experiments with quantum physics

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.

• Standard model of particle physics
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LHC experiments observe quantum entanglement at the highest energy yet

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 analog 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 in Nature , the ATLAS collaboration reports how it has 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 , currently available on the arXiv preprint server, 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 also observed spin entanglement between top quarks 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.

Measurements of polarization and spin correlation and observation of entanglement in top quark pairs using lepton+jets events from proton-proton collisions at √s = 13 TeV, arXiv (2024). DOI: 10.48550/arxiv.2409.11067

Journal information: arXiv , Nature

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Quantum experiments with entangled photons win the 2022 nobel prize in physics.

Physicists Alain Aspect, John Clauser and Anton Zeilinger laid the groundwork for quantum technology

Experiments on a bizarre feature of quantum physics known as entanglement (illustrated here as two objects entangled into one) have netted the 2022 Nobel Prize in physics. When two particles are entangled, what happens to one determines what happens to the other — even if the particles are far apart.

Nicolle R. Fuller/NSF

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By James R. Riordon

October 4, 2022 at 10:47 am

Tests of quantum weirdness and its potential real-world applications have been recognized with the 2022 Nobel Prize in physics. 

At some level we are all subject to quantum rules that even Albert Einstein struggled to come to terms with. For the most part, these rules play out behind the scenes in transistors that make up computer chips, lasers and even in the chemistry of atoms and molecules in materials all around us. Applications that stem from this year’s Nobel Prize take advantage of quantum features at larger scales. They include absolutely secure communications and quantum computers that may eventually solve problems that no conceivable conventional computer could complete in the lifetime of the universe.

This year’s prize is shared among three physicists. Alain Aspect and John Clauser confirmed that the rules of quantum mechanics, as weird and difficult to believe as they are, really do rule the world, while Anton Zeilinger has taken advantage of strange quantum behavior to develop rudimentary applications that no conventional technology can match. Each laureate will take home a third of the prize money, which totals 10 million Swedish kronor, worth roughly $915,000 as of October 4.

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“Today, we honor three physicists whose pioneering experiments showed us that the strange world of entanglement … is not just the micro-world of atoms, and certainly not the virtual world of science fiction or mysticism, but it’s the real world that we all live in,” said Thors Hans Hansson, a member of the Nobel Committee for Physics, at a press conference announcing the award on October 4 at the Royal Swedish Academy of Sciences ( SN: 11/5/10 ). 

“It was certainly very exciting to learn about the three laureates,” says physicist Jerry Chow of IBM Quantum in Yorktown Heights, N.Y. “Aspect, Zeilinger and Clauser — they’re all very, very well known in our quantum community, and their work is something that’s really been a big part of many people’s research efforts over many years.”

Aspect, of the Université Paris-Saclay and École Polytechnique in France, and Clauser, who now runs a company in California, showed that there are no secret back channels of communication that explain how two particles can exist as a single entity, even though they are far apart ( SN: 12/29/14 ). 

The experiments of Zeilinger, of the University of Vienna, that rely on that quantum behavior include demonstrations of communications, absolutely secure encryption and components crucial for quantum computers. He pioneered another, widely misunderstood, application — quantum teleportation. Unlike the teleportation of people and objects in science fiction, the effect involves the perfect transmission of information about a quantum object from one place to another. 

“I was always interested in quantum mechanics from the very first moments when I read about it,” Zeilinger said via phone at the news conference announcing the award. “I was actually struck by some of the theoretical predictions, because they did not fit the usual intuitions which one might have.”

The discovery of quantum behavior that rules the world at small scales, like the motion of an electron around an atom, revolutionized physics at the beginning of the 20th century. Many leading scientists, most famously including Einstein, acknowledged that quantum theories worked, but argued that they couldn’t be the true description of the world because they involved, at best, calculating the probabilities that something would happen ( SN: 1/12/22 ). To Einstein, this meant that there was some hidden information that experiments were too crude to uncover.

Others believed that quantum behavior, derogatively called weirdness, though difficult to understand, had no secret ways of transmitting information. It was largely a matter of opinion and debate until physicist John Bell proposed a test in the 1960s to prove that there were no hidden channels of communication among quantum objects ( SN: 12/29/14 ). At the time it wasn’t clear that an experiment to perform the test was possible.

Clauser was the first to develop a practical experiment to confirm Bell’s test, although there remained loopholes his experiment couldn’t check that left room for doubt. (His interest in science developed early. In 1959 and 1960, Clauser competed in the National Science Fair , now known as the International Science and Engineering Fair ( SN: 5/23/59 ). The fair is run by the Society for Science, which publishes Science News .) 

Aspect took the idea further to eliminate any chance that quantum mechanics had some hidden underpinnings of classical physics ( SN: 1/11/86 ). The experiments of Clauser and Aspect involved creating pairs of photons that were entangled, meaning that they were essentially a single object. As the photons moved in different directions, they remained entangled. That is, they continue to exist as a single, extended object. Measuring the characteristics of one instantly reveals characteristics of the other, no matter how far apart they may be. 

Entanglement is a delicate state of affairs and is difficult to maintain, but the results of the experiments of Clauser and Aspect show that quantum effects cannot be explained with any hidden variables that would be signs of non-quantum underpinnings.

To Chow, the significance of this research is twofold. “There’s really an element of showing, from a philosophical point, that quantum mechanics is real,” he says. “But then, from the more practical standpoint … this same beautiful theory of quantum mechanics gives a different set of rules by which information is processed.” That, in turn, opens up new avenues for next-generation technologies like quantum computers and communications ( SN: 12/3/20 ). 

Zeilinger’s experiments take advantage of entanglement to achieve feats that would not be possible without the effects that Clauser and Aspect confirmed. He has extended the experiments from the lab to intercontinental distances , opening up the possibility that entanglement can be put to practical use ( SN: 5/31/12 ). Because interacting with one of a pair of entangled particles affects the other, they can become key components in secure communications and encryption. An outsider trying to listen in on a quantum communique would be revealed because they would break the entanglement as they snooped.

Quantum computers that rely on entangled particles have also become a topic of active research. Instead of the ones and zeros of conventional computers, quantum computers encode information and perform calculations that are blends of both one and zero. In theory, they can perform some calculations that no digital computer could ever match. Zeilinger’s quantum teleportation experiments offer a route to transfer the information that such quantum computers rely on ( SN: 1/17/98 ). 

“This [award] is a very nice and positive surprise to me,” says Nicolas Gisin, a physicist at the University of Geneva in Switzerland. “This prize is very well-deserved, but comes a bit late. Most of that work was done in the [1970s and 1980s], but the Nobel Committee was very slow, and now is rushing after the boom of quantum technologies.” 

That boom is happening on a global scale, Gisin says. “In the U.S. and in Europe and in China, billions — literally billions of dollars are poured into this field. So, it’s changing completely,” he says. “Instead of having a few individuals pioneering the field, now we have really huge crowds of physicists and engineers that work together.”

Although some of the most esoteric quantum applications are in their infancy, the experiments of Clauser, Aspect and Zeilinger bring quantum mechanics, and its strange implications, to the macroscopic world. Their contributions validate some of the key, once controversial ideas of quantum mechanics and promise novel applications that may someday be commonplace in daily life, in ways that even Einstein couldn’t deny.

Maria Temming contributed reporting to this story.

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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.

2024-22088  |  INT/EXT  |   Newsroom

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Experiments on ‘entangled’ quantum particles won the physics nobel prize.

Physicists Alain Aspect, John Clauser and Anton Zeilinger share the award

Experiments on entanglement — a strange feature of quantum physics — have netted three scientists the 2022 Nobel Prize in physics. When two particles are entangled (illustrated), what happens to one determines what happens to the other — even when the second one is far away.

Nicolle R. Fuller/NSF

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By James R. Riordon and Maria Temming

October 5, 2022 at 10:23 am

For their tests of quantum weirdness and its real-world uses, three scientists will share the 2022 Nobel Prize in physics.

Quantum physics is the science of super small things. It governs how atoms and even tinier particles behave. Such itty-bitty bits of matter don’t obey the same rules as larger objects. One especially strange feature of quantum physics is “entanglement.” When two particles are entangled, everything about them — from their speed to the way they spin — is perfectly connected. If you know the state of one particle, then you know the state of the other. This is true even when the linked particles are very far apart.

When this idea was first proposed, physicists like Albert Einstein were skeptical. Math might allow entanglement in theory, they thought. But there should be no way such linked particles could exist in the real world.

This year’s Nobel Prize winners show that, in fact, it does. And it could lead to many new technologies. Completely secure systems of communication, for instance. Or quantum computers that solve problems that stump any ordinary computer.

Each of this year’s winners will take home a third of the prize money, which totals 10 million Swedish kronor (worth roughly $900,000).

One winner is Alain Aspect. He works at the Université Paris-Saclay and École Polytechnique in France. Another is John Clauser, who runs a company in California. These two confirmed that the rules of quantum physics really do rule the world.

Anton Zeilinger, the third winner, works at the University of Vienna in Austria. He has taken advantage of the quantum strangeness confirmed by Aspect and Clauser to develop new technologies.

“Today, we honor three physicists whose pioneering experiments showed us that the strange world of entanglement … is not just the micro-world of atoms, and certainly not the virtual world of science fiction or mysticism,” said Thors Hans Hansson. “It’s the real world that we all live in.” Hansson is a member of the Nobel Committee for Physics, which chose the winners. He spoke at an October 4 press conference at the Royal Swedish Academy of Sciences in Stockholm. It’s where the award was announced.

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“It was certainly very exciting to learn about the three laureates,” says Jerry Chow. He’s a physicist at IBM Quantum in Yorktown Heights, N.Y. “They’re all very, very well known in our quantum community. And their work is something that’s really been a big part of many people’s research efforts over many years.”

Proving entanglement

The discovery that quantum rules govern tiny things like atoms and electrons shook up early 20th century physics. Many leading scientists, such as Einstein, thought the math of quantum physics worked in theory . But they weren’t sure it could truly describe the real world. Ideas like entanglement were just too weird. How could you really know the state of one particle by looking at another?

Einstein suspected the quantum weirdness of entanglement was an illusion. There must be some classical physics that could explain how it worked — like the secret to a magic trick. Lab tests, he suspected, were just too crude to uncover that hidden information.

Other scientists believed there was no secret to entanglement. Quantum particles had no hidden back channels for sending information. Some particles could just become perfectly linked, and that was that. It was the way the world worked.

In the 1960s, physicist John Bell came up with a test to prove there was no hidden communication between quantum objects. Clauser was the first one to develop an experiment to run this test. His results supported Bell’s idea about entanglement. Linked particles just are .

But Clauser’s test had some loopholes. These left room for doubt. Aspect ran another test that ruled out any chance quantum strangeness could be cleared up by some hidden explanation.

Clauser and Aspect’s experiments involved pairs of light particles, or photons . They created pairs of entangled photons. This meant the particles acted like a single object. As the photons moved apart, they stayed entangled. That is, they kept acting as a single, extended object. Measuring the features of one instantly revealed those of the other. This was true no matter how far apart the photons got.

Entanglement is fragile and hard to maintain. But Clauser and Aspect’s work showed that quantum effects could not be explained by classical physics.

Zeilinger’s experiments show the practical uses of these effects. For instance, he has used entanglement to create absolutely secure encryption and communication. Here’s how it works: Interacting with one entangled particle affects another. So, anyone trying to peek at secret quantum information would break the particles’ entanglement as soon as they snooped. That means nobody can spy on a quantum message without getting caught.  

Zeilinger has also pioneered another use for entanglement. That is quantum teleportation . This isn’t like people popping from one place to another in science fiction and fantasy. The effect involves sending information from one place to another about a quantum object.

Quantum computers are another technology that would rely on entangled particles. Normal computers process data using ones and zeroes. Quantum computers would use bits of information that are each a blend of one and zero. In theory, such machines could run calculations that no normal computer can.

Quantum boom

“This [award] is a very nice and positive surprise to me,” says Nicolas Gisin. He’s a physicist at the University of Geneva in Switzerland. “This prize is very well-deserved. But comes a bit late. Most of that work was done in the [1970s and 1980s]. But the Nobel Committee was very slow and now is rushing after the boom of quantum technologies.”

That boom is happening around the world, Gisin says. “Instead of having a few individuals pioneering the field, now we have really huge crowds of physicists and engineers that work together.”

Some of the most cutting-edge uses of quantum physics are still in their infancy. But the three new Nobel laureates have helped transform this strange science from an abstract curiosity into something useful. Their work validates some key, once-contested ideas of modern physics. Someday, it may also become a basic part of our daily lives, in ways not even Einstein could deny.

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The 12 Most Important and Stunning Quantum Experiments of 2019

Quantum computing seems to inch closer every year.

The smallest scale events have giant consequences. And no field of science demonstrates that better than quantum physics, which explores the strange behaviors of — mostly — very small things. In 2019, quantum experiments went to new and even stranger places and practical quantum computing inched ever closer to reality, despite some controversies. These were the most important and surprising quantum events of 2019.

Google claims "quantum supremacy"

If one quantum news item from 2019 makes the history books, it will probably be a big announcement that came from Google: The tech company announced that it had achieved " quantum supremacy ." That's a fancy way of saying that Google had built a computer that could perform certain tasks faster than any classical computer could. (The category of classical computers includes any machine that relies on regular old 1s and 0s, such as the device you're using to read this article.)

Google's quantum supremacy claim, if borne out, would mark an inflection point in the history of computing. Quantum computers rely on strange small-scale physical effects like entanglement , as well as certain basic uncertainties in the nano-universe, to perform their calculations. In theory, that quality gives these machines certain advantages over classical computers. They can easily break classical encryption schemes, send perfectly encrypted messages, run some simulations faster than classical computers can and generally solve hard problems very easily. The difficulty is that no one's ever made a quantum computer fast enough to take advantage of those theoretical advantages — or at least no one had, until Google's feat this year.

Not everyone buys the tech company's supremacy claim though. Subhash Kak, a quantum skeptic and researcher at Oklahoma State University, laid out several of the reasons in this article for Live Science .

Read more about Google's achievement of quantum supremacy .

The kilogram goes quantum

Another 2019 quantum inflection point came from the world of weights and measures. The standard kilogram, the physical object that defined the unit of mass for all measurements, had long been a 130-year-old, platinum-iridium cylinder weighing 2.2 lbs. and sitting in a room in France. That changed this year.

The old kilo was pretty good, barely changing mass over the decades. But the new kilo is perfect: Based on the fundamental relationship between mass and energy, as well as a quirk in the behavior of energy at quantum scales, physicists were able to arrive at a definition of the kilogram that won't change at all between this year and the end of the universe.

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Read more about the perfect kilogram .

Reality broke a little

A team of physicists designed a quantum experiment that showed that facts actually change depending on your perspective on the situation. Physicists performed a sort of "coin toss" using photons in a tiny quantum computer, finding that the results were different at different detectors, depending on their perspectives.

"We show that, in the micro-world of atoms and particles that is governed by the strange rules of quantum mechanics, two different observers are entitled to their own facts," the experimentalists wrote in an article for Live Science . "In other words, according to our best theory of the building blocks of nature itself, facts can actually be subjective."

Read more about the lack of objective reality .

Entanglement got its glamour shot

For the first time, physicists made a photograph of the phenomenon Albert Einstein described as "spooky action at a distance," in which two particles remain physically linked despite being separated across distances. This feature of the quantum world had long been experimentally verified, but this was the first time anyone got to see it .

Read more about the unforgettable image of entanglement .

Something big went in multiple directions

In some ways the conceptual opposite of entanglement, quantum superposition is enables a single object to be in two (or more) places at once, a consequence of matter existing as both particles and waves. Typically, this is achieved with tiny particles like electrons.

But in a 2019 experiment, physicists managed to pull off superposition at the largest scale ever : using hulking, 2,000-atom molecules from the world of medical science known as "oligo-tetraphenylporphyrins enriched with fluoroalkylsulfanyl chains."

Read about the macro-scale achievement of superposition .

Heat crossed the vacuum

Under normal circumstances, heat can cross a vacuum in only one manner: in the form of radiation. (That's what you're feeling when the sun's rays cross space to beat on your face on a summer day.) Otherwise, in standard physical models, heat moves in two manners: First, energized particles can knock into other particles and transfer their energy. (Wrap your hands around a warm cup of tea to feel this effect.) Second, a warm fluid can displace a colder fluid. (That's what happens when you turn the heater on in your car, flooding the interior with warm air.) So without radiation, heat can't cross a vacuum.

But quantum physics, as usual, breaks the rules. In a 2019 experiment, physicists took advantage of the fact that at the quantum scale, vacuums aren't truly empty. Instead, they're full of tiny, random fluctuations that pop into and out of existence. At a small enough scale, the researchers found, heat can cross a vacuum by jumping from one fluctuation to the next across the apparently empty space.

Read more about heat leaping across the quantum vacuum of space .

Cause and effect might have gone backward

This next finding is far from an experimentally verified discovery, and it's even well outside the realm of traditional quantum physics. But researchers working with quantum gravity — a theoretical construct designed to unify the worlds of quantum mechanics and Einstein's general relativity — showed that under certain circumstances an event might cause an effect that occurred earlier in time.

Certain very heavy objects can influence the flow of time in their immediate vicinity due to general relativity. We know this is true. And quantum superposition dictates that objects can be in multiple places at once. Put a very heavy object (like a big planet) in a state of quantum superposition, the researchers wrote, and you can design oddball scenarios where cause and effect take place in the wrong order .

Read more about cause and effect reversing .

Quantum tunneling cracked

Physicists have long known about a strange effect known as "quantum tunneling," in which particles seem to pass through seemingly impassable barriers . It's not because they're so small that they find holes, though. In 2019, an experiment showed how this really happens.

Quantum physics says that particles are also waves, and you can think of those waves as probability projections for the location of the particle. But they're still waves. Smash a wave against a barrier in the ocean, and it will lose some energy, but a smaller wave will appear on the other side. A similar effect occurs in the quantum world, the researchers found. And as long as there's a bit of probability wave left on the far side of the barrier, the particle has a chance of making it through the obstruction, tunneling through a space where it seems it should not fit.

Read more about the amazing quantum tunneling effect .

Metallic hydrogen may have appeared on Earth

This was a big year for ultra-high-pressure physics. And one of the boldest claims came from a French laboratory, which announced that it had created a holy grail substance for materials science: metallic hydrogen . Under high enough pressures, such as those thought to exist at the core of Jupiter, single-proton hydrogen atoms are thought to act as an alkali metal. But no one had ever managed to generate pressures high enough to demonstrate the effect in a lab before. This year, the team said they'd seen it at 425 gigapascals (4.2 million times Earth's atmospheric pressure at sea level). Not everyone buys that claim , however.

Read more about metallic hydrogen .

We beheld the quantum turtle

Zap a mass of supercooled atoms with a magnetic field , and you'll see "quantum fireworks": jets of atoms firing off in apparently random directions. Researchers suspected there might be a pattern in the fireworks, but it wasn't obvious just from looking. With the aid of a computer, though, researchers discovered a shape to the fireworks effect: a quantum turtle . No one's yet sure why it takes that shape, however.

Read more about the quantum turtle .

A tiny quantum computer turned back time

Time's supposed to move in only one direction: forward. Spill some milk on the ground, and there's no way to perfectly dry out the dirt and return that same clean milk back into the cup. A spreading quantum wave function doesn't unspread.

Except in this case, it did. Using a tiny, two-qubit quantum computer, physicists were able to write an algorithm that could return every ripple of a wave to the particle that created it — unwinding the event and effectively turning back the arrow of time .

Read more about reversing time's arrow .

Another quantum computer saw 16 futures

A nice feature of quantum computers, which rely on superpositions rather than 1s and 0s, is their ability to play out multiple calculations at once. That advantage is on full display in a new quantum prediction engine developed in 2019. Simulating a series of connected events, the researchers behind the engine were able to encode 16 possible futures into a single photon in their engine . Now that's multitasking!

Read more about the 16 possible futures .

• The 10 Weirdest Science Studies of 2019
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• What's That? Your Physics Questions Answered

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Spooky action up close —

A “no math” (but seven-part) guide to modern quantum mechanics, welcome to "the curious observer’s guide to quantum mechanics"–featuring particle/wave duality..

Miguel F. Morales - Jan 10, 2021 2:00 pm UTC

Some technical revolutions enter with drama and a bang, others wriggle unnoticed into our everyday experience. And one of the quietest revolutions of our current century has been the entry of quantum mechanics into our everyday technology. It used to be that quantum effects were confined to physics laboratories and delicate experiments. But modern technology increasingly relies on quantum mechanics for its basic operation, and the importance of quantum effects will only grow in the decades to come.

As such, the time has come to explain quantum mechanics—or, at least, its basics.

My goal in this seven(!)-part series is to introduce the strangely beautiful effects of quantum mechanics and explain how they’ve come to influence our everyday world. Each edition will include a guided hike into the quantum mechanical woods where we’ll admire a new—and often surprising—effect. Once back at the visitor’s center, we’ll talk about how that effect is used in technology and where to look for it.

Embarking on a series of quantum mechanics articles can be intimidating. Few things trigger more fear than “a simple introduction to physics.” But to the intrepid and brave, I will make a few promises before we start:

• No math. While the language of quantum mechanics is written using fairly advanced math, I don’t believe one has to read Japanese before you can appreciate Japanese art. Our journey will focus on the beauty of the quantum world.
• No philosophy. There has been a fascination with the ‘meaning’ of quantum mechanics, but we’ll leave that discussion for pints down at the pub. Here we will focus on what we see.
• Everything we encounter will be experimentally verified. While some of the results might be surprising, nothing we encounter will be speculative.

If you choose to follow me through this series of articles, we will see quantum phenomena on galactic scales, watch particles blend and mix, and see how these effects give rise to both our current technology and advances that are on the verge of making it out of the lab.

So put on your mental hiking boots, grab your binoculars, and follow me as we set out to explore the quantum world.

What is quantum mechanics?

My Mom once asked me, “What is quantum mechanics?” This question has had me stumped for a while now. My best answer so far is that quantum mechanics is the study of how small particles move and interact. But that’s an incomplete answer, since quantum effects can be important on galactic scales too. And it is doubly unsatisfactory because many effects like superconductivity are caused by the blending and mixing of multiple particles.

In many ways, the role of quantum mechanics can be understood in analogy with Newtonian gravity and Einstein’s general relativity. Both describe gravity, but general relativity is more correct—it describes how the Universe works in every situation we’ve managed to test. But 99.99 percent of the time, Newtonian gravity and general relativity give the same answer, and Newtonian gravity is much easier to use. So unless we’re near a black hole, or making precision measurements of time with an optical clock, Newtonian gravity is good enough.

Similarly classical mechanics and quantum mechanics both describe motions and interactions. Quantum mechanics is more right, but most of the time classical mechanics is good enough.

What I find fascinating is that "good enough" increasingly isn’t. Much of the technology developed in this century is starting to rely on quantum mechanics—classical mechanics is no longer accurate enough to understand how these inventions work.

So let’s start today’s hike with a deceptively simple question, “How do particles move?”

Kitchen quantum mechanics

Some of the experiments we will see require specialized equipment, but let’s start with an experiment you can do at home. Like a cooking show, I’ll explain how to do it, but you are encouraged to follow along and do the experiment for yourself. (Share your photos in the discussion below. Bonus points for setting the experiment up in your cubicle/place of work/other creative setting.)

To study how particles move, we need a good particle pea shooter to make lots of particles for us to play with. It turns out a laser pointer, in addition to entertaining the cat, is a great source of particles. It makes copious amounts of photons, all moving in nearly the same direction and with nearly the same energy (as indicated by their color).

If we look at the light from a laser pointer, it exits the end of the laser pointer and moves in a straight line until it hits an obstacle and scatters (or hits a mirror and bounces). At this point, it is tempting to guess that we know how particles move: they exit the end of the laser like little ball bearings and move in a straight line until they hit something. But as good observers, let’s make sure.

Let’s challenge the particles with an obstacle course by cutting thin slits in aluminum foil with razor blades. In the aluminum foil I’ve made a couple of different cuts. The first is a single slit, a few millimeters long. For the second I’ve stacked two razor blades together and used them to cut two parallel slits a few tenths of a millimeter apart.

In a darkened room, I setup my laser pointer to shoot across the room and hit a blank wall. As expected I see a spot (provided the cat’s not around). Next, I put the single slit in the aluminum foil in the laser’s path and look at the pattern on the wall. When we send the light through the single slit, we see that the beam dramatically expands in the direction perpendicular to the slit—not along the slit.

Interesting. But let’s press on.

Now let’s put the closely spaced slits into the laser beam. The light is again spread out, but now there is a stripey pattern.

Congratulations! You’ve just spotted a quantum mechanical effect! (whoo hoo animated emoji) This is the classic double-slit experiment. The stripey pattern is called interference, and is a telltale signature of quantum mechanics. We will see a lot of stripes like these.

Now you have probably seen interference like this before, since water and sound waves show exactly this kind of striping.

In the photo above, each ball creates waves that move out in a circle. But a wave has both a peak and a trough. In some places the peak of the wave from one of the balls always coincides with the trough from the other (and vice versa). In these areas the waves always cancel out and the water is calm. In other locations the peaks of the waves from both balls always arrive together and add up to make a wave that is extra tall. In these locations the troughs also add up to be extra deep.

So does the fact that we are seeing stripes when our laser pointer goes through two slits mean that particles are waves? To answer that question, we’re going to have to look more closely.

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How Do Scientists Conduct Quantum Experiments?

To conduct quantum science experiments, researchers often work with the smallest objects—and some of the most fragile and sensitive phenomena—in nature. This requires specialized tools and techniques that have advanced in sophistication since the field of quantum mechanics emerged in the early 1900s.

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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.

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 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.

• ATLAS Nature paper
• CMS first study
• CMS second study

View original article here .

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July 2, 2021

AI Designs Quantum Physics Experiments beyond What Any Human Has Conceived

Originally built to speed up calculations, a machine-learning system is now making shocking progress at the frontiers of experimental quantum physics

By Anil Ananthaswamy

Kotryna Zukauskaite

Q uantum physicist Mario Krenn remembers sitting in a café in Vienna in early 2016, poring over computer printouts, trying to make sense of what MELVIN had found. MELVIN was a machine-learning algorithm Krenn had built, a kind of artificial intelligence. Its job was to mix and match the building blocks of standard quantum experiments and find solutions to new problems. And it did find many interesting ones. But there was one that made no sense. “The first thing I thought was, ‘My program has a bug because the solution cannot exist,’” Krenn says.

MELVIN had seemingly solved the problem of creating highly complex entangled states involving multiple photons (entangled states being those that once made Albert Einstein invoke the specter of “spooky action at a distance”) . Krenn, Anton Zeilinger of the University of Vienna and their colleagues had not explicitly provided MELVIN the rules needed to generate such complex states, yet it had found a way. Eventually Krenn realized that the algorithm had rediscovered a type of experimental arrangement that had been devised in the early 1990s. But those experiments had been much simpler. MELVIN had cracked a far more complex puzzle. “When we understood what was going on, we were immediately able to generalize [the solution],” says Krenn, who is now at the University of Toronto.

Since then, other teams have started performing the experiments identified by MELVIN, allowing them to test the conceptual underpinnings of quantum mechanics in new ways. Meanwhile Krenn, working with colleagues in Toronto, has refined their machine-learning algorithms. Their latest effort, an AI called THESEUS, has upped the ante: it is orders of magnitude faster than MELVIN, and humans can readily parse its output. While it would take Krenn and his colleagues days or even weeks to understand MELVIN’s meanderings, they can almost immediately figure out what THESEUS is saying. “It is amazing work,” says theoretical quantum physicist Renato Renner of the Institute for Theoretical Physics at the Swiss Federal Institute of Technology Zurich, who reviewed a 2020 study about THESEUS but was not directly involved in these efforts.

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Krenn stumbled on this entire research program somewhat by accident when he and his colleagues were trying to figure out how to experimentally create quantum states of photons entangled in a very particular manner. When two photons interact, they become entangled, and both can be mathematically described only using a single shared quantum state. If you measure the state of one photon, the measurement instantly fixes the state of the other even if the two are kilometers apart (hence Einstein’s derisive comments on entanglement being “spooky”).

In 1989 three physicists—Daniel Greenberger, the late Michael Horne and Zeilinger— described an entangled state that came to be known as GHZ (after their initials). It involved four photons, each of which could be in a quantum superposition of, say, two states, 0 and 1 (a quantum state called a qubit). In their paper, the GHZ state involved entangling four qubits such that the entire system was in a two-dimensional quantum superposition of states 0000 and 1111. If you measured one of the photons and found it in state 0, the superposition would collapse, and the other photons would also be in state 0. The same went for state 1. In the late 1990s Zeilinger and his colleagues experimentally observed GHZ states using three qubits for the first time .

Krenn and his colleagues were aiming for GHZ states of higher dimensions. They wanted to work with three photons, where each photon had a dimensionality of three, meaning it could be in a superposition of three states: 0, 1 and 2. This quantum state is called a qutrit . The entanglement the team was after was a three-dimensional GHZ state that was a superposition of states 000, 111 and 222. Such states are important ingredients for secure quantum communications and faster quantum computing. In late 2013 the researchers spent weeks designing experiments on blackboards and doing the calculations to see if their setups could generate the required quantum states. But each time they failed. “I thought, ‘This is absolutely insane. Why can’t we come up with a setup?’” Krenn says.

To speed up the process, Krenn first wrote a computer program that took an experimental setup and calculated the output. Then he upgraded the program to allow it to incorporate in its calculations the same building blocks that experimenters use to create and manipulate photons on an optical bench: lasers, nonlinear crystals, beam splitters, phase shifters, holograms, and the like. The program searched through a large space of configurations by randomly mixing and matching the building blocks, performed the calculations and spat out the result. MELVIN was born. “Within a few hours the program found a solution that we scientists—three experimentalists and one theorist—could not come up with for months,” Krenn says. “That was a crazy day. I could not believe that it happened.” Then he gave MELVIN more smarts. Anytime it found a setup that did something useful, MELVIN added that setup to its toolbox. “The algorithm remembers that and tries to reuse it for more complex solutions,” Krenn says.

It was this more evolved MELVIN that left Krenn scratching his head in a Viennese café. He had set it running with an experimental toolbox that contained two crystals, each capable of generating a pair of photons entangled in three dimensions. Krenn’s naive expectation was that MELVIN would find configurations that combined these pairs of photons to create entangled states of at most nine dimensions. But “it actually found one solution, an extremely rare case, that has much higher entanglement than the rest of the states,” Krenn says.

Eventually he figured out that MELVIN had used a technique that multiple teams had developed nearly three decades ago. In 1991 Xin Yu Zou, Li Jun Wang and Leonard Mandel, all then at the University of Rochester, designed one method. And in 1994 Zeilinger, then at the University of Innsbruck in Austria, and his colleagues came up with another . Conceptually these experiments attempted something similar, but the configuration that Zeilinger and his colleagues devised is simpler to understand. It starts with one crystal that generates a pair of photons (A and B). The paths of these photons go right through another crystal, which can also generate two photons (C and D). The paths of photon A from the first crystal and of photon C from the second overlap exactly and lead to the same detector. If that detector clicks, it is impossible to tell whether the photon originated from the first or the second crystal. The same goes for photons B and D.

A phase shifter is a device that effectively increases the path a photon travels as some fraction of its wavelength. If you were to introduce a phase shifter in one of the paths between the crystals and kept changing the amount of phase shift, you could cause constructive and destructive interference at the detectors. For example, each of the crystals could be generating, say, 1,000 pairs of photons per second. With constructive interference, the detectors would register 4,000 pairs of photons per second. And with destructive interference, they would detect none: the system as a whole would not create any photons even though individual crystals would be generating 1,000 pairs a second. “That is actually quite crazy, when you think about it,” Krenn says.

MELVIN’s funky solution involved such overlapping paths. What had flummoxed Krenn was that the algorithm had only two crystals in its toolbox. And instead of using those crystals at the beginning of the experimental setup, it had wedged them inside an interferometer (a device that splits the path of, say, a photon into two and then recombines them). After much effort, he realized that the setup MELVIN had found was equivalent to one involving more than two crystals, each generating pairs of photons, such that their paths to the detectors overlapped. The configuration could be used to generate high-dimensional entangled states.

Quantum physicist Nora Tischler, who was a Ph.D. student working with Zeilinger on an unrelated topic when MELVIN was being put through its paces, was paying attention to these developments. “It was kind of clear from the beginning [that such an] experiment wouldn’t exist if it hadn’t been discovered by an algorithm,” she says.

Besides generating complex entangled states, the setup using more than two crystals with overlapping paths can be employed to perform a generalized form of Zeilinger’s 1994 quantum interference experiments with two crystals. Aephraim Steinberg, an experimentalist who is a Toronto colleague of Krenn’s but has not worked on these projects, is impressed by what the AI found. “This is a generalization that (to my knowledge) no human dreamed up in the intervening decades and might never have done,” he says. “It’s a gorgeous first example of the kind of new explorations these thinking machines can take us on.”

In one such generalized configuration with four crystals, each generating a pair of photons, and overlapping paths leading to four detectors, quantum interference can create situations where either all four detectors click (constructive interference) or none of them do so (destructive interference). Until recently, carrying out such an experiment had remained a distant dream. Then, in a March preprint paper, a team led by Lan-Tian Feng of the University of Science and Technology of China, in collaboration with Krenn, reported that they had fabricated the entire setup on a single photonic chip and performed the experiment. The researchers collected data for more than 16 hours: a feat made possible because of the photonic chip’s incredible optical stability, something that would have been impossible to achieve in a larger-scale tabletop experiment. For starters, the setup would require a square meter’s worth of optical elements precisely aligned on an optical bench, Steinberg says. Besides, “a single optical element jittering or drifting by a thousandth of the diameter of a human hair during those 16 hours could be enough to wash out the effect,” he says.

During their early attempts to simplify and generalize what MELVIN had found, Krenn and his colleagues realized that the solution resembled abstract mathematical forms called graphs, which contain vertices and edges and are used to depict pairwise relations between objects. For these quantum experiments, every path a photon takes is represented by a vertex. And a crystal, for example, is represented by an edge connecting two vertices. MELVIN first produced such a graph and then performed a mathematical operation on it. The operation, called perfect matching, involves generating an equivalent graph in which each vertex is connected to only one edge. This process makes calculating the final quantum state much easier, although it is still hard for humans to understand.

That changed with MELVIN’s successor THESEUS, which generates much simpler graphs by winnowing the first complex graph representing a solution that it finds down to the bare minimum number of edges and vertices (such that any further deletion destroys the setup’s ability to generate the desired quantum states). Such graphs are simpler than MELVIN’s perfect matching graphs, so it is even easier to make sense of any AI-generated solution. Renner is particularly impressed by THESEUS’s human-interpretable outputs. “The solution is designed in such a way that the number of connections in the graph is minimized,” he says. “And that’s naturally a solution we can better understand than if you had a very complex graph.”

Eric Cavalcanti of Griffith University in Australia is both impressed by the work and circumspect about it. “These machine-learning techniques represent an interesting development. For a human scientist looking at the data and interpreting it, some of the solutions may look like ‘creative’ new solutions. But at this stage, these algorithms are still far from a level where it could be said that they are having truly new ideas or coming up with new concepts,” he says. “On the other hand, I do think that one day they will get there. So these are baby steps—but we have to start somewhere.” Steinberg agrees. “For now they are just amazing tools,” he says. “And like all the best tools, they’re already enabling us to do some things we probably wouldn’t have done without them.”

Anil Ananthaswamy is author of The Edge of Physics (Houghton Mifflin Harcourt, 2010), The Man Who Wasn't There (Dutton, 2015), Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality (Dutton, 2018), and Why Machines Learn: The Elegant Math Behind AI (Dutton, 2024).

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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

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• Published: 29 April 2021

Learning models of quantum systems from experiments

• Antonio A. Gentile   ORCID: orcid.org/0000-0002-1763-9746 1 , 2   na1 ,
• Brian Flynn 1 , 3   na1 ,
• Sebastian Knauer   ORCID: orcid.org/0000-0002-5790-4575 1 , 4   na1 ,
• Nathan Wiebe 5 , 6 ,
• Stefano Paesani   ORCID: orcid.org/0000-0001-5709-0906 1 ,
• Christopher E. Granade 7 ,
• John G. Rarity 1 ,
• Raffaele Santagati   ORCID: orcid.org/0000-0001-9645-0580 1 , 8 , 9   na1 &
• Anthony Laing 1  

Nature Physics volume  17 ,  pages 837–843 ( 2021 ) Cite this article

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As Hamiltonian models underpin the study and analysis of physical and chemical processes, it is crucial that they are faithful to the system they represent. However, formulating and testing candidate Hamiltonians for quantum systems from experimental data is difficult, because one cannot directly observe which interactions are present. Here we propose and demonstrate an automated protocol to overcome this challenge by designing an agent that exploits unsupervised machine learning. We first show the capabilities of our approach to infer the correct Hamiltonian when studying a nitrogen-vacancy centre set-up. In preliminary simulations, the exact model is known and is correctly inferred with success rates up to 59%. When using experimental data, 74% of protocol instances retrieve models that are deemed plausible. Simulated multi-spin systems, characterized by a space of 10 10 possible models, are also investigated by incorporating a genetic algorithm in our protocol, which identifies the target model in 85% of instances. The development of automated agents, capable of formulating and testing modelling hypotheses from limited prior assumptions, represents a fundamental step towards the characterization of large quantum systems.

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QDataSet, quantum datasets for machine learning

Electronic structure prediction of multi-million atom systems through uncertainty quantification enabled transfer learning

Quantum machine learning using atom-in-molecule-based fragments selected on the fly

Data availability.

Data and analysis scripts are available at https://figshare.com/s/caf2581da7eb2414fc7f . The data are presented in CSV format with analysis in Python using Pandas and NumPy libraries.

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Our code, written in Python and implemented via the Portable Batch System for parallel processing, is available at https://github.com/flynnbr11/QMLA , with documentation at https://quantum-model-learning-agent.readthedocs.io/en/latest/ .

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Acknowledgements

We thank C. Bonato, F. Jelezko, F. Marquardt, T. Fösel, C. Woods, J. Wang, M. G. Thompson, A. Paiement, A. Cimarelli, P. Yard and J. Bulmer for useful discussions and feedback. A.A.G., S.K., S.P. and R.S. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC), programme grant no. EP/L024020/1. B.F. acknowledges support from Airbus and EPSRC grant EP/P510427/1. N.W. was funded by a grant from Google Quantum AI, the NQI center for Quantum Co-Design, the Pacific Northwest National Laboratory LDRD programme and the ‘Embedding Quantum Computing into Many-Body Frameworks for Strongly Correlated Molecular and Materials Systems’ project, funded by the US Department of Energy (DOE). J.G.R. acknowledges support from EPSRC (EP/M024458/1). A.L. acknowledges fellowship support from EPSRC (EP/N003470/1). This work is supported by the UK Hub in Quantum Computing and Simulation, part of the UK National Quantum Technologies Programme with funding from UKRI EPSRC grant no. EP/T001062/1, and by the European project QuCHIP (Quantum Simulation on a Photonic Chip; grant agreement no. 641039). This work was carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol ( http://www.bristol.ac.uk/acrc/ ).

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These authors contributed equally: Antonio A. Gentile, Brian Flynn, Sebastian Knauer, Raffaele Santagati.

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Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering, University of Bristol, Bristol, UK

Antonio A. Gentile, Brian Flynn, Sebastian Knauer, Stefano Paesani, John G. Rarity, Raffaele Santagati & Anthony Laing

Qu & Co BV, Amsterdam, The Netherlands

Antonio A. Gentile

Quantum Engineering Centre for Doctoral Training, University of Bristol, Bristol, UK

Brian Flynn

Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of New South Wales, Sydney, New South Wales, Australia

Sebastian Knauer

Department of Physics, University of Washington, Seattle, WA, USA

Nathan Wiebe

Pacific Northwest National Laboratory, Richland, WA, USA

Quantum Architectures and Computation Group, Microsoft Research, Redmond, WA, USA

Christopher E. Granade

International Iberian Nanotechnology Laboratory (INL), Braga, Portugal

• Raffaele Santagati

Boehringer-Ingelheim Quantum Lab, Wien, Austria

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R.S., A.A.G. and N.W. conceived the methodology. B.F., A.A.G. and R.S. performed simulations with support from N.W., S.P. and C.E.G. S.K. built the set-up and performed the experiments under the guidance of J.G.R. A.A.G., R.S., B.F. and S.P. analysed and interpreted the data with support from N.W., S.K. and C.E.G. A.A.G., R.S., B.F., S.K., N.W., S.P., C.E.G. and A.L. wrote the manuscript. R.S. and A.L. supervised the project.

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Gentile, A.A., Flynn, B., Knauer, S. et al. Learning models of quantum systems from experiments. Nat. Phys. 17 , 837–843 (2021). https://doi.org/10.1038/s41567-021-01201-7

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“This is a wonderful book to dip into. … it begins with the Aharanov­–Bohm effect, runs through the Born Rule and the Copenhagen Interpretation … before ending with Zero-point Energy. … Kaiser on Feynman Diagrams nicely brings all those elements together, as does Lyre on Gauge Symmetry. … a book everyone interested in the history and philosophy of quantum physics should have on their desk or even at their bedside … . it should be on hand electronically via your library’s online access.” (Steven French, Metascience, June, 2011)

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Experiments Spell Doom for Decades-Old Explanation of Quantum Weirdness

October 20, 2022

Theories that propose a natural trigger for the collapse of the quantum wave function appear to themselves be collapsing.

Michal Bednarski for Quanta Magazine

Introduction

How does objective reality emerge from the palette of possibilities supplied by quantum mechanics? That question — the deepest and most vexed issue posed by the theory — is still the subject of arguments a century old. Possible explanations for how observations of the world yield definite, “classical” results, drawing on different interpretations of what quantum mechanics means, have only multiplied over those hundred or so years.

But now we may be ready to eliminate at least one set of proposals. Recent experiments have mobilized the extreme sensitivity of particle physics instruments to test the idea that the “collapse” of quantum possibilities into a single classical reality is not just a mathematical convenience but a real physical process — an idea called “physical collapse.” The experiments find no evidence of the effects predicted by at least the simplest varieties of these collapse models.

It’s still too early to say definitively that physical collapse does not occur. Some researchers believe that the models could yet be modified to escape the constraints placed on them by the experiments’ null results. But while “it is always possible to rescue any model,” said Sandro Donadi , a theoretical physicist at the National Institute for Nuclear Physics (INFN) in Trieste, Italy, who led one of the experiments, he doubts that “the community will keep modifying the models [indefinitely], since there will not be too much to learn by doing that.” The noose seems to be tightening on this attempt to resolve the biggest mystery of quantum theory.

What Causes Collapse?

Physical collapse models aim to resolve a central dilemma of conventional quantum theory. In 1926 Erwin Schrödinger asserted that a quantum object is described by a mathematical entity called a wave function, which encapsulates all that can be said about the object and its properties. As the name implies, a wave function describes a kind of wave — but not a physical one. Rather, it is a “probability wave,” which allows us to predict the various possible outcomes of measurements made on the object, and the chance of observing any one of them in a given experiment.

Detectors such as this one, originally designed for neutrino research, have taken on a second job testing the predictions of physical-collapse theories.

Nick Hubbard, Sanford Underground Research Facility

If many measurements are made on such objects when they’re prepared in an identical manner, the wave function always correctly predicts the statistical distribution of outcomes. But there’s no way of knowing what the outcome of any single measurement will be — quantum mechanics offers only probabilities. What determines a specific observation? In 1932, the mathematical physicist John von Neumann proposed that, when a measurement is made, the wave function is “collapsed” into one of the possible outcomes. The process is essentially random but biased by the probabilities it encodes. Quantum mechanics itself doesn’t appear to predict the collapse, which has to be manually added to the calculations.

As an ad hoc mathematical trick, it works well enough. But it seemed (and continues to seem) to some researchers to be an unsatisfactory sleight of hand. Einstein famously likened it to God playing dice to decide what becomes “real” — what we actually observe in our classical world. The Danish physicist Niels Bohr, in his so-called Copenhagen interpretation, simply pronounced the issue out of bounds, saying that physicists just had to accept a fundamental distinction between the quantum and classical regimes. In contrast, in 1957 the physicist Hugh Everett asserted that wave function collapse is just an illusion and that in fact all outcomes are realized in a near-infinite number of branching universes — what physicists now call “ many worlds .”

The truth is that “the fundamental cause of the wave function collapse is yet unknown,” said Inwook Kim , a physicist at the Lawrence Livermore National Laboratory in California. “Why and how does it occur?”

In 1986, the Italian physicists Giancarlo Ghirardi, Alberto Rimini and Tullio Weber suggested an answer. What if, they said, Schrödinger’s wave equation was not the whole story? They posited that a quantum system is constantly prodded by some unknown influence that can induce it to spontaneously jump into one of the system’s possible observable states, on a timescale that depends on how big the system is. A small, isolated system, such as an atom in a quantum superposition (a state in which several measurement outcomes are possible), will stay that way for a very long time. But bigger objects — a cat, say, or an atom when it interacts with a macroscopic measuring device — collapse into a well-defined classical state almost instantaneously. This so-called GRW model (after the trio’s initials) was the first physical-collapse model; a later refinement known as the continuous spontaneous localization (CSL) model involved gradual, continuous collapse rather than a sudden jump. These models are not so much interpretations of quantum mechanics as additions to it, said the physicist Magdalena Zych of the University of Queensland in Australia.

What is it that causes this spontaneous localization via wave function collapse? The GRW and CSL models don’t say; they just suggest adding a mathematical term to the Schrödinger equation to describe it. But in the 1980s and ’90s, the mathematical physicists Roger Penrose of the University of Oxford and Lajos Diósi of Eötvös Loránd University in Budapest independently proposed a possible cause of the collapse: gravity. Loosely speaking, their idea was that if a quantum object is in a superposition of locations, each position state will “feel” the others via their gravitational interaction. It is as if this attraction causes the object to measure itself, forcing a collapse. Or if you look at it from the perspective of general relativity, which describes gravity, a superposition of localities deforms the fabric of space-time in two different ways at once, a circumstance that general relativity can’t accommodate. As Penrose has put it, in a standoff between quantum mechanics and general relativity, quantum will crack first.

The Test of Truth

These ideas have always been highly speculative. But, in contrast to explanations of quantum mechanics like the Copenhagen and Everett interpretations, physical-collapse models have the virtue of making observable predictions — and thus being testable and falsifiable.

If there is indeed a background perturbation that provokes quantum collapse — whether it comes from gravitational effects or something else — then all particles will be continuously interacting with this perturbation, whether they are in a superposition or not. The consequences should in principle be detectable. The interaction should create a “permanent zigzagging of particles in space” comparable to Brownian motion, said Catalina Curceanu, a physicist at INFN.

Current physical-collapse models suggest that this diffusive motion is only very slight. Nonetheless, if the particle is electrically charged, the motion will generate electromagnetic radiation in a process called bremsstrahlung. A lump of matter should thus continuously emit a very faint stream of photons, which typical versions of the models predict to be in the X-ray range. Donadi and his colleague Angelo Bassi have shown that emission of such radiation is expected from any model of dynamical spontaneous collapse, including the Diósi-Penrose model.

Yet “while the idea is simple, in practice the test is not so easy,” said Kim. The predicted signal is extremely weak, which means that an experiment must involve an enormous number of charged particles to get a detectable signal. And the background noise — which comes from sources such as cosmic rays and radiation in the environment — must be kept low. Those conditions can only be satisfied by the most extremely sensitive experiments, such as those designed to detect dark matter signals or the elusive particles called neutrinos.

In 1996, Qijia Fu of Hamilton College in New York —  then just an undergraduate — proposed using germanium-based neutrino experiments to detect a CSL signature of X-ray emission. (Weeks after he submitted his paper, he was struck by lightning on a hiking trip in Utah and killed.) The idea was that the protons and electrons in germanium should emit the spontaneous radiation, which ultrasensitive detectors would pick up. Yet only recently have instruments with the required sensitivity come online.

In 2020, a team in Italy, including Donadi, Bassi and Curceanu, along with Diósi in Hungary, used a germanium detector of this sort to test the Diósi-Penrose model. The detectors, created for a neutrino experiment called IGEX, are shielded from radiation by virtue of their location underneath Gran Sasso, a mountain in the Apennine range of Italy.

Catalina Curceanu, seen here at a TedX event in Rome in 2016, used particle-physics detectors to put stringent limits on collapse theories.

After carefully subtracting the remaining background signal — mostly natural radioactivity from the rock — the physicists saw no emission at a sensitivity level that ruled out the simplest form of the Diósi-Penrose model. They also placed strong bounds on the parameters within which various CSL models might still work. The original GRW model lies just within this tight window: It survived by a whisker.

In a paper published this August , the 2020 result was confirmed and strengthened by an experiment called the Majorana Demonstrator, which was established primarily to search for hypothetical particles called Majorana neutrinos (which have the curious property of being their own antiparticles). The experiment is housed in the Sanford Underground Research Facility, which lies almost 5,000 feet underground in a former gold mine in South Dakota. It has a larger array of high-purity germanium detectors than IGEX, and they can detect X-rays down to low energies. “Our limit is much more stringent compared to the previous work,” said Kim, a member of the team.

A Messy End

Although physical-collapse models are badly ailing, they’re not quite dead. “The various models make very different assumptions about the nature and the properties of the collapse,” said Kim. Experimental tests have now excluded most plausible possibilities for these values, but there’s still a small island of hope.

Continuous spontaneous localization models propose that the physical entity perturbing the wave function is some sort of “noise field,” which the current tests assume is white noise: uniform at all frequencies. That’s the simplest assumption. But it’s possible that the noise might be “colored,” for example by having some high-frequency cutoff. Curceanu said that testing these more complicated models will require measuring the emission spectrum at higher energies than has been possible so far.

The Gerda experiment features a large tank that will hold the germanium detectors inside a protective layer of liquid argon. The hall surrounding the tank will be filled with purified water.

Kai Freund/LNGS-INFN.

The Majorana Demonstrator experiment is now winding down, but the team is forming a new collaboration with an experiment called Gerda , based at Gran Sasso, to create another experiment probing neutrino mass. Called Legend , it will have more massive and thus more sensitive germanium detector arrays. “Legend may be able to push the limits on CSL models further,” said Kim. There are also proposals for testing these models in space-based experiments, which won’t suffer from noise produced by environmental vibrations.

Falsification is hard work, and rarely reaches a tidy end point. Even now, according to Curceanu, Roger Penrose — who was awarded the 2020 Nobel Prize in Physics for his work on general relativity — is working on a version of the Diósi-Penrose model in which there’s no spontaneous radiation at all.

All the same, some suspect that for this view of quantum mechanics, the writing is on the wall. “What we need to do is to rethink what are these models trying to achieve,” said Zych, “and see if the motivating problems may not have a better answer through a different approach.” While few would argue that the measurement problem is no longer an issue, we have also learned much, in the years since the first collapse models were proposed, about what quantum measurement entails. “I think we need to go back to the question of what these models were created for decades ago,” she said, “and take seriously what we have learned in the meantime.”

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