apply it to problems that require measuring subatomic-scale distances with incredible accuracy," McCuller says.

“When NSF first invested in building the twin LIGO detectors in the late 1990s, we were enthusiastic about the potential to observe gravitational waves,” says NSF Director Sethuraman Panchanathan. “Not only did these detectors make possible groundbreaking discoveries, they also unleashed the design and development of novel technologies. This is truly exemplar of the DNA of NSF—curiosity-driven explorations coupled with use-inspired innovations. Through decades of continuing investments and expansion of international partnerships, LIGO is further poised to advance rich discoveries and technological progress."

The laws of quantum physics dictate that particles, including photons, will randomly pop in and out of empty space, creating a background hiss of quantum noise that brings a level of uncertainty to LIGO's laser-based measurements. Quantum squeezing, which has roots in the late 1970s, is a method for hushing quantum noise or, more specifically, for pushing the noise from one place to another with the goal of making more precise measurements.

The term squeezing refers to the fact that light can be manipulated like a balloon animal. To make a dog or giraffe, one might pinch one section of a long balloon into a small precisely located joint. But then the other side of the balloon will swell out to a larger, less precise size. Light can similarly be squeezed to be more precise in one trait, such as its frequency, but the result is that it becomes more uncertain in another trait, such as its power. This limitation is based on a fundamental law of quantum mechanics called the uncertainty principle , which states that you cannot know both the position and momentum of objects (or the frequency and power of light) at the same time.

Since 2019, LIGO's twin detectors have been squeezing light in such a way as to improve their sensitivity to the upper frequency range of gravitational waves they detect. But, in the same way that squeezing one side of a balloon results in the expansion of the other side, squeezing light has a price. By making LIGO's measurements more precise at the high frequencies, the measurements became less precise at the lower frequencies.

"At some point, if you do more squeezing, you aren't going to gain much. We needed to prepare for what was to come next in our ability to detect gravitational waves," Barsotti explains.

Now, LIGO's new frequency-dependent optical cavities—long tubes about the length of three football fields—allow the team to squeeze light in different ways depending on the frequency of gravitational waves of interest, thereby reducing noise across the whole LIGO frequency range.

Vacuum tube hosting LIGO's 300-meter filter cavity used to implement frequency-dependent squeezing. Each LIGO facility, one in Hanford, Washington, and the other in Livingston Louisiana, has its own such cavity. (Image credit: M.J. Doherty)

"Before, we had to choose where we wanted LIGO to be more precise," says LIGO team member Rana Adhikari, a professor of physics at Caltech. "Now we can eat our cake and have it too. We've known for a while how to write down the equations to make this work, but it was not clear that we could actually make it work until now. It's like science fiction."

Uncertainty in the Quantum Realm

Each LIGO facility is made up of two 4-kilometer-long arms connected to form an "L" shape. Laser beams travel down each arm, hit giant suspended mirrors, and then travel back to where they started. As gravitational waves sweep by Earth, they cause LIGO's arms to stretch and squeeze, pushing the laser beams out of sync . This causes the light in the two beams to interfere with each other in a specific way, revealing the presence of gravitational waves.

However, the quantum noise that lurks inside the vacuum tubes that encase LIGO's laser beams can alter the timing of the photons in the beams by minutely small amounts. McCuller likens this uncertainty in the laser light to a can of BBs. "Imagine dumping out a can full of BBs. They all hit the ground and click and clack independently. The BBs are randomly hitting the ground, and that creates a noise. The light photons are like the BBs and hit LIGO's mirrors at irregular times," he said in a Caltech interview .

The squeezing technologies that have been in place since 2019 make "the photons arrive more regularly, as if the photons are holding hands rather than traveling independently," McCuller said. The idea is to make the frequency, or timing, of the light more certain and the amplitude, or power, less certain as a way to tamp down the BB-like effects of the photons. This is accomplished with the help of specialized crystals that essentially turn one photon into a pair of two entangled , or connected, photons with lower energy. The crystals don't directly squeeze light in LIGO's laser beams; rather, they squeeze stray light in the vacuum of the LIGO tubes, and this light interacts with the laser beams to indirectly squeeze the laser light.

"The quantum nature of the light creates the problem, but quantum physics also gives us the solution," Barsotti says.

A look at the technology that creates squeezed light in LIGO's vacuum chamber. The picture was taken from one of the chamber's viewports at a time when the squeezer was operational and pumped with green light. (Image credit: Georgia Mansell/LIGO Hanford Observatory)

An Idea That Began Decades Ago

The concept for squeezing itself dates back to the late 1970s, beginning with theoretical studies by the late Russian physicist Vladimir Braginsky; Caltech's Kip Thorne, Richard P. Feynman Professor of Theoretical Physics, Emeritus; and Carlton Caves, a former Caltech graduate student and research fellow now at the University of New Mexico. The researchers had been thinking about the limits of quantum-based measurements and communications, and this work inspired one of the first experimental demonstrations of squeezing in 1986 by H. Jeff Kimble, Caltech's William L. Valentine Professor of Physics, Emeritus. Kimble compared squeezed light to a cucumber; the certainty of the light measurements are pushed into only one direction, or feature, turning "quantum cabbages into quantum cucumbers," he wrote in an article in Caltech's Engineering & Science magazine in 1993 .

In 2002, researchers began thinking about how to squeeze light in the LIGO detectors, and, in 2008, the first experimental demonstration of the technique was achieved at the 40-meter test facility at Caltech. In 2010, MIT researchers developed a preliminary design for a LIGO squeezer, which they tested at LIGO's Hanford site. Parallel work done at the GEO600 detector in Germany also convinced researchers that squeezing would work. Nine years later, in 2019, after many trials and careful teamwork, LIGO began squeezing light for the first time .

"We went through a lot of troubleshooting," says Sheila Dwyer, who has been working on the project since 2008, first as a graduate student at MIT and then as a scientist at the LIGO Hanford Observatory beginning in 2013. "Squeezing was first thought of in the late 1970s, but it took decades to get it right."

Too Much of a Good Thing

However, as noted earlier, there is a tradeoff that comes with squeezing. By moving the quantum noise out of the timing, or frequency, of the laser light, the researchers put the noise into the amplitude, or power, of the laser light. The more powerful laser beams then push LIGO's heavy mirrors around causing a rumbling of unwanted noise corresponding to lower frequencies of gravitational waves. These rumbles mask the detectors' ability to sense low-frequency gravitational waves.

"Even though we are using squeezing to put order into our system, reducing the chaos, it doesn't mean we are winning everywhere," says Dhruva Ganapathy, a graduate student at MIT and one of four co-lead authors of the new study. "We are still bound by the laws of physics." The other three lead authors of the study are MIT graduate student Wenxuan Jia, LIGO Livingston postdoctoral scholar Masayuki Nakano, and MIT postdoctoral scholar Victoria Xu.

Unfortunately, this troublesome rumbling becomes even more of a problem when the LIGO team turns up the power on its lasers. "Both squeezing and the act of turning up the power improve our quantum-sensing precision to the point where we are impacted by quantum uncertainty," McCuller says. "Both cause more pushing of photons, which leads to the rumbling of the mirrors. Laser power simply adds more photons, while squeezing makes them more clumpy and thus rumbly."

A view at the source of squeezed light in LIGO's vacuum chamber, taken when the chamber holding the technology was open for maintenance. (Image credit: Wenxuan Jia/MIT)

The solution is to squeeze light in one way for high frequencies of gravitational waves and another way for low frequencies. It's like going back and forth between squeezing a balloon from the top and bottom and from the sides.

This is accomplished by LIGO's new frequency-dependent squeezing cavity, which controls the relative phases of the light waves in such a way that the researchers can selectively move the quantum noise into different features of light (phase or amplitude) depending on the frequency range of gravitational waves.

"It is true that we are doing this really cool quantum thing, but the real reason for this is that it's the simplest way to improve LIGO's sensitivity," Ganapathy says. "Otherwise, we would have to turn up the laser, which has its own problems, or we would have to greatly increase the sizes of the mirrors, which would be expensive."

LIGO's partner observatory, Virgo, will likely also use frequency-dependent squeezing technology within the current run, which will continue until roughly the end of 2024. Next-generation larger gravitational-wave detectors, such as the planned ground-based Cosmic Explorer , will also reap the benefits of squeezed light.

With its new frequency-dependent squeezing cavity, LIGO can now detect even more black hole and neutron star collisions. Ganapathy says he's most excited about catching more neutron star smashups. "With more detections, we can watch the neutron stars rip each other apart and learn more about what's inside."

"We are finally taking advantage of our gravitational universe," Barsotti says. "In the future, we can improve our sensitivity even more. I would like to see how far we can push it."

The Physical Review X study is titled " Broadband quantum enhancement of the LIGO detectors with frequency-dependent squeezing ." Many additional researchers contributed to the development of the squeezing and frequency-dependent squeezing work, including Mike Zucker of MIT and GariLynn Billingsley of Caltech, the leads of the "Advanced LIGO Plus" upgrades that includes the frequency-dependent squeezing cavity; Daniel Sigg of LIGO Hanford Observatory; Adam Mullavey of LIGO Livingston Laboratory; and David McClelland's group from the Australian National University.

The LIGO–Virgo–KAGRA Collaboration operates a network of gravitational-wave detectors in the United States, Italy, and Japan. LIGO Laboratory is operated by Caltech and MIT, and is funded by the NSF with contributions to the Advanced LIGO detectors from Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council), and Australia (Australian Research Council). Virgo is managed by the European Gravitational Observatory (EGO) and is funded by the Centre national de la recherche scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and the National Institute for Subatomic Physics (Nikhef) in the Netherlands. KAGRA is hosted by the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo and co-hosted by the National Astronomical Observatory of Japan (NAOJ) and the High Energy Accelerator Research Organization (KEK).

Written by Whitney Clavin, Caltech

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How scientists are using quantum squeezing to push the limits of their sensors

Fuzziness may rule the quantum realm, but it can be manipulated to our advantage.

Sophia Chen archive page

Researchers install a new quantum squeezing device into one of LIGO’s gravitational wave detectors.

When two black holes spiral inward and collide, they shake the very fabric of space, producing ripples in space-time that can travel for hundreds of millions of light-years. Since 2015, scientists have been observing these so-called gravitational waves to help them study fundamental questions about the cosmos, including the origin of heavy elements such as gold and the rate at which the universe is expanding.

But detecting gravitational waves isn’t easy. By the time they reach Earth and the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), in Louisiana and Washington state, the ripples have dissipated into near silence. LIGO’s detectors must sense motions on the scale of one ten-thousandth the width of a proton to stand a chance.

LIGO has confirmed 90 gravitational wave detections so far, but physicists want to detect more, which will require making the experiment even more sensitive. And that is a challenge.

“The struggle of these detectors is that every time you try to improve them, you actually can make things worse, because they are so sensitive,” says Lisa Barsotti, a physicist at the Massachusetts Institute of Technology.

Nevertheless, Barsotti and her colleagues recently pushed past this challenge, creating a device that will allow LIGO’s detectors to detect far more black hole mergers and neutron star collisions. The device belongs to a growing class of instruments that use quantum squeezing—a practical way for researchers dealing with systems that operate by the fuzzy rules of quantum mechanics to manipulate those phenomena to their advantage.

Physicists describe objects in the quantum realm in terms of probabilities—for example, an electron is not located here or there but has some likelihood of being in each place, locking into one only when its properties are measured. Quantum squeezing can manipulate the probabilities, and researchers are increasingly using it to exert more control over the act of measurement, dramatically improving the precision of quantum sensors like the LIGO experiment.

“In precision sensing applications where you want to detect super-small signals, quantum squeezing can be a pretty big win,” says Mark Kasevich, a physicist at Stanford University who applies quantum squeezing to make more precise magnetometers, gyroscopes, and clocks with potential applications for navigation. Creators of commercial and military technology have begun dabbling in the technique as well: the Canadian startup Xanadu uses it in its quantum computers, and last fall, DARPA announced Inspired , a program for developing quantum squeezing technology on a chip. Let’s take a look at two applications where quantum squeezing is already being used to push the limits of quantum systems.

Taking control of uncertainty

The key concept behind quantum squeezing is the phenomenon known as Heisenberg’s uncertainty principle. In a quantum-mechanical system, this principle puts a fundamental limit on how precisely you can measure an object’s properties. No matter how good your measurement devices are, they will suffer a fundamental level of imprecision that is part of nature itself. In practice, that means there’s a trade-off. If you want to track a particle’s speed precisely, for example, then you must sacrifice precision in knowing its location, and vice versa. “Physics imposes limits on experiments, and especially on precision measurement,” says John Robinson, a physicist at the quantum computing startup QuEra.

By “squeezing” uncertainty into properties they aren’t measuring, however, physicists can gain precision in the property they want to measure. Theorists proposed using squeezing in measurement as early as the 1980s. Since then, experimental physicists have been developing the ideas; over the last decade and a half, the results have matured from sprawling tabletop prototypes to practical devices. Now the big question is what applications will benefit. “We’re just understanding what the technology might be,” says Kasevich. “Then hopefully our imagination will grow to help us find what it’s really going to be good for.”

LIGO is blazing a trail to answer that question, by enhancing the detectors’ ability to measure extremely tiny distances. The observatory registers gravitational waves with L-shaped machines capable of sensing tiny motions along their four-kilometer-long arms. At each machine, researchers split a laser beam in two, sending a beam down each arm to reflect off a set of mirrors. In the absence of a gravitational wave, the crests and troughs of the constituent light waves should completely cancel each other out when the beams are recombined. But when a gravitational wave passes through, it will alternately stretch and compress the arms so that the split light waves are slightly out of phase.

The resulting signals are subtle, though—so subtle that they risk being drowned out by the quantum vacuum, the irremovable background noise of the universe, caused by particles flitting in and out of existence. The quantum vacuum introduces a background flicker of light that enters LIGO’s arms, and this light pushes the mirrors, shifting them on the same scale as the gravitational waves LIGO aims to detect.

Barsotti’s team can’t get rid of this background flicker, but quantum squeezing allows them to exert limited control over it. To do so, the team installed a 300-meter-long cavity in each of LIGO’s two L-shaped detectors. Using lasers, they can create an engineered quantum vacuum, in which they can manipulate conditions to increase their level of control over either how bright the flicker can be or how randomly it occurs in time. Detecting higher-frequency gravitational waves is harder when the rhythm of the flickering is more random, while lower-frequency gravitational waves get drowned out when the background light is brighter. In their engineered vacuum, noisy particles still show up in their measurements, but in ways that don’t do as much to disturb the detection of gravitational waves.“ You can [modify] the vacuum by manipulating it in a way that is useful to you,” she explains.

The innovation was decades in the making: through the 2010s, LIGO incorporated incrementally more sophisticated forms of quantum squeezing based on theoretical ideas developed in the 1980s. With these latest squeezing innovations, installed last year, the collaboration expects to detect gravitational waves up to 65% more frequently than before.

Quantum squeezing has also improved precision in timekeeping. Working at the University of Colorado Boulder with physicist Jun Ye, a pioneer in atomic clock technology, Robinson and his team made a clock that will lose or gain at most a second in 14 billion years. These super-precise clocks tick slightly differently in different gravitational fields, which could make them useful for sensing how Earth’s mass redistributes itself as a result of seismic or volcanic activity . They could also potentially be used to detect certain proposed forms of dark matter , the hypothesized substance that physicists think permeates the universe, pulling on objects with its gravity.

The clock Robinson’s team developed, a type called an optical atomic clock, uses 10,000 strontium atoms. Like all atoms, strontium emits light at specific signature frequencies as electrons around the atom’s nucleus jump between different energy levels. A fixed number of crests and troughs in one of these light waves corresponds to a second in their clock. “You’re saying the atom is perfect,” says Robinson. “The atom is my reference.” The “ticking” of this light is far steadier than the vibrating quartz crystal in a wristwatch, for example, which expands and contracts at different temperatures to tick at different rates.

In practice, the tick in the Robinson team’s clock comes not from the light the electrons emit but from how the whole system evolves over time. The researchers first put each strontium atom in a “superposition” of two states: one in which the atom’s electrons are all at their lowest energy levels and another in which one of the electrons is in an excited state. This means each atom has some probability of being in either state but is not definitively in either one—similar to how a coin flipping in the air has some probability of being either heads or tails, but is neither.

Then they measure how many atoms are in each state. The act of measurement puts the atoms definitively in one state or the other, equivalent to letting the flipping coin land on a surface. Before they measure the atoms, even if they intend to wind up with a 50-50 mixture, they cannot precisely dictate how many atoms will end up in each state. That’s because in addition to the system’s change over time, there is also inherent uncertainty in the state of the individual atoms. Robinson’s team uses quantum squeezing to more reliably determine their final states by reducing these intrinsic fluctuations. Specifically, they manipulate the uncertainties in the direction of each atom’s spin, a property of many quantum particles that has no classical counterpart. Squeezing improved the clock’s precision by a factor of 1.5.

To be sure, gravitational waves and ultra-precise clocks are niche academic applications. But there is interest in adapting the approach to other, potentially more mainstream uses, including quantum computers, navigation, and microscopy.

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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

Dissecting the quantum illusion: debunking the cheshire cat effect, time-bending experiment: physicists reveal quantum nature of light in a new dimension, efficient quantum-mechanical interface leads to a strong interaction between light and matter, photons traverse optical obstacles as both a wave and particle simultaneously, fastest laser blast – 67 quintillionths of a second, dynamics of a system of ultracold potassium atoms, evidence of elusive majorana fermions raises possibilities for quantum computing, quantum entanglement of 8 photons successfully accomplished by physicists, quantum physicists take a step forward in understanding quantum inseparability.

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?

Does this mean that a four-dimensional space-time really does exist, but only in the extremely small spaces around atomic particles/nucleus

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Physicists Have Manipulated 'Quantum Light' For The First Time, in a Huge Breakthrough

For the first time, an international team of physicists has successfully manipulated small numbers of light particles – known as photons – that have a strong relationship with each other.

That may sound a little obscure, but i t's a fundamental breakthrough in the quantum realm that could lead to technology we currently can't even dream of. Imagine lasers, but with quantum sensitivity, for medical imaging.

"This opens the door to the manipulation of what we can call 'quantum light'," says physicist Sahand Mahmoodian from the University of Sydney.

"This fundamental science opens the pathway for advances in quantum-enhanced measurement techniques and photonic quantum computing ."

While physicists are getting very good at controlling quantum entangled atoms , it's proved far more challenging to achieve the same thing with light.

In this new experiment, a team from the University of Sydney and the University of Basel in Switzerland shot both a single photon and a pair of bound photons at a quantum dot (an artificially created atom) and could measure a direct time delay between the photon on its own and the ones that were bound.

"The device we built induced such strong interactions between photons that we were able to observe the difference between one photon interacting with it compared to two," says physicist Natasha Tomm , joint lead author, from the University of Basel.

"We observed that one photon was delayed by a longer time compared to two photons. With this really strong photon-photon interaction, the two photons become entangled in the form of what is called a two-photon bound state."

They set up this bound state using stimulated emission – a phenomenon first described by Albert Einstein in 1916, and which forms the basis of modern lasers. (Fun fact: laser stands for Light Amplification by Stimulated Emission of Radiation.)

Inside a laser, an electrical current or light source is used to hype up electrons within the atoms of an optical material such as glass or crystal.

This excitement bumps the electrons up an orbit in their atom's nucleus. And when they come back down to their regular state, they emit energy in the form of photons. These are the "stimulated" emissions and this process means all the resulting photons have identical wavelengths, unlike normal white light, which is a mix of different frequencies (colors).

A mirror is then used to bounce the old and new photons back towards the atoms, stimulating more identical photons to be produced.

These photons move in unison, travelling with the same speed and direction, and build up until eventually they overcome the mirrors and the optical medium and blast free in a perfectly synchronized beam of light that can stay sharply focused over long distances.

All of that occurs in milliseconds when you push the button on your laser pointer (thanks, Einstein).

This type of cool interaction between light and matter is the basis for all kinds of incredible technology, such as GPS, computers, medical imaging, and global communications networks. Even LIGO, the laser interferometer gravitational-wave observatory that detected gravitational waves for the first time in 2015 is based on lasers.

But all of this technology still requires a whole lot of photons, which limits how sensitive they can be.

The new breakthrough has now achieved stimulated emission and detection for single photons, as well as small groups of photons from a single atom, leading to them becoming strongly correlated – in other words, 'quantum light'. And that's a huge step forward.

"By demonstrating that we can identify and manipulate photon-bound states, we have taken a vital first step towards harnessing quantum light for practical use," says Mahmoodian .

The next steps, she explains , are to use the approach to generate states of light that can make better quantum computers .

"This experiment is beautiful, not only because it validates a fundamental effect – stimulated emission – at its ultimate limit, but it also represents a huge technological step towards advanced applications," adds Tomm .

"We can apply the same principles to develop more-efficient devices that give us photon bound states. This is very promising for applications in a wide range of areas: from biology to advanced manufacturing and quantum information processing."

The research has been published in Nature Physics.

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Even the heaviest particles experience the usual quantum weirdness, new experiment shows

Associate Professor of Physics, University of Sydney

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Bruce Yabsley works for the School of Physics at the University of Sydney, and receives funding from the Australian Research Council. He is a member of the ATLAS Collaboration at CERN, in Geneva, Switzerland; and the Belle II Collaboration at KEK in Tsukuba, Japan.

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One of the most surprising predictions of physics is entanglement, a phenomenon where objects can be some distance apart but still linked together. The best-known examples of entanglement involve tiny chunks of light (photons), and low energies.

At the Large Hadron Collider in Geneva, the world’s largest particle accelerator, an experiment called ATLAS has just found entanglement in pairs of top quarks: the heaviest particles known to science.

The results are described in a new paper from my colleagues and me in the ATLAS collaboration, published today in Nature.

What is entanglement?

In everyday life, we think of objects as being either “separate” or “connected”. Two balls a kilometre apart are separate. Two balls joined by a piece of string are connected.

When two objects are “entangled”, there is no physical connection between them – but they are not truly separate either. You can make a measurement of the first object, and that is enough to know what the second object is doing, even before you look at it.

The two objects form a single system, even though there is nothing connecting them together. This has been shown to work with photons on opposite sides of a city.

The idea will be familiar to fans of the recent streaming series 3 Body Problem, based on Liu Cixin’s sci-fi novels. In the show, aliens have sent a tiny supercomputer to Earth, to mess with our technology and to allow them to communicate with us. Because this tiny object is entangled with a twin on the alien homeworld, the aliens can communicate with it and control it – even though it is four light-years away.

That part of the story is science fiction: entanglement doesn’t really allow you to send signals faster than light. (It seems like entanglement should allow you to do this, but according to quantum physics this isn’t possible. So far, all of our experiments are consistent with that prediction.)

But entanglement itself is real. It was first demonstrated for photons in the 1980s , in what was then a cutting-edge experiment .

Today you can buy a box from a commercial provider that will spit out entangled pairs of photons. Entanglement is one of the properties described by quantum physics, and is one of the properties that scientists and engineers are trying to exploit to create new technologies, such as quantum computing.

Since the 1980s, entanglement has also been seen with atoms, with some subatomic particles, and even with tiny objects undergoing very, very slight vibrations. These examples are all at low energies.

The new development from Geneva is that entanglement has been seen in pairs of particles called top quarks, where there are vast amounts of energy in a very small space.

So what are quarks?

Matter is made of molecules; molecules are made of atoms; and an atom is made of light particles called electrons orbiting a heavy nucleus in the centre, like the Sun in the centre of the solar system. We already knew this from experiments by about 1911.

We then learned that the nucleus is made up of protons and neutrons, and by the 1970s we discovered that protons and neutrons are made up of even smaller particles called quarks.

There are six types of quark in total: the “up” and “down” quarks that make up protons and neutrons, and then four heavier ones. The fifth quark, the “beauty” or “bottom” quark, is about four-and-a-half times heavier than a proton, and when we found it we thought it was very heavy. But the sixth and final quark, the “top”, is a monster: slightly heavier than a tungsten atom, and 184 times the mass of a proton.

No one knows why the top quark is so massive . The top quark is an object of intense study at the Large Hadron Collider, for exactly this reason. (In Sydney, where I am based, most of our work on the ATLAS experiment is focused on the top quark.)

We think the very large mass may be a clue. Maybe the top quark is so massive because the top quark feels new forces, beyond the four we already know about. Or maybe it has some other connection to “new physics”.

We know that the laws of physics, as we currently understand them, are incomplete. Studying the way the top quark behaves may show us the way to something new.

So does entanglement mean that top quarks are special?

Probably not. Quantum physics says that entanglement is common, and that all sorts of things can be entangled.

But entanglement is also fragile. Many quantum physics experiments are done at ultra-cold temperatures, to avoid “bumping” the system and disturbing it. And so, up to now, entanglement has been demonstrated in systems where scientists can set up the right conditions to make the measurements.

For technical reasons, the top quark’s very large mass makes it a good laboratory for studying entanglement. (The new ATLAS measurement would not have been possible for the other five types of quark.)

But top quark pairs won’t be the basis of a convenient new technology: you can’t pick up the Large Hadron Collider and carry it around. Nevertheless, top quarks do provide a new kind of tool to conduct experiments with, and entanglement is interesting in itself, so we’ll keep looking to see what else we find.

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Deriving Fundamental Constants from Three-Beam Collisions

A long-standing prediction of quantum electrodynamics is that high-energy photons can scatter off each other. However, this process has yet to be observed because dedicated experiments have an extremely low signal-to-noise ratio. Now Alexander Macleod at the Extreme Light Infrastructure, Czech Republic, and Ben King at the University of Plymouth, UK, have designed an experiment that could achieve a high-enough signal-to-noise ratio to measure the phenomenon [ 1 ]. Researchers could use such measurements to derive the values of fundamental constants in quantum electrodynamics and then set constraints on various extensions to the standard model of particle physics.

Conventionally, scientists have looked for evidence of photon–photon scattering by colliding pairs of laser beams. Macleod and King instead propose colliding three laser beams: an x-ray beam and two high-power optical beams. The two optical beams provide the photons that scatter off each other, and the x-ray beam imparts a momentum kick to the scattered photons. This kick alters the trajectory of the photons and spatially separates them from much of the experimental background. As a result, in the detection region, the signal-to-noise ratio is higher than that of two-beam setups.

Macleod and King consider how their setup could be realized in two currently existing research facilities: the European X-Ray Free-Electron Laser facility in Germany, as part of the planned BIREF@HIBEF experiment, and the SPring-8 Angstrom Compact Free Electron Laser in Japan. They then show how the technology used in these facilities should be sufficient to measure photon–photon scattering. Macleod says that such a demonstration would be important for researchers working on “high-power lasers, strong-field physics, and quantum electrodynamics.”

–Ryan Wilkinson

Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.

A. J. MacLeod and B. King, “Fundamental constants from photon-photon scattering in three-beam collisions,” Phys. Rev. A 110 , 032216 (2024) .

Fundamental constants from photon-photon scattering in three-beam collisions

A. J. MacLeod and B. King

Phys. Rev. A 110 , 032216 (2024)

Published September 18, 2024

Subject Areas

Signatures of Gravitational Atoms from Black Hole Mergers

Gravitational-wave signals from black hole mergers could reveal the presence of “gravitational atoms”—black holes surrounded by clouds of axions or other light bosons. Read More »

Gamma-Ray Burst Tightens Constraints on Quantum Gravity

An analysis of the brightest gamma-ray burst ever observed reveals no difference in the propagation speed of different frequencies of light—placing some of the tightest constraints on certain violations of general relativity. Read More »

Flavor Profiling the Highest-Energy Neutrinos

A way to determine the flavors of ultrahigh-energy cosmic neutrinos observed by future detectors could help scientists understand the origin of these elusive particles. Read More »

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NASA's Plan to Turn the ISS Into a Quantum Laser Lab

Later this summer, physicists at the Argonne and Fermi national laboratories will exchange quantum information across 30 miles of optical fiber running beneath the suburbs of Chicago. One lab will generate a pair of entangled photons—particles that have identical states and are linked in such a way that what happens to one happens to the other—and send them to their colleagues at the other lab, who will extract the quantum information carried by these particles of light. By establishing this two-way link, the labs will become the first nodes in what the researchers hope will one day be a quantum internet linking quantum computers around the nation.

A quantum web is loaded with potential. It would enable ultra-secure data transmission through quantum encryption. Astronomers could study distant galaxies in unprecedented detail by combining the rare intergalactic photons collected by individual optical telescopes to create a distributed superscope. Linking small quantum computers could create a quantum cloud and rapidly scale our computing abilities. The problem is that quantum information hates long-distance travel. Send entangled photons out into the real world through optical fiber and, in less than 50 miles, environmental interference will destroy their quantum state. But if the photons were relayed through a satellite instead, they could be sent to destinations hundreds—and potentially thousands—of miles away. So in 2018, NASA partnered with MIT’s Lincoln Laboratory to develop the technologies needed to make it happen.

The goal of the National Space Quantum Laboratory program, sometimes referred to as Quantum Technology in Space, is to use a laser system on the International Space Station to exchange quantum information between two devices on Earth without a physical link. The refrigerator-sized module would be attached to the outside of the space station and would generate the entangled photons that carry the quantum information to Earth. The demonstration would pave the way for a satellite that could take entangled particles generated in local quantum networks and send them to far-flung locations.

“In the future, we will likely see quantum information from Argonne routed through a sequence of satellites to another location across the country, or the world,” says David Awschalom, a senior scientist and the quantum group leader at Argonne National Laboratory. “Much like with existing telecommunications, developing a global quantum network may involve a combination of space- and ground-based platforms.”

NASA is not the first to take quantum technologies to space. In 2016 China launched a satellite that sent a pair of entangled photons to two cities more than 700 miles apart. It was a critical test for long-distance quantum key distribution, which uses particles to encrypt information in a way that is almost impossible to break . It demonstrated that entangled particles could survive the journey from space to Earth by randomly sending photons to two ground stations and comparing when they arrived. If two photons arrived at the same time, they must have been entangled.

It was a groundbreaking demonstration, but “you can’t use that to generate a quantum network, because the photons are arriving at random times, and it wasn’t sending any quantum information,” says Scott Hamilton, who leads the Optical Communications Technology group at MIT’s Lincoln Lab. In this sense, what NASA is pursuing is totally different. The agency wants to use a technique called entanglement swapping to send quantum information carried by entangled particles from one node on the ground to another. This requires being able to send entangled photons with very precise timing and measure them without destroying the information they carry.

The Top New Features Coming to Apple’s iOS 18 and iPadOS 18

Entanglement is the source of many of the advantages of a quantum network, since it allows for information to be exchanged between two particles no matter how far apart they happen to be—what Einstein famously called “spooky action at a distance.” These particles are typically photons, which can be thought of as the envelopes carrying letters full of quantum information. But this information is notoriously delicate. Too much interference from the outside world will cause the information in the quantum missives to disappear like vanishing ink.

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Typically, entangled photons are generated from a single source. A laser is fired at a special kind of crystal, and two identical photons pop out; one copy stays with the sender, the other goes to the receiver. The problem is that entangled photons can’t be amplified as they travel from sender to receiver, which limits how far they can travel before the information they carry is destroyed. Entanglement swapping is the art of entangling photons generated from two different sources, which allows the photons to be passed from node to node in a network similar to how a repeater relays optical or radio signals in a classical system.

“Entanglement swapping is a necessity to propagate entanglement over large distances,” says Babak Saif, an optical physicist at NASA’s Goddard Flight Center. “It’s the first step toward a quantum internet.”

In NASA’s system, a pair of entangled photons is generated on the International Space Station and another pair of entangled photons is generated at a ground station on Earth. One of the photons from space and one of the photons generated on Earth are sent to a quantum device that performs a bell measurement, which determines the state of each photon. This simultaneous measurement causes the remaining photons from their respective pairs—the one in space and the other on Earth—to become entangled, despite being generated by different sources. The next step is to send the remaining photon in space to a different ground station on Earth and repeat the process. This entangles the photons at each ground station and establishes a connection between the two quantum devices without a physical link.

It all sounds good in theory, but Saif says just getting the timing right is a major challenge. Entanglement swapping requires both photons—the one from space and the one from Earth—to arrive in the measurement system on Earth at the exact same time. Moreover, the photons need to be able to hit a small receiver with perfect accuracy. Achieving this level of precision from a spacecraft 250 miles away moving 17,000 miles per hour is every bit as hard as it sounds. To make it happen, NASA needs a damn good space laser.

NASA’s last major experiment in space laser communications was in 2013, when the agency sent data to and from a satellite orbiting the moon. The experiment was a huge success and allowed researchers to send data from the lunar satellite to Earth at over 600 megabits per second—that’s faster than the internet connections in most homes. But the lunar laser link wasn’t long for this world. Shortly after the experiment, NASA plowed the satellite into the moon so researchers could study the dust it kicked up on impact.

“Unfortunately, they crashed a perfectly good laser communication system on purpose,” says David Israel, the Exploration and Space Communications Projects Division architect at NASA’s Goddard Flight Center. But he says the experiment laid the groundwork for the Laser Communications Relay Demonstration (LCRD) satellite, which is scheduled to launch early next year. This new satellite will spend its first few years in orbit relaying laser communications from a ground station in California to one in Hawaii so Israel and his colleagues can study how the weather affects laser communications.

The long-term vision is to transition the satellite from an experiment to a data relay for future missions. Israel says its first operational user will be the ILLUMA-T experiment, an acronym so tortuous that I am not even going to spell it out here. ILLUMA-T is a laser communication station that is scheduled to be installed on the International Space Station in 2022 and will relay data through the LCRD satellite to the ground to experiment with laser cross-links in space. “The goal is to connect it to the onboard systems so that LCRD and ILLUMA-T are not so much experiments anymore, but another path to get data to and from the space station,” says Israel.

Together, ILLUMA-T and the LCRD satellite will lay the foundation for an optical communications network in space, which will enable the next generation of lunar explorers to send back high-definition video from the surface of the moon. But they will also be used as test beds to qualify the laser technologies needed for NASA’s quantum communication ambitions. “Since we were already building an optical thing for the space station, the idea was, why not go the extra mile and make it quantum enhanced?” says Nasser Barghouty, who leads the Quantum Sciences and Technology Group at NASA.

Hamilton and his colleagues at MIT Lincoln Lab are already building a tabletop prototype of the quantum systems that could be connected to ILLUMA-T. He says it will be used to demonstrate entanglement swapping on Earth and that a space-ready version could be ready within five years. But whether or not the system will ever be installed on the space station is an open question.

Earlier this year, Hamilton, Barghouty, and other quantum physicists gathered for a workshop at the University of California, Berkeley, to discuss the future of quantum communications at NASA. One of the main topics of discussion was whether to start with a quantum communication demo on the space station or proceed directly to a quantum communication satellite. While the space station is a useful test platform for advanced technologies, its low orbit means it can only see a relatively small portion of the Earth’s surface at a time. To establish a quantum link between locations that are thousands of miles apart requires a satellite orbiting higher than the ISS.

NASA’s plan to build a quantum satellite link is referred to as “Marconi 2.0,” a nod to the Italian inventor Guglielmo Marconi, who was the first to achieve a long-distance radio transmission. Barghouty says the main idea behind Marconi 2.0 is to establish a space-based quantum link between Europe and North America by the mid- to late-2020s. But the details are still being discussed. “Marconi 2.0 is not a specific mission, but a vaguely defined class of missions,” says Barghouty. “There are a lot of variations on the concept.”

Hamilton says he expects NASA will have a finalized road map for its quantum communication program in the next year or two. In the meantime, he and his colleagues are focused on building the technologies that will make the first long-distance quantum network possible. Although the exact form this network will take is still being discussed, one thing is for certain—the road to a quantum internet passes through space.

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Published: 17 September 2024

Quantum-inspired clustering with light

Miguel Varga 1 ,
Pablo Bermejo 2 ,
Ruben Pellicer-Guridi 1 , 2 ,
Román Orús 2 , 3 , 4 &
Gabriel Molina-Terriza 1 , 2 , 3

Scientific Reports volume 14 , Article number: 21726 ( 2024 ) Cite this article

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Information technology
Optics and photonics
Quantum physics

This article introduces a novel approach to perform the simulation of a single qubit quantum-inspired algorithm using laser beams. Leveraging the polarization states of photonic qubits, and inspired by variational quantum eigensolvers, we develop a variational quantum-inspired algorithm implementing a clustering procedure following the approach proposed by some of us in SciRep 13, 13284 (2023). A key aspect of our research involves the utilization of non-orthogonal states within the photonic domain, harnessing the potential of polarization schemes to reproduce unitary circuits. By mapping these non-orthogonal states into polarization states, we achieve an efficient and versatile quantum information processing unit which serves as a clustering device for a diverse set of datasets.

Variational quantum and quantum-inspired clustering

Efficient generation of entangled multiphoton graph states from a single atom

Exactly solving the Kitaev chain and generating Majorana-zero-modes out of noisy qubits

Introduction.

In recent years, the field of quantum computing has witnessed a surge in novel proposals for quantum algorithms, many of which are strategically tailored to thrive in the Noisy Intermediate-Scale Quantum (NISQ) era, a period characterized by the presence of error-prone quantum hardware 1 , 2 . While quantum technology continues to evolve, the quest for more robust and reliable quantum algorithms remains paramount. Amidst this dynamic landscape, the ability to simulate quantum algorithms using classical computers for practical applications has not lost its relevance. As quantum algorithms diversify across different quantum computing architectures, a parallel trend is emerging: the development of increasingly efficient classical simulation methods. A good example of these methods are Tensor Networks 3 , which have proven recently capable of simulating the complex dynamics of many-qubit systems 4 , 5 . In addition, classical platforms are continually being innovated to replicate qubit behaviors, adding to the repertoire of simulation tools 6 , 7 .

In this paper, we introduce an alternative approach to simulate variational quantum algorithms 8 by leveraging the capabilities of a photonic device as a dedicated processing unit. By using light, we can replicate single-qubit rotations, therefore being able to implement tasks such as Variational Quantum Eigensolvers (VQE) 9 and Variational Quantum Clustering (VQC) 10 . Photonic quantum devices offer some inherent characteristics which make them compatible for such purpose: (i) they present a processing unit which can be entirely mapped to the logical gates of a single-qubit circuit, (ii) they provide accurate control over noise, and (iii) they can be eventually scaled-up introducing actual quantum regimes for the input state. Using such architecture, we implement a photonic version of the quantum clustering algorithm Ref. 10 for a single qubit. Clustering methods represent an important research avenue in classical ML, since it can be applied to many practical purposes 11 , 12 , 13 . This particular algorithm has a simple structure that allows to implement a clustering variant on NISQ devices, so that it can be run on a single qubit-like device as in this case. Therefore, our photonic classical device mimicks the behavior of a quantum circuit for a single qubit. Our results are a first test of the capabilities of such a classical platform to simulate quantum algorithms.

The algorithm at the core of this experiment is an unsupervised quantum clustering algorithm, designed for the classification of data points within a given dataset where no prior information about the system is available 14 . In such a scenario there is no training stage, and the implementation follows that of Ref. 10 , where a variational quantum circuit is optimized self-consistently so as to minimize the distance between points and cluster centroids. As explained in that reference, the procedure iteratively optimizes a set of variational parameters based on a reference cost function to determine the optimal configuration for the dataset. Notably, this approach relies solely on the intrinsic features of the dataset without any prior labeling.

To achieve this, the first step is to design a cost function. This cost function is built in such a way that its minimization provides the configuration of the optimal classification. The reference Hamiltonian, specifically constructed for this purpose, is given by:

In this equation, N is the number of datapoints, subscripts i , j represent each data point within the dataset, and a , b denote the family labels associated to points i and j , respectively. The maximum number of families, k , is fixed beforehand. \(\delta _{a,b}\) is the Dirac delta retaining only contributions when points i and j belong to the same cluster. In our coordinate system, each state \(\vec {x}_i\) is represented by a point \((\psi ,\chi )\) at the surface of the Poincaré polarization sphere, as is shown in Fig. 1 (b). Family labels a , b also represent points defined on the Poincaré sphere, belonging to the maximally orthogonal states set, whose number of elements depend on the number of clusters, k , involved in the classification 10 . In each iteration, and before calculating the value of the cost function, we have to determine to which cluster belongs each datapoint. This is done by calculating the fidelity \(f_i^a\) of each state \(\vec {x}_i\) with respect to every cluster label a and asigning to \(\vec {x}_i\) the closest label. The cost function takes into account the distance between data points \(d(\vec {x}_i, \vec {x}_j)\) , given by the l \(^2\) -norm, the distance between a data point and the centroid of its corresponding cluster \(d(\vec {x}_i, \vec {c}_i)\) , also given by the l \(^2\) -norm, as well as the fidelity \(f_i^a \equiv | {\langle {\psi _i}\rangle }{\psi ^a} |^2\) between a variational quantum state \(\vert \psi _i\rangle\) for datapoint \(\vec {x}_i\) and a reference state \(\vert \psi ^a\rangle\) for a given cluster \(a \in k\) . The cluster a to which point i belongs is assigned among the k possible families so that the quantity \(f_i^c\) \(\forall {c}\in k\) is maximized when \(c=a\) . Therefore, all fidelities \(f_i^c\) \(\forall {c}\in k\) are precomputed for each data point before calculating the cost function (one could alleviate this calculation by incorporating information concerning, for example, k , so that \(f_i^a\) can be bounded). This fidelity is then influenced by the position of each data point in the Poincaré polarization sphere, which is determined by a set of variational parameters. In addition, \(\lambda\) serves as a regularization parameter, allowing for different penalizations of distances between dataset points and considering the relative importance of distances between data points and their respective cluster centroids. Indeed, the variational nature of the algorithm entirely falls within the function \(f_i^a\) .

This minimization is built so that points i and j originally belonging to the same family (and thus having a small \(d(\vec {x}_i,\vec {x}_j)\) ) are constrained to stay in the same cluster by diminishing the value \(\left( 1-f_i^a\right) \left( 1-f_j^a\right)\) . If points belonging to the same familiy, with such small distance \(d(\vec {x}_i,\vec {x}_j)\) , were assigned to different families, this contribution would not exist in the cost function, but points that do not belong to the same family could be pushed to stay in the same cluster by proximity, adding up a larger contribution (larger \(d(\vec {x}_i,\vec {x}_j)\) and larger \(\left( 1-f_i^a\right) \left( 1-f_j^a\right)\) ) to the cost than the one with correct label assignment.

Our photonic implementation simulates the optimization of the above cost function for a single qubit, using a variational quantum-inspired circuit. Our method consists of 2 distinct working units: a classical one, and a quantum-inspired one, which we simulate in our case with a diode laser. The classical computer will take care of upgrading the variational parameters driving the quantum circuit by means of a classical optimizer aiming at minimizing the cost function, as in a regular optimization problem. The quantum-inspired circuit will be then modulated based on the upgrade of the variational parameters, which will enter the circuit as rotations in the polarization of the light.

Experimental setup

To implement our quantum-inspired circuit, we have set a series of waveplates that modulate the polarization of an 808nm diode laser. These waveplates will adjust the laser beam’s polarization based on variational parameters. Starting from a specific initial polarization state, the combination of waveplates will gradually transform it. The ultimate goal is to measure the polarization of a specific quantum state using a polarimeter, effectively allowing us to position polarized states throughout the Poincaré sphere, akin to the Bloch sphere of a single qubit. In essence, the system will serve as a versatile tool for transforming the positions of dataset points. It begins with an initial configuration and is manipulated to achieve an optimal configuration in which a particular cost function, like the one in Eq. ( 1 ), is minimized. The underlying concept of using an 808nm diode laser stems from its simplicity and its capacity to map a single-qubit quantum circuit to the laser’s operational principles. By replicating quantum logical gates through a combination of waveplates and facilitating the measurement process with a polarimeter, we can recreate the dynamics of the qubit. In our case, we use this setting to implement the photonic simulation of variational quantum clustering.

[Color online] ( a ) Scheme of optical setup. Experimental stages are visualized in different colors. In yellow, the state initialization part. In magenta, the states manipulation section. And finally, in cyan, the measurement stage is depicted. SMF: single mode optical fiber. FC: fiber collimator. P: linear polarizator. \(\text {H}_\text {x}\) : half wave plate. \(\text {Q}_\text {x}\) : quarter wave plate. Pol.: polarimeter. ( b ) Example of cluster classification for a configuration of 2 clusters of 200 points defined by \(\frac{d}{\sigma }=8\) , where d is the distance between centers and \(\sigma\) the width of the Gaussian blobs.

The optical setup used to initialize, manipulate and read the states is shown in Fig. 1 . In the state preparation sequence (yellow part of Fig. 1 ), after half wave plate \(H_0\) and polarizer P are applied in the incoming beam, the initial state of the system can be represented as

which corresponds to horizontal polarization. This configuration, \(H_0 + P\) , is fixed in order to allow maximum intensity and a stable polarization.

The two first plates, \(Q_{in}\) and \(H_{in}\) , are used to transform the states from the data feature space to the Poincaré sphere. They are mounted on separate rotary stages (RSW60C-T3A from Zaber). These motors have a maximum accuracy of \(\hbox {0.08}^{\circ }\) and a maximum speed of \(\hbox {450}^{\circ }\) /s. The state \(|\phi _o\rangle\) , defined by the angles \((\psi ,\chi )\) , is determined by the angles \((\alpha , \beta )\) corresponding to the fast axis orientation of \(Q_{in}\) and \(H_{in}\) , respectively. This bijection was done by means of a look-up-table.

After initialization, the state \(\vert \phi _o\rangle\) is modified by subsequent sets of m half and quarter wave plates \(\{\hat{H}_k,\hat{Q}_k\}\) (magenta part of the Fig. 1 ). These wave plates work in the same way as the initial ones, that is, for ideal waveplates their Jones matrices are given by

for the half wave plates, and

for the quarter wave plates. While experimentally, the waveplates can depart from this idealized model, this allows us to simulate the behaviour of our experimental system. Here, \(\beta _k\) and \(\alpha _k\) are the angle of rotation of the waveplates. They act as the variational parameters to be optimized in the variational circuit optimizing the cost function from Eq.( 1 ). Therefore, the final state \(\vert \phi _f\rangle\) of the system is given by

or, in terms of the initial horizontal polarization state \(\vert \psi _{in}\rangle\) ,

These output states \(\vert \phi _o\rangle\) are directly measured by the polarimeter (cyan part of Fig. 1 ). In the case of upgrading this experiment to more qubits, the polarimeter should be replaced by a complete tomography of the output state. In our case, this is simplified as the polarimeter provides the value of the Stokes parameters \(s_0, s_1, s_2, s_3\) corresponding to a point in the Poincaré polarization sphere. The readout of this point in the sphere allows for the self-consistent optimization of the variational parameters in the circuit, allowing in turn to minimize the clustering cost function and therefore implement an unsupervised classification of the points in the dataset.

The results for the case of two clusters with \(\sim 100\) points are shown in Fig. 1 (b). One can observe that the algorithm successfully automatically classifies the points in the two different clusters. The ratio of success of the algorithm for this particular case is of \(100\%\) . In this case, the classification task is conducted on top of a random exploration of the initialization step, in order to favour the exploration of the parameters space.

Numerical analysis

To better understand the phase space of the variational parameters and provide a better intuition for more complex quantum optimization processes, we performed a numerical analysis for a 4 Gaussian blobs dataset, classified with 2 single variational layers, generating 3 different optimization paths in hyperparameter space (as shown in Fig. ( 2 a, b, c). We present in the figure the landscape of the cost function, which as expected is a complex shape with many local minima. The initialization of the algorithm would start at a random point in the landscape and then, subsequently, the optimization algorithm would provide the rotations of the Poincare sphere which would optimize the cost function, providing a path in the landscape.

Notice that while there is a single absolute minima, most of the local minima also provide a good classification. The three examples shown fall into the local minima with success ratios of \(92.5\%\) , \(95\%\) and \(100\%\) . The final classification can be shown in Fig. 3 with the corresponding evolution of the cost function. One can observe that, after just 10 iterations, the cost function typically arrives to a stable solution, while it may need up to 30 iterations to reach the minimum. Only one of the optimization paths ends up displaying perfect classification, corresponding to the one with the smallest cost. This result is a consequence of how sensible variational algorithms are to initialization 15 , 16 , which has become one of the main features to look at in the search for non-classical simulability of variational algorithms 17 .

[Color online] Numerical example for the clusterization of a 4 clusters distribution. Top: Colormap of the cost function value with respect of the hiperparameters \(\alpha\) and \(\beta\) corresponding to the rotation angle of a half and a quarter wave-plate, respectively. Bottom: insets’ detail of the trajectories taken to get to the local minima of the cost function.

Numerical cost function evolution for 4 Gaussian blobs: 3 different trajectories corresponding to the insets of Fig. 2 .

As mentioned earlier, one could use more complex minimization algorithms, but in our case a combination of Monte Carlo and steepest descent has provided good results. In this work, we focus on the capabilities of the method to reproduce efficiently a single qubit algorithm, showing the main features of this clustering scheme and opening the way for further tuning strategies.

Experimental results

The methods described before were implemented in our clustering experiment, using more clusters, in order to test the experimental limitations of the system. The main results are summarized in the plots in Fig. 4 . Similarly as in Fig. 1 (b) the figure shows the capability of the photonic clustering implemented to automatically identify configurations with different quantity of clusters. Additionally, in Fig. 5 we provide the experimental evolution of the cost function for the four different configurations tested. These results can be compared with the numerical results that we provided earlier. It can be observed that the experimental errors do not significantly affect the expected behaviour of the optimization process.

[Color online] Experimental clustering results: ( a ) 3 gaussian blobs, ( b ) 4 gaussian blobs and (5) 5 gaussian globes. Colors correspond to the different clusters identified by the experiment.

[Color online] Experimental cost function evolution corresponding to the distributions of the figures 1 and 4 . The cost values where normalized between 0 and 1.

The results presented above constitute a first proof of principle of the validity of using classical photonic systems to simulate quantum clustering. The experiment can be further expanded in many different directions. For instance, it should be possible to classify more complex datasets. However, an increasing number of clusters would considerably lengthen the experimental time, since we have to perform a complete tomographic reconstruction of the states. A potential solution to this limitation could be training on a few data points in our dataset and inferring the position of the remaining points after training by their relative positions within the original dataset. Another alternative approach could be to sequentially add and evaluate points in order to obtain a proper classification minimizing the data required. In addition, one may also explore the possibility of simulating a multi-qubit quantum circuit, by generating and manipulating quantum states consisting of more than one photon.

Conclusions and outlook

In this paper we have introduced a novel quantum-inspired clustering method, based on the photonic simulation of single-qubit dynamics. We have shown how it is possible to optimize the polarization degree of freedom of a laser beam, in the same way that one can optimize the quantum state of a single qubit, so as to minimize a clustering cost function. We have implemented the experiment and shown the validity of our ideas, performing automatic clustering of random data, with no prior information, scattered in up to five zones, with perfect accuracy.

Our experiment is based on encoding the information on the polarization of light, akin to using a single qubit in a quantum algorithm. The rotation matrix introduced by the wave plates in our experiment, can be cast in the quantum circuit in terms of general Pauli rotations around the main axis of a qubit. This can be useful in order to build a bridge between this specific implementation and the virtual environments commonly used in academia and industry with access to quantum hardware (Pennylane, Qiskit, etc...) 18 , 19 . There exist several restrictions in the amount and type of logical gates allowed in actual quantum hardware, so a mapping between the functioning of this optical circuit and universal sets of gates is advisable.

Our work can be further expanded in many directions, as discussed previously. We believe that a promising path is the simulation of multi-qubit systems. In addition, the flexibility of the variational circuit allows for a wide variety of applications. With this in mind, one could for instance build up diverse cost functions for different purposes using the same experimental arrangement, so that we could indeed use a laser beam to implement different types of quantum-inspired machine learning strategies. Indeed, when writing up this paper, we noticed the proposal of a very similar set-up for the realization of VQE algorithms using photonic devices 20 . Last but not least, our scheme allows the introduction of other features, such as data reuploading 21 , 22 . Data reuploading was already proposed in similar contexts, such as in Ref. 23 , where this was used to outperform kernel methods using very few quantum resources, as low as one single qubit. Even without reuploading, introducing several layers of gates in the circuit helps in practice in the convergence of the variational optimization, allowing for small changes of the different parameters at each iteration. All these topics will be the subject of future investigations.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Jerbi, S., Fiderer, L. J., Poulsen Nautrup, H., Kübler, J. M., Briegel, H. J. & Dunjko, V. Quantum machine learning beyond kernel methods (2023) https://doi.org/10.1038/s41467-023-36159-y Nature Communications 14 , 517

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Acknowledgements

We acknowledge Donostia International Physics Center (DIPC), Centro de Física de Materiales, Ikerbasque, Basque Government, Diputación de Gipuzkoa, Spanish Ministry of Science and Innovation and European Innovation Council (EIC) for constant support, as well as insightful discussions with the teams from Multiverse Computing, DIPC and CFM on the algorithms and technical implementations. This work was supported by the Spanish Ministry of Science and Innovation through the PLEC2021-008251 project. We also acknowledge support from PTI-001 from CSIC and the Ministry of Science through project PID2022-143268NB-I00.

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Miguel Varga, Ruben Pellicer-Guridi & Gabriel Molina-Terriza

Donostia International Physics Center, Paseo Manuel de Lardizabal 4, San Sebastián, E-20018, Spain

Pablo Bermejo, Ruben Pellicer-Guridi, Román Orús & Gabriel Molina-Terriza

Ikerbasque Foundation for Science, Maria Diaz de Haro 3, Bilbao, E-48013, Spain

Román Orús & Gabriel Molina-Terriza

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M.V. set up the experiment. M.V and P.B. wrote the clustering code. M.V. and P.B. analyzed the results and wrote the manuscript. All authors reviewed the manuscript.

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Varga, M., Bermejo, P., Pellicer-Guridi, R. et al. Quantum-inspired clustering with light. Sci Rep 14 , 21726 (2024). https://doi.org/10.1038/s41598-024-73053-z

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TNO launches Qu-STAR to pioneer quantum internet via space

TNO launched a new project called Qu-STAR to define the role of space in quantum information networks. The quantum internet, envisioned to connect quantum devices globally, currently faces distance limitations when relying exclusively on ground networks, restricting the exchange of quantum information to just a few hundred kilometers. Satellites present a promising solution to achieve global connectivity, yet the comprehensive architecture for a global quantum internet remains undefined. Qu-STAR will have the added benefit of strengthening the position of the Netherlands in this emerging field, leveraging the country's strong expertise in quantum technology, free space optics, and photonics.

Quantum internet enabled by space

Quantum computing holds the potential to tackle some of today's most significant challenges. To harness its full power, a network is needed that can link quantum devices around the world. Ground networks can only transmit quantum information over relatively short distances, making them insufficient for global connections. Satellites offer a promising solution for achieving worldwide reach, but the design of a comprehensive global quantum internet is still missing.

‘We believe that a collaborative, open approach will yield the best results, and welcome other organizations to join us in this initiative.’

Kees Buijsrogge

Director TNO Space

TNO will collaborate with Airbus Central Research and Technology to take up this task, aiming to define such a design. They will seek collaboration with stakeholders globally, in particular by contributing to the Quantum Internet Alliance and its recently launched Special Interest Group on Space. More broadly, the results of this will be openly accessible to the community.

TNO brings its extensive expertise in optics for laser satellite communications, quantum technology, and novel ICT (Information Communication Technology) infrastructures. Airbus, a key player in all pillars of quantum technology with a focus on aerospace applications, has developed advanced knowledge in ground-satellite optical and quantum communication links and quantum communication networks.

Qu-STAR is the direct follow-up of the letter of intent that Airbus and TNO signed last November at the Conférence Érasme Descartes in presence of the ambassadors of France and the Netherlands.

Kees Buijsrogge, director TNO Space said: ‘Our goal is to advance technologies and make a significant impact in achieving a global quantum internet. We believe that a collaborative, open approach will yield the best results, and welcome other organizations to join us in this initiative.’

Enabling a global quantum internet via space

Quantum computing is expected to solve some of the biggest challenges of our time. From predictive analysis to material design, from pharmaceutical development to the processing of enormous data sets. Yet building a sufficiently powerful quantum computer is extremely challenging.

Charlotte Postma

Contact me to partner with TNO and grow knowledge and technology in Space Situational Awareness and Quantum Laser Satellite Communication. If you are an optical satellite manufacturer, I am very happy to also take you through what TNO can offer in terms of state-of-the-art instrument calibration campaigns.

Looking for an expert?

Get inspired, this is our time: industrialising tno's optical satellite communication technology.

This is our time: working on breakthroughs in laser satellite communication

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TNO joins European collaboration to develop global quantum internet enabled from space

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Could interstellar quantum communications involve Earth or solve the Fermi paradox?

by David Appell , Phys.org

Thus far, the search for extraterrestrial intelligence (SETI) has used strategies based on classical science—listening for radio waves, telescopes watching for optical signals, telescopes in orbit scouring light from the atmospheres of exoplanets, scanning for laser light that might come from aliens. Could a quantum mechanical approach do better?

Latham Boyle says maybe. "It's interesting that our galaxy (and the sea of cosmic background radiation in which it's embedded) 'does' permit interstellar quantum communication in certain frequency bands," he says.

A researcher at the Higgs Center for Theoretical Physics at the University of Edinburgh in Scotland, Boyle has investigated the possibility and says, "But whereas our current telescopes are big enough to allow interstellar 'classical' communication, interstellar 'quantum' communication requires huge telescopes—much bigger than anything we've built so far."

Further, his analysis leads to another potential solution to the Fermi paradox.

For interstellar communication, Boyle wrote "it is natural to ask whether it is also possible to send or receive interstellar quantum communications." His preprint was released on the arXiv preprint server and has been submitted to a peer-reviewed journal.

The idea is to use entangled qubit pairs, one kept by the sender and the other sent to Earth. A few years ago it was discovered that two quantum particles could retain a quantum coherence over interstellar and even galactic distances, even entangled with one another—somehow linked so that determining a property of one entangled qubit immediately determines that of the other.

This strange connection has already been demonstrated between photons over a thousand kilometers apart, with one on Earth's surface and the other in a spacecraft orbiting the planet.

A qubit is a unit of quantum information. Quantum mechanics allows, via quantum superposition, for a particle like a photon to be in two states at once, for example, spin up and spin down. Whereas in classical communication, a photon is in a single state, a bit, that is, either spin up or spin down, but not both at the same time. The qubit's difference makes them more powerful for many applications.

Boyle concentrated on the physical requirements and limitations of sending and detecting such a qubit signal, beginning with the "quantum capacity" of a transmission—the maximum rate at which a quantum communications channel can transmit quantum information.

Much is already known about quantum communications channels from studies and experiments of quantum teleportation, quantum cryptography, quantum entanglement and other quantum phenomena. Protocols based on quantum communication are exponentially faster than those based on classical communication—channels passing one bit at a time from transmitter to receiver—for some tasks.

Using known constraints on the quantum capacity for so-called quantum erasure channels , and properties of the interstellar medium, Boyle was able to obtain two important results: a quantum capacity greater than zero requires the exchanged photons lie within certain allowed frequency bands, and that the effective diameter of both the sending and receiving telescopes must be greater than a value which is proportional to the square root of the photon's wavelength multiplied by the distance between the telescopes.

According to Boyle's analysis, a quantum capacity that doesn't vanish requires the exchanged photons to have a wavelength less than 26.5 cm, mostly to avoid complications with the cosmic microwave background.

Moreover, while classical communications can happen if the receiver receives only a tiny percentage of the photons transmitted (as with radio signals), quantum communications requires that a majority of the photons sent be detected in the receiver's telescope .

For a ground-based telescope, that diameter would be enormous. The photon's wavelength must be at least 320 nm to get through Earth's atmosphere, and given that the distance to our nearest star, Proxima Centauri, is 4.25 light-years, Boyle finds a ground-based telescope would need to be at least 100 kilometers in diameter.

Needless to say, that's a vast difference from the largest ground-based telescope now under construction, the European Extremely Large Telescope under construction in Chile, which will have a diameter of 0.04 km (40 meters).

"In fact," Boyle said, "the required telescopes are so large that if the extraterrestrial sender has a big enough transmitting telescope, they can necessarily also see that we have not yet built a sufficiently large receiving telescope, so they would know that it doesn't yet make sense to communicate with us."

And that's maybe we haven't heard from them, he notes. "In other words, the assumption that extraterrestrials communicate quantum mechanically seems sufficient to explain the Fermi paradox."

Above the atmosphere, shorter wavelengths could be utilized that would require a smaller telescope, perhaps on the moon or at Earth's L2 Lagrange Point, but even gamma rays with wavelengths of order 0.001 nm would still require telescope diameters of about 200 meters.

The telescope need not be a single dish—it could be many small dishes packed close together (either on earth or in space), but they would have to close together, "like the cells in a honeycomb," Boyle said.

A series of relays or quantum repeaters could also be placed on the line between the sender and the target, but for diameters less than 100 meters the repeater telescopes would need to be placed every tenth of an astronomical unit, which includes inside our own solar system. Keeping them in alignment might be a problem (for them at first, not us).

A missing piece is how the receiver would know that an arriving signal is quantum mechanical instead of classical, "viz." part of an entangled pair, if aliens and humans start off with no prior communication . "I think that answer is at least one additional paper in its own right," Boyle said.

Journal information: arXiv

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