experiments for entanglement

First Experimental Proof That Quantum Entanglement Is Real

A Q&A with Caltech alumnus John Clauser on his first experimental proof of quantum entanglement.

When scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement in the 1930s, they were perplexed. Disturbingly, entanglement required two separated particles to remain connected without being in direct contact. In fact, Einstein famously called entanglement “spooky action at a distance,” because the particles seemed to be communicating faster than the speed of light.

Born on December 1, 1942, John Francis Clauser is an American theoretical and experimental physicist known for contributions to the foundations of quantum mechanics, in particular the Clauser–Horne–Shimony–Holt inequality. Clauser was awarded the 2022 Nobel Prize in Physics, jointly with Alain Aspect and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.”

To explain the bizarre implications of entanglement, Einstein, along with Boris Podolsky and Nathan Rosen (EPR), argued that “hidden variables” should be added to quantum mechanics. These could be used to explain entanglement, and to restore “locality” and “causality” to the behavior of the particles. Locality states that objects are only influenced by their immediate surroundings. Causality states that an effect cannot occur before its cause, and that causal signaling cannot propagate faster than light speed. Niels Bohr famously disputed EPR’s argument, while Schrödinger and Wendell Furry, in response to EPR, independently hypothesized that entanglement vanishes with wide-particle separation.

Unfortunately, at the time, no experimental evidence for or against quantum entanglement of widely separated particles was available. Experiments have since proven that entanglement is very real and fundamental to nature. Furthermore, quantum mechanics has now been proven to work, not only at very short distances but also at very great distances. Indeed, China’s quantum-encrypted communications satellite, Micius, (part of the Quantum Experiments at Space Scale (QUESS) research project) relies on quantum entanglement between photons that are separated by thousands of kilometers.

John Clauser Second Quantum Entanglement Experiment

The very first of these experiments was proposed and executed by Caltech alumnus John Clauser (BS ’64) in 1969 and 1972, respectively. His findings are based on Bell’s theorem, devised by CERN theorist John Bell. In 1964, Bell ironically proved that EPR’s argument actually led to the opposite conclusion from what EPR had originally intended to show. Bell demonstrated that quantum entanglement is, in fact, incompatible with EPR’s notion of locality and causality.

In 1969 , while still a graduate student at Columbia University , Clauser, along with Michael Horne, Abner Shimony, and Richard Holt, transformed Bell’s 1964 mathematical theorem into a very specific experimental prediction via what is now called the Clauser–Horne–Shimony–Holt (CHSH) inequality ( Their paper has been cited more than 8,500 times on Google Scholar .) In 1972, when he was a postdoctoral researcher at the University of California Berkeley and Lawrence Berkeley National Laboratory , Clauser and graduate student Stuart Freedman were the first to prove experimentally that two widely separated particles (about 10 feet apart) can be entangled.

Clauser went on to perform three more experiments testing the foundations of quantum mechanics and entanglement, with each new experiment confirming and extending his results. The Freedman–Clauser experiment was the first test of the CHSH inequality. It has now been tested experimentally hundreds of times at laboratories around the world to confirm that quantum entanglement is real.

Clauser’s work earned him the 2010 Wolf Prize in physics. He shared it with Alain Aspect of the Institut d’ Optique and Ecole Polytechnique and Anton Zeilinger of the University of Vienna and the Austrian Academy of Sciences “for an increasingly sophisticated series of tests of Bell’s inequalities, or extensions thereof, using entangled quantum states,” according to the award citation.

Here, John Clauser answers questions about his historical experiments.

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech’s Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing! My reception at UC Berkeley was lukewarm at best and was only possible through the kindness and tolerance of Professors Charlie Townes [PhD ’39, Nobel Laureate ’64] and Howard Shugart [BS ’53], who allowed me to continue my experiments there.

In my correspondence with John Bell , he expressed exactly the opposite sentiment and strongly encouraged me to do an experiment. John Bell’s 1964 seminal work on Bell’s theorem was originally published in the terminal issue of an obscure journal, Physics , and in an underground physics newspaper, Epistemological Letters . It was not until after the 1969 CHSH paper and the 1972 Freedman–Clauser results were published in the Physical Review Letters that John Bell finally openly discussed his work. He was aware of the taboo on questioning quantum mechanics’ foundations and had never discussed it with his CERN co-workers.

What made you want to carry through with the experiments anyway?

Part of the reason that I wanted to test the ideas was because I was still trying to understand them. I found the predictions for entanglement to be sufficiently bizarre that I could not accept them without seeing experimental proof. I also recognized the fundamental importance of the experiments and simply ignored the career advice of my faculty. Moreover, I was having a lot of fun doing some very challenging experimental physics with apparatuses that I built mostly using leftover physics department scrap. Before Stu Freedman and I did the first experiment, I also personally thought that Einstein’s hidden-variable physics might actually be right, and if it is, then I wanted to discover it. I found Einstein’s ideas to be very clear. I found Bohr’s rather muddy and difficult to understand.

What did you expect to find when you did the experiments?

In truth, I really didn’t know what to expect except that I would finally determine who was right—Bohr or Einstein. I admittedly was betting in favor of Einstein but did not actually know who was going to win. It’s like going to the racetrack. You might hope that a certain horse will win, but you don’t really know until the results are in. In this case, it turned out that Einstein was wrong. In the tradition of Caltech’s Richard Feynman and Kip Thorne [BS ’62], who would place scientific bets, I had a bet with quantum physicist Yakir Aharonov on the outcome of the Freedman–Clauser experiment. Curiously, he put up only one dollar to my two. I lost the bet and enclosed a two-dollar bill and congratulations when I mailed him a preprint with our results.

I was very sad to see that my own experiment had proven Einstein wrong. But the experiment gave a 6.3-sigma result against him [a five-sigma result or higher is considered the gold standard for significance in physics]. But then Dick Holt and Frank Pipkin’s competing experiment at Harvard (never published) got the opposite result. I wondered if perhaps I had overlooked some important detail. I went on alone at UC Berkeley to perform three more experimental tests of quantum mechanics. All yielded the same conclusions. Bohr was right, and Einstein was wrong. The Harvard result did not repeat and was faulty. When I reconnected with my Columbia faculty, they all said, “We told you so! Now stop wasting money and go do some real physics.” At that point in my career, the only value in my work was that it demonstrated that I was a reasonably talented experimental physicist. That fact alone got me a job at Lawrence Livermore National Lab doing controlled-fusion plasma physics research.

Can you help us understand exactly what your experiments showed?

In order to clarify what the experiments showed, Mike Horne and I formulated what is now known as Clauser–Horne Local Realism [ 1974 ]. Additional contributions to it were subsequently offered by John Bell and Abner Shimony , so perhaps it is more properly called Bell–Clauser–Horne–Shimony Local Realism . Local Realism was very short-lived as a viable theory. Indeed, it was experimentally refuted even before it was fully formulated. Nonetheless, Local Realism is heuristically important because it shows in detail what quantum mechanics is not .

Local Realism assumes that nature consists of stuff, of objectively real objects, i.e., stuff you can put inside a box. (A box here is an imaginary closed surface defining separated inside and outside volumes.) It further assumes that objects exist whether or not we observe them. Similarly, definite experimental results are assumed to obtain, whether or not we look at them. We may not know what the stuff is, but we assume that it exists and that it is distributed throughout space. Stuff may evolve either deterministically or stochastically. Local Realism assumes that the stuff within a box has intrinsic properties, and that when someone performs an experiment within the box, the probability of any result that obtains is somehow influenced by the properties of the stuff within that box. If one performs say a different experiment with different experimental parameters, then presumably a different result obtains. Now suppose one has two widely separated boxes, each containing stuff. Local Realism further assumes that the experimental parameter choice made in one box cannot affect the experimental outcome in the distant box. Local Realism thereby prohibits spooky action-at-a-distance. It enforces Einstein’s causality that prohibits any such nonlocal cause and effect. Surprisingly, those simple and very reasonable assumptions are sufficient on their own to allow derivation of a second important experimental prediction limiting the correlation between experimental results obtained in the separated boxes. That prediction is the 1974 Clauser–Horne (CH) inequality.

The 1969 CHSH inequality’s derivation had required several minor supplementary assumptions, sometimes called “loopholes.” The CH inequality’s derivation eliminates those supplementary assumptions and is thus more general. Quantum entangled systems exist that disagree with the CH prediction, whereby Local Realism is amenable to experimental disproof. The CHSH and CH inequalities are both violated, not only by the first 1972 Freedman–Clauser experiment and my second 1976 experiment but now by literally hundreds of confirming independent experiments. Various labs have now entangled and violated the CHSH inequality with photon pairs, beryllium ion pairs, ytterbium ion pairs, rubidium atom pairs, whole rubidium-atom cloud pairs, nitrogen vacancies in diamonds, and Josephson phase qubits.

Testing Local Realism and the CH inequality was considered by many researchers to be important to eliminate the CHSH loopholes. Considerable effort was thus marshaled, as quantum optics technology improved and permitted. Testing the CH inequality had become a holy grail challenge for experimentalists. Violation of the CH inequality was finally achieved first in 2013 and again in 2015 at two competing laboratories: Anton Zeilinger’s group at the University of Vienna, and Paul Kwiat’s group at the University of Illinois at Urbana–Champaign. The 2015 experiments involved 56 researchers! Local Realism is now soundly refuted! The agreement between the experiments and quantum mechanics now firmly proves that nonlocal quantum entanglement is real.

What are some of the important technological applications of your work?

One application of my work is to the simplest possible object defined by Local Realism—a single bit of information. Local Realism shows that a single quantum mechanical bit of information, a “qubit,” cannot always be localized in a space-time box. This fact provides the fundamental basis of quantum information theory and quantum cryptography. Caltech’s quantum science and technology program, the 2019 $1.28-billion U.S. National Quantum Initiative, and the 2019 $400 million Israeli National Quantum Initiative all rely on the reality of entanglement. The Chinese Micius quantum-encrypted communications satellite system’s configuration is almost identical to that of the Freedman–Clauser experiment. It uses the CHSH inequality to verify entanglement’s persistence through outer space.

Can you tell us more about your family’s strong connection with Caltech?

My dad, Francis H. Clauser [BS ’34, MS ’35, PhD ’37, Distinguished Alumni Award ’66] and his brother Milton U. Clauser [BS ’34, MS ’35, PhD ’37] were PhD students at Caltech under Theodore von Kármán . Francis Clauser was Clark Blanchard Millikan Professor of Engineering at Caltech (Distinguished Faculty Award ’80) and chair of Caltech’s Division of Engineering and Applied Science. Milton U. Clauser’s son, Milton J. Clauser [PhD ’66], and grandson, Karl Clauser [BS ’86] both went to Caltech. My mom, Catharine McMillan Clauser was Caltech’s humanities librarian, where she met my dad. Her brother, Edwin McMillan [BS ’28, MS ’29], is a Caltech alum and ’51 Nobel Laureate. The family now maintains Caltech’s “Milton and Francis Doctoral Prize” awarded at Caltech commencements.

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The interactions and balances of topological vortex fields cover all short-distance and long-distance contributions, and are the basis of the formation and evolution of cosmic matter. 1.According to the topological vortex field theory, not only light, almost all rays and particles have electric effects. 2.The nature of electricity is perfect fluid.It has no shear stresses,viscosity,or heat conduction.Electric current generates heat because it interacts with vortex current. 3.Entanglement is one of the forms of interaction between vortexes. 4.If you are interested, please see https://zhuanlan.zhihu.com/p/463666584 . Good luck to your team.

The physical characteristics of the fluid vortex center are suitable to be described by energy rather than mass. According to the topological vortex field theory, there are two types of vortex centers: one is constant temperature and the other is variable temperature.

The expansion of space is due to star dynamics or relativity is a part of this.So,entanglement is a natural property present uniformly in universe for quantum particle after adding,saý GR(but not essential beside an arbitrary rational standard taken for measurement in the experiment);hence speed of light has no relation for entanglment of quantum particles at any two points in the same galaxy,or else.These facts have well proven before by many experimentations and are in due course of appĺied field,thus nothing to state about noble prize of current year in physics,but taken as secoded phenomena of the principle of entanglement.Congratulations to the physicists for their good presentations and wise works,granted late.

Fascinating!!!

Quantum Entanglement is perhaps the strangest phenomenon in physics, when some small particles may communicate with each other instantly and over vast distances. How can that happen without violating the maximum speed in the universe, the speed of light?

As this article says, the two ways physicists have used to answer this, that the particles contain hidden variables of unknown natures and that the universe is completely deterministic with all results predefined, have been shown to be incorrect.

Perhaps concepts in String Theory can help. There are 11 possible dimensions in String Theory and I suggest one of them leads a way around, what Einstein called this “spooky action at a distance”. Specifics on this can be found by searching YouTube for “Quantum Entanglement – A String Theory Way”

Bùt credìt of research on particle physics goes for quark-gluon to the America,charm quark to the CERN particĺe physicists.Thus,spin or magnetism required for entanglement has been done in parallel is an established work.

GR in connection to star dynamics is well proven concept taken in all kinds of measurements.

Metaphysics in Quantum Computation field is usual natural part has also been proven and established by experimentation.

All these are distinct works present ofcourse in fractional forms,but commonly adopted jointly in Quantum Computation.

So,alĺ these discoveries with their applications express happiness on behalf of this year’s Physics Nobel Prize with gratefulness to community and all with thanks.

It’s so fascinating contemplating the theme and variations of line of sight communication being moot through the newer mechanical developments

Energy can not be created or destroyed. Our thoughts are forms of energy. And scripture says “as a man think so is he”. Negative thoughts create depression, lack and poverty. Positive thoughts create abundance, wealth and prosperity.

FTL effects and hidden variable are not clearly ruled out and failure of localism could arise from FTL effects, it seems. “Non-local” with “hidden variables” still point to invisible FTL gravity effects, I believe.

Just to clarify, I wanted to note that it still seems “non-localism” and “hidden variables” can fit FTL gravity effects.

Refraction of fire and chief fields to contain high density gravity using quantum Magnetic codings will intensify the field of gravity to project

Quantum energy and its distant entanglement might be a breakthrough for holistic medical science. So therefore mysteries of working of homeopathic remedies on living organisms including humans could be explained and placebo effects of homeopathic remedies can further be explored. Diversity of conventional medical treatment can be boxed into single holistic approach. Thumbs up to marvelous discovery.

I have found a name for what goes on in my mind

ha ha ha… It’s the bizarre world where those embarrassments attempt to qualify as an authority by making word salad… to use their deleterious language once reserved for those sacrifices for the greater good, fire pits, abattoirs, and bomb vests. China still kills them, an economic champion, at what sacrifice? But you may be talking of mirror neurons, that not critical part of physical motion that allows instant … ok. entanglement for such as line dancers, but don’t confuse that with your critical thinking. Remember mirror neurons don’t really care, it’s a temporary allowing of one’s trust to be like another, not forebrain activity.

There is, of course, an information ‘matrix’ associated with the isotropic energy substrate underlying all measurable phenomena. ‘Particles’, therefore, isuue from this substrate and have, ipso facto, access to the information at any point of manifestation. It seems to me. So, no problem really with ‘Spooky Action’.

‘Particles’ isuue from this substrate put this notion well. The interactions and balances of topological vortex fields cover all short-distance and long-distance contributions, wich are the substrate of the formation and evolution of cosmic matter.

There are many here who are eminently more qualified than myself but it seems “apparent” that particles simultaneously exist in a different dimension and in that different dimension are essentially quite local.

you guys are just figuring this whole thing out now, this whole thing had been figured out a long time ago by ancient spiritualism, probably over 10 000 years ago. ancient spirituality had been trying to tell humanity that there is another dimension( “invisible reality”) which is the source of all things happening in this universe and outside the universe, they call it “the all”, some spiritual traditions call it the infinite consciousness, non-duality, the timeless dimension, the formless dimension and more. What’s happening in this universe of relativity is ultimately an illusion because people perceive reality as separate entities and the dimension I’m talking about is beyond forms, time, and space which all the dualistic categories of this universe and mental principles ceases to exist and what left is pure energy, the existence of this present moment(now).thats what science is trying to figure out and spirituality had already figure this whole thing out very long time ago. if someone wants to figure out what’s going on in quantum entanglement, I highly recommend you to access spirituality and non-dual teachings. it is not surprising that science is shocked about this because this whole had been figured out a long time ago, it’s just that science is catching up with spirituality. whatever is happening in the phenomenon of quantum entanglement that seems spooky is governed by that invisible reality called infinite conscioussness, which you cannot understand conceptually but realize as the oness.

Pure drivel, to start with Einstien who this fraudulent author misquotes, said quantum entanglement DOESN’T occur and there was no spooky action at a distance… completely misquoting others and besmirching their names by such slanders is common among such complete frauds as By CALIFORNIA INSTITUTE OF TECHNOLOGY or Not?

Just saying, anyone else ever seen a supposedly academic publication without a long list of authors, and co-authors all wanting credit for the publication as well as a long list of citations? No? Also the long list of word salad comments, anything false spawns false, proof of contraction is abundant, no such thing as quantum entanglement.

i suspect the quantum entanglement experiments are flawed but I have not found the details of these experiments. My skepticism arises from theories surrounding the origin of the universe. Black holes are gravitationally sorted spheres with the densest particles in the center. In order to have a big bang black holes (remnants of adjacent universes would have to collide. The resulting explosion propelled particles into space while preserving some of the more dense particles from the core which formed the early galaxies. The bulk of the mass shot into space the gavitational force decreasing with greater volume and distance from the center dense particles resulting in acceleration. No dark matter required. I am also skeptical of the atomic clock experiment which showed time slows with speed. all the experiment shows is that atomic radius is not constant. As an atom approaches the dense matter from the big bang at the center of the earth its radius deceases. I also suspect the current pole rotation we are in is tied to pre big bang dense matter at the center of the earth, and does not involve liquid iron suddenly changing direction. If quantum entanglement is real the experimental proceedures should be published and available to the layman.

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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.

"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

Atlas probes higgs interaction with the heavi..., atlas probes uncharted territory with lhc run..., accelerator report: no summer holidays for th..., also on physics, alice probes the strong interaction three-bod..., na62 experiment at cern observes ultra-rare p..., cms experiment at cern weighs in on the w bos..., cosmic count exceeds expectation, alice does the double-slit, na62 announces its first search for long-live..., amber releases its first results.

Proving that Quantum Entanglement is Real

A Q&A with Caltech alumnus John Clauser on his first experimental proof of quantum entanglement

In the 1930s when scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement, they were perplexed. Entanglement, disturbingly, required two separated particles to remain connected without being in direct contact. Einstein famously called entanglement "spooky action at a distance," since the particles seemed to be communicating faster than the speed of light.

To explain the bizarre implications of entanglement, Einstein, along with Boris Podolsky and Nathan Rosen (EPR), argued that "hidden variables" should be added to quantum mechanics to explain entanglement, and to restore "locality" and "causality" to the behavior of the particles. Locality states that objects are only influenced by their immediate surroundings. Causality states that an effect cannot occur before its cause, and that causal signaling cannot propagate faster than light-speed. Niels Bohr famously disputed EPR's argument, while Schrödinger and Wendell Furry, in response to EPR, independently hypothesized that entanglement vanishes with wide-particle separation.

Unfortunately, no experimental evidence for or against quantum entanglement of widely separated particles was available then. Experiments have since proven that entanglement is very real and fundamental to nature. Moreover, quantum mechanics has now been proven to work, not only at very short distances but also at very great distances. Indeed, China's quantum-encrypted communications satellite, Micius, relies on quantum entanglement between photons that are separated by thousands of kilometers.

The very first of these experiments was proposed and executed by Caltech alumnus John Clauser (BS '64) in 1969 and 1972, respectively. His findings are based on Bell's theorem, devised by CERN theorist John Bell. In 1964, Bell ironically proved that EPR's argument actually led to the opposite conclusion from what EPR had originally intended to show. Bell showed that quantum entanglement is, in fact, incompatible with EPR's notion of locality and causality.

In 1969 , while still a graduate student at Columbia University, Clauser, along with Michael Horne, Abner Shimony, and Richard Holt, transformed Bell's 1964 mathematical theorem into a very specific experimental prediction via what is now called the Clauser–Horne–Shimony–Holt (CHSH) inequality ( Their paper has been cited more than 8,500 times on Google Scholar .) In 1972, when he was a postdoctoral researcher at UC Berkeley and Lawrence Berkeley National Laboratory, Clauser and graduate student Stuart Freedman were the first to prove experimentally that two widely separated particles (about 10 feet apart) can be entangled. Clauser went on to perform three more experiments testing the foundations of quantum mechanics and entanglement, with each new experiment confirming and extending his results. The Freedman–Clauser experiment was the first test of the CHSH inequality. It has now been tested experimentally hundreds of times at laboratories around the world to confirm that quantum entanglement is real.

Clauser's work earned him the 2010 Wolf Prize in physics. He shared it with Alain Aspect of the Institut d' Optique and Ecole Polytechnique and Anton Zeilinger of the University of Vienna and the Austrian Academy of Sciences "for an increasingly sophisticated series of tests of Bell's inequalities, or extensions thereof, using entangled quantum states," according to the award citation.

Here, John Clauser answers questions about his historical experiments.

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech's Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing! My reception at UC Berkeley was lukewarm at best and was only possible through the kindness and tolerance of Professors Charlie Townes [PhD '39, Nobel Laureate '64] and Howard Shugart [BS '53], who allowed me to continue my experiments there.

In my correspondence with John Bell , he expressed exactly the opposite sentiment and strongly encouraged me to do an experiment. John Bell's 1964 seminal work on Bell's theorem was originally published in the terminal issue of an obscure journal, Physics , and in an underground physics newspaper, Epistemological Letters . It was not until after the 1969 CHSH paper and the 1972 Freedman–Clauser results were published in the Physical Review Letters that John Bell finally openly discussed his work. He was aware of the taboo on questioning quantum mechanics' foundations and had never discussed it with his CERN co-workers.

What made you want to carry through with the experiments anyway?

Part of the reason that I wanted to test the ideas was because I was still trying to understand them. I found the predictions for entanglement to be sufficiently bizarre that I could not accept them without seeing experimental proof. I also recognized the fundamental importance of the experiments and simply ignored the career advice of my faculty. Moreover, I was having a lot of fun doing some very challenging experimental physics with apparatuses that I built mostly using leftover physics department scrap. Before Stu Freedman and I did the first experiment, I also personally thought that Einstein's hidden-variable physics might actually be right, and if it is, then I wanted to discover it. I found Einstein's ideas to be very clear. I found Bohr's rather muddy and difficult to understand.

What did you expect to find when you did the experiments?

In truth, I really didn't know what to expect except that I would finally determine who was right—Bohr or Einstein. I admittedly was betting in favor of Einstein but did not actually know who was going to win. It's like going to the racetrack. You might hope that a certain horse will win, but you don't really know until the results are in. In this case, it turned out that Einstein was wrong. In the tradition of Caltech's Richard Feynman and Kip Thorne [BS '62], who would place scientific bets, I had a bet with quantum physicist Yakir Aharonov on the outcome of the Freedman–Clauser experiment. Curiously, he put up only one dollar to my two. I lost the bet and enclosed a two-dollar bill and congratulations when I mailed him a preprint with our results.

I was very sad to see that my own experiment had proven Einstein wrong. But the experiment gave a 6.3-sigma result against him [a five-sigma result or higher is considered the gold standard for significance in physics]. But then Dick Holt and Frank Pipkin's competing experiment at Harvard (never published) got the opposite result. I wondered if perhaps I had overlooked some important detail. I went on alone at UC Berkeley to perform three more experimental tests of quantum mechanics. All yielded the same conclusions. Bohr was right, and Einstein was wrong. The Harvard result did not repeat and was faulty. When I reconnected with my Columbia faculty, they all said, "We told you so! Now stop wasting money and go do some real physics." At that point in my career, the only value in my work was that it demonstrated that I was a reasonably talented experimental physicist. That fact alone got me a job at Lawrence Livermore National Lab doing controlled-fusion plasma physics research.

Can you help us understand exactly what your experiments showed?

Local Realism assumes that nature consists of stuff, of objectively real objects, i. e., stuff you can put inside a box. (A box here is an imaginary closed surface defining separated inside and outside volumes.) It further assumes that objects exist whether or not we observe them. Similarly, definite experimental results are assumed to obtain, whether or not we look at them. We may not know what the stuff is, but we assume that it exists and that it is distributed throughout space. Stuff may evolve either deterministically or stochastically. Local Realism assumes that the stuff within a box has intrinsic properties, and that when someone performs an experiment within the box, the probability of any result that obtains is somehow influenced by the properties of the stuff within that box. If one performs say a different experiment with different experimental parameters, then presumably a different result obtains. Now suppose one has two widely separated boxes, each containing stuff. Local Realism further assumes that the experimental parameter choice made in one box cannot affect the experimental outcome in the distant box. Local Realism thereby prohibits spooky action-at-a-distance. It enforces Einstein's causality that prohibits any such nonlocal cause and effect. Surprisingly, those simple and very reasonable assumptions are sufficient on their own to allow derivation of a second important experimental prediction limiting the correlation between experimental results obtained in the separated boxes. That prediction is the 1974 Clauser–Horne (CH) inequality.

The 1969 CHSH inequality's derivation had required several minor supplementary assumptions, sometimes called "loopholes." The CH inequality's derivation eliminates those supplementary assumptions and is thus more general. Quantum entangled systems exist that disagree with the CH prediction, whereby Local Realism is amenable to experimental disproof. The CHSH and CH inequalities are both violated, not only by the first 1972 Freedman–Clauser experiment and my second 1976 experiment but now by literally hundreds of confirming independent experiments. Various labs have now entangled and violated the CHSH inequality with photon pairs, beryllium ion pairs, ytterbium ion pairs, rubidium atom pairs, whole rubidium-atom cloud pairs, nitrogen vacancies in diamonds, and Josephson phase qubits.

Testing Local Realism and the CH inequality was considered by many researchers to be important to eliminate the CHSH loopholes. Considerable effort was thus marshaled, as quantum optics technology improved and permitted. Testing the CH inequality had become a holy grail challenge for experimentalists. Violation of the CH inequality was finally achieved first in 2013 and again in 2015 at two competing laboratories: Anton Zeilinger's group at the University of Vienna, and Paul Kwiat's group at the University of Illinois at Urbana–Champaign. The 2015 experiments involved 56 researchers! Local Realism is now soundly refuted! The agreement between the experiments and quantum mechanics now firmly proves that nonlocal quantum entanglement is real.

What are some of the important technological applications of your work?

One application of my work is to the simplest possible object defined by Local Realism—a single bit of information. Local Realism shows that a single quantum mechanical bit of information, a "qubit," cannot always be localized in a space-time box. This fact provides the fundamental basis of quantum information theory and quantum cryptography. Caltech's quantum science and technology program, the 2019 $1.28-billion U.S. National Quantum Initiative, and the 2019 $400 million Israeli National Quantum Initiative all rely on the reality of entanglement. The Chinese Micius quantum-encrypted communications satellite system's configuration is almost identical to that of the Freedman–Clauser experiment. It uses the CHSH inequality to verify entanglement's persistence through outer space.

Can you tell us more about your family's strong connection with Caltech?

My dad, Francis H. Clauser [BS '34, MS '35, PhD '37, Distinguished Alumni Award '66] and his brother Milton U. Clauser [BS '34, MS '35, PhD '37] were PhD students at Caltech under Theodore von Kármán . Francis Clauser was Clark Blanchard Millikan Professor of Engineering at Caltech (Distinguished Faculty Award '80) and chair of Caltech's Division of Engineering and Applied Science. Milton U. Clauser's son, Milton J. Clauser [PhD '66], and grandson, Karl Clauser [BS '86] both went to Caltech. My mom, Catharine McMillan Clauser was Caltech's humanities librarian, where she met my dad. Her brother, Edwin McMillan [BS '28, MS '29], is a Caltech alum and '51 Nobel Laureate. The family now maintains Caltech's "Milton and Francis Doctoral Prize" awarded at Caltech commencements.

John Clauser in 1976 standing with his second quantum entanglement experiment at UC Berkeley

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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

illustration of a two entangled particles

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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“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.

black and white image of John Clauser at work in a lab

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.

Alain Aspect points to an equation on a projector screen

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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Light from ancient quasars helps confirm quantum entanglement

Results are among the strongest evidence yet for “spooky action at a distance.”.

Last year, physicists at MIT, the University of Vienna, and elsewhere provided strong support for quantum entanglement, the seemingly far-out idea that two particles, no matter how distant from each other in space and time, can be inextricably linked, in a way that defies the rules of classical physics.

Take, for instance, two particles sitting on opposite edges of the universe. If they are truly entangled, then according to the theory of quantum mechanics their physical properties should be related in such a way that any measurement made on one particle should instantly convey information about any future measurement outcome of the other particle — correlations that Einstein skeptically saw as “spooky action at a distance.”

In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.

But what if such correlations were the result not of quantum entanglement, but of some other hidden, classical explanation? Such “what-ifs” are known to physicists as loopholes to tests of Bell’s inequality, the most stubborn of which is the “freedom-of-choice” loophole: the possibility that some hidden, classical variable may influence the measurement that an experimenter chooses to perform on an entangled particle, making the outcome look quantumly correlated when in fact it isn’t.

Last February, the MIT team and their colleagues significantly constrained the freedom-of-choice loophole, by using 600-year-old starlight to decide what properties of two entangled photons to measure. Their experiment proved that, if a classical mechanism caused the correlations they observed, it would have to have been set in motion more than 600 years ago, before the stars’ light was first emitted and long before the actual experiment was even conceived.

Now, in a paper published today in Physical Review Letters , the same team has vastly extended the case for quantum entanglement and further restricted the options for the freedom-of-choice loophole. The researchers used distant quasars, one of which emitted its light 7.8 billion years ago and the other 12.2 billion years ago, to determine the measurements to be made on pairs of entangled photons. They found correlations among more than 30,000 pairs of photons, to a degree that far exceeded the limit that Bell originally calculated for a classically based mechanism.

“If some conspiracy is happening to simulate quantum mechanics by a mechanism that is actually classical, that mechanism would have had to begin its operations — somehow knowing exactly when, where, and how this experiment was going to be done — at least 7.8 billion years ago. That seems incredibly implausible, so we have very strong evidence that quantum mechanics is the right explanation,” says co-author Alan Guth , the Victor F. Weisskopf Professor of Physics at MIT.

“The Earth is about 4.5 billion years old, so any alternative mechanism — different from quantum mechanics — that might have produced our results by exploiting this loophole would’ve had to be in place long before even there was a planet Earth, let alone an MIT,” adds David Kaiser , the Germeshausen Professor of the History of Science and professor of physics at MIT. “So we’ve pushed any alternative explanations back to very early in cosmic history.”

Guth and Kaiser’s co-authors include Anton Zeilinger and members of his group at the Austrian Academy of Sciences and the University of Vienna, as well as physicists at Harvey Mudd College and the University of California at San Diego.

A decision, made billions of years ago

In 2014, Kaiser and two members of the current team, Jason Gallicchio and Andrew Friedman, proposed an experiment to produce entangled photons on Earth — a process that is fairly standard in studies of quantum mechanics. They planned to shoot each member of the entangled pair in opposite directions, toward light detectors that would also make a measurement of each photon using a polarizer. Researchers would measure the polarization, or orientation, of each incoming photon’s electric field, by setting the polarizer at various angles and observing whether the photons passed through — an outcome for each photon that researchers could compare to determine whether the particles showed the hallmark correlations predicted by quantum mechanics.

The team added a unique step to the proposed experiment, which was to use light from ancient, distant astronomical sources, such as stars and quasars, to determine the angle at which to set each respective polarizer. As each entangled photon was in flight, heading toward its detector at the speed of light, researchers would use a telescope located at each detector site to measure the wavelength of a quasar’s incoming light. If that light was redder than some reference wavelength, the polarizer would tilt at a certain angle to make a specific measurement of the incoming entangled photon — a measurement choice that was determined by the quasar. If the quasar’s light was bluer than the reference wavelength, the polarizer would tilt at a different angle, performing a different measurement of the entangled photon.

In their previous experiment, the team used small backyard telescopes to measure the light from stars as close as 600 light years away. In their new study, the researchers used much larger, more powerful telescopes to catch the incoming light from even more ancient, distant astrophysical sources: quasars whose light has been traveling toward the Earth for at least 7.8 billion years — objects that are incredibly far away and yet are so luminous that their light can be observed from Earth.

Tricky timing

On Jan. 11, 2018, “the clock had just ticked past midnight local time,” as Kaiser recalls, when about a dozen members of the team gathered on a mountaintop in the Canary Islands and began collecting data from two large, 4-meter-wide telescopes: the William Herschel Telescope and the Telescopio Nazionale Galileo, both situated on the same mountain and separated by about a kilometer.

One telescope focused on a particular quasar, while the other telescope looked at another quasar in a different patch of the night sky. Meanwhile, researchers at a station located between the two telescopes created pairs of entangled photons and beamed particles from each pair in opposite directions toward each telescope.

In the fraction of a second before each entangled photon reached its detector, the instrumentation determined whether a single photon arriving from the quasar was more red or blue, a measurement that then automatically adjusted the angle of a polarizer that ultimately received and detected the incoming entangled photon.

“The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”

Demystifying a mirage

The researchers ran their experiment twice, each for around 15 minutes and with two different pairs of quasars. For each run, they measured 17,663 and 12,420 pairs of entangled photons, respectively. Within hours of closing the telescope domes and looking through preliminary data, the team could tell there were strong correlations among the photon pairs, beyond the limit that Bell calculated, indicating that the photons were correlated in a quantum-mechanical manner.

Guth led a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.

He calculated that, for the best of the two runs, the probability that a mechanism based on classical physics could have achieved the observed correlation was about 10 to the minus 20 — that is, about one part in one hundred billion billion, “outrageously small,” Guth says. For comparison, researchers have estimated the probability that the discovery of the Higgs boson was just a chance fluke to be about one in a billion.

“We certainly made it unbelievably implausible that a local realistic theory could be underlying the physics of the universe,” Guth says.

And yet, there is still a small opening for the freedom-of-choice loophole. To limit it even further, the team is entertaining ideas of looking even further back in time, to use sources such as cosmic microwave background photons that were emitted as leftover radiation immediately following the Big Bang, though such experiments would present a host of new technical challenges.

“It is fun to think about new types of experiments we can design in the future, but for now, we are very pleased that we were able to address this particular loophole so dramatically. Our experiment with quasars puts extremely tight constraints on various alternatives to quantum mechanics. As strange as quantum mechanics may seem, it continues to match every experimental test we can devise,” Kaiser says.

This research was supported in part by the Austrian Academy of Sciences, the Austrian Science Fund, the U.S. National Science Foundation, and the U.S. Department of Energy.

Paper: “Cosmic Bell Test Using Random Measurement Settings from High-Redshift Quasars”

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What is quantum entanglement? A physicist explains Einstein’s ‘spooky action at a distance’

The 2022 Nobel Prize in physics recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.

In the simplest terms, quantum entanglement means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is “spinning” in one direction or another.

The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, seemingly breaking a fundamental law of the universe . Albert Einstein famously called the phenomenon “spooky action at a distance.”

Having spent the better part of two decades conducting experiments rooted in quantum mechanics , I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel winners, Alain Aspect , John Clauser and Anton Zeilinger , physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.

However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons – who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.

Existing in multiple states at once

To truly understand the spookiness of quantum entanglement, it is important to first understand quantum superposition . Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.

For example, many particles have an attribute called spin that is measured either as “up” or “down” for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.

There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, but is itself unpredictable .

Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.

Two entangled particles

The spookiness of quantum entanglement emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.

To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero.

In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper that describes a thought experiment designed to illustrate a seeming absurdity of quantum entanglement that challenged a foundational law of the universe.

A simplified version of this thought experiment , attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.

This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin “collapse” into either up or down – instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?

Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property – dubbed hidden variables – that determined the state of a particle before measurement . But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.

Disproving a theory

It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.

Bell produced an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.

The experiments of the 2022 Nobel laureates, particularly those of Alain Aspect , were the first tests of the Bell inequality . The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and many follow-up experiments have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.

Importantly, there is also no conflict with special relativity, which forbids faster-than-light communication . The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles cannot use the phenomenon to pass along information faster than the speed of light.

Today, physicists continue to research quantum entanglement and investigate potential practical applications . Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.

Andreas Muller , Associate Professor of Physics, University of South Florida

This article is republished from The Conversation under a Creative Commons license. Read the original article .

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

18 September 2024 | By

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.

This press release was originally published on the CERN Press website ( English ).

About the banner image : Artistic visualisation of top-quark entanglement. The line between the particles emphasises the non-separability of the top-quark pair, which is produced by LHC collisions and recorded by ATLAS. (Image: Daniel Dominguez/CERN)

Observation of quantum entanglement with top quarks at the ATLAS detector , Nature , 18 September 2024
ATLAS achieves highest-energy detection of quantum entanglement , ATLAS Physics Briefing , 23 September 2023

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What is quantum entanglement? A physicist explains the science of Einstein’s ‘spooky action at a distance’

Associate Professor of Physics, University of South Florida

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The 2022 Nobel Prize in physics recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.

Existing in multiple states at once

Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.

Two entangled particles

The spookiness of quantum entanglement emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.

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Light from ancient quasars helps confirm quantum entanglement

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In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.

“If some conspiracy is happening to simulate quantum mechanics by a mechanism that is actually classical, that mechanism would have had to begin its operations — somehow knowing exactly when, where, and how this experiment was going to be done — at least 7.8 billion years ago. That seems incredibly implausible, so we have very strong evidence that quantum mechanics is the right explanation,” says co-author Alan Guth, the Victor F. Weisskopf Professor of Physics at MIT.

“The Earth is about 4.5 billion years old, so any alternative mechanism — different from quantum mechanics — that might have produced our results by exploiting this loophole would’ve had to be in place long before even there was a planet Earth, let alone an MIT,” adds David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “So we’ve pushed any alternative explanations back to very early in cosmic history.”

A decision, made billions of years ago

Tricky timing

“The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”

Demystifying a mirage

Guth led a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.

“We certainly made it unbelievably implausible that a local realistic theory could be underlying the physics of the universe,” Guth says.

This research was supported in part by the Austrian Academy of Sciences, the Austrian Science Fund, the U.S. National Science Foundation, and the U.S. Department of Energy.

Share this news article on:

Press mentions.

Gizmodo reporter Ryan Mandelbaum writes that by studying ancient quasars, MIT scientists have uncovered evidence supporting quantum entanglement, the concept that two particles can become linked despite their distance in space and time. “We’ve outsourced randomness to the furthest quarters of the universe, tens of billions of light years away,” says Prof. David Kaiser.

Motherboard

Writing for Motherboard , Daniel Oberhaus highlights how MIT researchers have used light emitted by quasars billions of years ago to confirm the existence of quantum entanglement. Oberhaus explains that the findings suggest entanglement occurs “because if it didn’t exist the universe would somehow have to have ‘known’ 7.8 billion years ago that these MIT scientists would perform these experiments in 2018.”

Space.com reporter Chelsea Gohd writes that MIT researchers have used the light emitted by two ancient quasars to provide evidence of quantum entanglement, the theory that two particles can become linked across space and time. The researchers used ancient quasars to see if, “the correlation between particles can be explained by classical mechanics stemming from earlier than 600 years ago.”

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Proving that Quantum Entanglement is Real

A Q&A with Caltech alumnus John Clauser on his first experimental proof of quantum entanglement

Here, John Clauser answers questions about his historical experiments.

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech's Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing! My reception at UC Berkeley was lukewarm at best and was only possible through the kindness and tolerance of Professors Charlie Townes [PhD '39, Nobel Laureate '64] and Howard Shugart [BS '53], who allowed me to continue my experiments there.

What made you want to carry through with the experiments anyway?

Part of the reason that I wanted to test the ideas was because I was still trying to understand them. I found the predictions for entanglement to be sufficiently bizarre that I could not accept them without seeing experimental proof. I also recognized the fundamental importance of the experiments and simply ignored the career advice of my faculty. Moreover, I was having a lot of fun doing some very challenging experimental physics with apparatuses that I built mostly using leftover physics department scrap. Before Stu Freedman and I did the first experiment, I also personally thought that Einstein's hidden-variable physics might actually be right, and if it is, then I wanted to discover it. I found Einstein's ideas to be very clear. I found Bohr's rather muddy and difficult to understand.

What did you expect to find when you did the experiments?

I was very sad to see that my own experiment had proven Einstein wrong. But the experiment gave a 6.3-sigma result against him [a five-sigma result or higher is considered the gold standard for significance in physics]. But then Dick Holt and Frank Pipkin's competing experiment at Harvard (never published) got the opposite result. I wondered if perhaps I had overlooked some important detail. I went on alone at UC Berkeley to perform three more experimental tests of quantum mechanics. All yielded the same conclusions. Bohr was right, and Einstein was wrong. The Harvard result did not repeat and was faulty. When I reconnected with my Columbia faculty, they all said, "We told you so! Now stop wasting money and go do some real physics." At that point in my career, the only value in my work was that it demonstrated that I was a reasonably talented experimental physicist. That fact alone got me a job at Lawrence Livermore National Lab doing controlled-fusion plasma physics research.

Can you help us understand exactly what your experiments showed?

Local Realism assumes that nature consists of stuff, of objectively real objects, i. e., stuff you can put inside a box. (A box here is an imaginary closed surface defining separated inside and outside volumes.) It further assumes that objects exist whether or not we observe them. Similarly, definite experimental results are assumed to obtain, whether or not we look at them. We may not know what the stuff is, but we assume that it exists and that it is distributed throughout space. Stuff may evolve either deterministically or stochastically. Local Realism assumes that the stuff within a box has intrinsic properties, and that when someone performs an experiment within the box, the probability of any result that obtains is somehow influenced by the properties of the stuff within that box. If one performs say a different experiment with different experimental parameters, then presumably a different result obtains. Now suppose one has two widely separated boxes, each containing stuff. Local Realism further assumes that the experimental parameter choice made in one box cannot affect the experimental outcome in the distant box. Local Realism thereby prohibits spooky action-at-a-distance. It enforces Einstein's causality that prohibits any such nonlocal cause and effect. Surprisingly, those simple and very reasonable assumptions are sufficient on their own to allow derivation of a second important experimental prediction limiting the correlation between experimental results obtained in the separated boxes. That prediction is the 1974 Clauser–Horne (CH) inequality.

Testing Local Realism and the CH inequality was considered by many researchers to be important to eliminate the CHSH loopholes. Considerable effort was thus marshaled, as quantum optics technology improved and permitted. Testing the CH inequality had become a holy grail challenge for experimentalists. Violation of the CH inequality was finally achieved first in 2013 and again in 2015 at two competing laboratories: Anton Zeilinger's group at the University of Vienna, and Paul Kwiat's group at the University of Illinois at Urbana–Champaign. The 2015 experiments involved 56 researchers! Local Realism is now soundly refuted! The agreement between the experiments and quantum mechanics now firmly proves that nonlocal quantum entanglement is real.

What are some of the important technological applications of your work?

One application of my work is to the simplest possible object defined by Local Realism—a single bit of information. Local Realism shows that a single quantum mechanical bit of information, a "qubit," cannot always be localized in a space-time box. This fact provides the fundamental basis of quantum information theory and quantum cryptography. Caltech's quantum science and technology program, the 2019 $1.28-billion U.S. National Quantum Initiative, and the 2019 $400 million Israeli National Quantum Initiative all rely on the reality of entanglement. The Chinese Micius quantum-encrypted communications satellite system's configuration is almost identical to that of the Freedman–Clauser experiment. It uses the CHSH inequality to verify entanglement's persistence through outer space.

Can you tell us more about your family's strong connection with Caltech?

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Explain it in 60 Seconds: Quantum entanglement

The CMS and ATLAS experiments recently detected quantum entanglement between top quarks in high-energy collisions at the LHC. What does that mean?

Quantum entanglement links subatomic particles in a way that appears to defy logic. When scientists measure one of a pair of entangled particles, the other seems to immediately know the result.

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At the Large Hadron Collider, scientists have been studying entangled pairs of fermions called top quarks.

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This strange phenomenon baffled Albert Einstein, who, along with Boris Podolski and Nathan Rosen, proposed in 1934 that quantum mechanics might be incomplete. He suggested that hidden variables must allow particles to communicate.

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Published: 16 June 2022

Learning interpretable representations of entanglement in quantum optics experiments using deep generative models

Daniel Flam-Shepherd ORCID: orcid.org/0000-0002-9568-3451 1 , 2 ,
Tony C. Wu 1 ,
Xuemei Gu 3 ,
Alba Cervera-Lierta ORCID: orcid.org/0000-0002-8835-2910 1 , 4 ,
Mario Krenn 1 , 2 , 4 , 5 &
Alán Aspuru-Guzik 1 , 2 , 4 , 6

Nature Machine Intelligence volume 4 , pages 544–554 ( 2022 ) Cite this article

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A preprint version of the article is available at arXiv.

Quantum physics experiments produce interesting phenomena such as interference or entanglement, which are the core properties of numerous future quantum technologies. The complex relationship between the setup structure of a quantum experiment and its entanglement properties is essential to fundamental research in quantum optics but is difficult to intuitively understand. We present a deep generative model of quantum optics experiments where a variational autoencoder is trained on a dataset of quantum optics experiment setups. In a series of computational experiments, we investigate the learned representation of our quantum optics variational autoencoder (QOVAE) and its internal understanding of the quantum optics world. We demonstrate that QOVAE learns an interpretable representation of quantum optics experiments and the relationship between the experiment structure and entanglement. We show QOVAE is able to generate novel experiments for highly entangled quantum states with specific distributions that match its training data. QOVAE can learn to generate specific entangled states and efficiently search the space of experiments that produce highly entangled quantum states. Importantly, we are able to interpret how QOVAE structures its latent space, finding curious patterns that we can explain in terms of quantum physics. The results demonstrate how we can use and understand the internal representations of deep generative models in a complex scientific domain. QOVAE and the insights from our investigations can be immediately applied to other physical systems.

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A framework for demonstrating practical quantum advantage: comparing quantum against classical generative models

Synergic quantum generative machine learning

Flexible learning of quantum states with generative query neural networks

Data availability.

The training data are available via GitHub at https://github.com/danielflamshep/qovae/blob/main/setups.smi (ref. 79 ).

Code availability

The code is available via GitHub at https://github.com/danielflamshep/qovae (ref. 79 ). The source code for the Melvin algorithm is available via GitHub at https://github.com/XuemeiGu/MelvinPython .

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Acknowledgements

A.A.-G. acknowledges support from the Canada 150 Research Chairs Program, the Canada Industrial Research Chair Program and from Google in the form of a Google Focused Award. M.K. acknowledges support from the FWF (Austrian Science Fund) via Erwin Schrödinger Fellowship no. J4309.

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Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, China

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Alba Cervera-Lierta, Mario Krenn & Alán Aspuru-Guzik

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D.F.-S. conceived the overall project, developed the approach and wrote the paper. D.F.-S. and T.W. designed and performed the investigations. A.C.-L. provided the technical advice. X.G. provided the technical advice and wrote the entanglement calculation code. M.K. built the dataset, provided technical advice and helped design the interpretability investigation and analysed the experiments. A.A.-G. led the project and provided the overall directions. All the authors participated in preparing the manuscript.

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Correspondence to Daniel Flam-Shepherd , Mario Krenn or Alán Aspuru-Guzik .

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Flam-Shepherd, D., Wu, T.C., Gu, X. et al. Learning interpretable representations of entanglement in quantum optics experiments using deep generative models. Nat Mach Intell 4 , 544–554 (2022). https://doi.org/10.1038/s42256-022-00493-5

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Positron Emission Tomography Could Be Aided by Entanglement

Center for Theranostics, Marian Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland

Medical scans using an imaging method called positron emission tomography (PET) are crucial for diagnosing diseases such as cancer and Alzheimer’s. In PET, electrons and positrons annihilate into pairs of photons inside a patient’s body, and the photons are detected and used to reconstruct images of body tissues. The photons in each pair are known to be quantum entangled in their polarization, and recent work has suggested that this entanglement could improve the quality of PET imaging. Such quantum-enhanced imaging could now be one step closer thanks to Julien Bordes and colleagues at the University of York, UK, who have observed that the photon entanglement is much more resilient than previously thought [ 1 ].

Currently, in PET, a patient receives an intravenous injection of biomolecules with attached radioactive atoms that emit a positron as they decay. The annihilation of such a positron with an electron in the patient’s body creates two photons that propagate in opposite directions, each with an energy of 511 keV—more than 100,000 times the energy of visible light. Such photons can penetrate through the patient’s body and produce signals in the PET detectors. These signals enable the determination of the distribution of electron–positron annihilations in the body and, in turn, the production of images showing how fast the administered biomolecules are metabolized in body tissues.

Over the past 70 years, PET has undergone continuous development, moving from blurred images obtained using only two detectors to dynamic, simultaneous full-body imaging obtained using state-of-the-art systems constructed from hundreds of thousands of crystal scintillators. However, the main principle of PET scanners has remained unchanged: They reconstruct the distribution of electron–positron annihilations by recording when and where the created photons interact with the detector system and how much energy they deposit.

A key challenge in PET imaging is filtering out events in which one or both photons from an annihilation scatter off an electron in the patient’s body before reaching the detectors (Fig. 1 ). Such filtering is crucial because these events account for about 90% of all detected photon pairs and cause PET images to be blurry. The scattered photons have less energy than the original photons, so some of the blurring events can be filtered out by rejecting photons with measured energies lower than 511 keV (by a margin greater than the detector’s energy resolution).

In 2014, scientists proposed that blurring events could be suppressed by carefully examining the difference in the polarization direction of two photons coming from the same annihilation [ 2 ]. This method works only if, after one photon scatters in the patient’s body, the photons do not remain entangled in their polarization and begin to propagate independently of each other [ 3 ]. Until a few years ago, this entanglement loss was widely thought to occur [ 4 ]. But in 2023, an experiment indicated that entanglement persisted after one photon from an entangled pair scattered [ 5 ]. This unexpected observation was confirmed by independent experiments for scattering angles up to 50°, with a hint of entanglement loss noted only at 50° [ 6 ].

Bordes and colleagues extended these studies to a scattering angle of 70°, which allowed them to observe the first clear evidence for entanglement loss at angles larger than 50°. Moreover, by considering the scattering of one of the two entangled photons as well as entanglement-loss effects, the team showed that the dependence of the degree of entanglement on the scattering angle is well described by a recently developed quantum theory [ 7 ]. This theory assumes the probability that a photon from an entangled pair scatters off an electron at a given angle depends not only on the photon’s momentum and polarization but also on the degree of entanglement between the photons.

The observation that the photons created in an electron–positron annihilation can remain entangled when one of them is scattered is a scientifically exciting discovery, but it is both bad and good news for medical diagnosis. It is bad news because it means that measuring the difference between photon polarizations cannot help in PET imaging by reducing the fraction of blurring events caused by scattering in the patient’s body. But it is potentially good news for the development of quantum-enhanced PET diagnosis because the possible diagnostic information about body tissues that is carried by entangled photons will not be lost if one photon scatters in the body.

The first full-scale PET system capable of entanglement-aided imaging has already been constructed using plastic scintillators [ 8 ]. Preliminary results with this system, known as J-PET, have demonstrated a dependence of the degree of entanglement on the type of material in which the electron–positron annihilations occur—a promising indication of the potential use of entanglement in PET diagnosis. Additionally, several groups are working on the development of technology for conventional, crystal-scintillator-based PET systems capable of entanglement-aided imaging [ 1 , 4 , 9 , 10 ].

In this decade, the advancement of PET is undergoing a paradigm shift toward completely new diagnostic parameters. Such parameters might be sensitive to how the electron–positron annihilations occur, given the dependence of the lifetime of positronium (a bound electron–positron state) and the degree of entanglement on the type of tissue [ 8 ]. The intriguing results on photon entanglement reported by Bordes and colleagues could act as an invitation to the broader scientific community to join this emerging field.

J. Bordes et al. , “First detailed study of the quantum decoherence of entangled gamma photons,” Phys. Rev. Lett. 133 , 132502 (2024) .
A. L. McNamara et al. , “Towards optimal imaging with PET: An in silico feasibility study,” Phys. Med. Biol. 59 , 7587 (2014) .
P. Moskal et al. , “Feasibility studies of the polarization of photons beyond the optical wavelength regime with the J-PET detector,” Eur. Phys. J. C 78 , 970 (2018) .
D. P. Watts et al. , “Photon quantum entanglement in the MeV regime and its application in PET imaging,” Nat. Commun. 12 , 2646 (2021) .
A. Ivashkin et al. , “Testing entanglement of annihilation photons,” Sci. Rep. 13 , 7559 (2023) .
S. Parashari et al. , “Closing the door on the ‘puzzle of decoherence’ of annihilation quanta,” Phys. Lett. B 852 , 138628 (2024) .
P. Caradonna, “Kinematic analysis of multiple Compton scattering in quantum-entangled two-photon systems,” Ann. Phys. 470 , 169779 (2024) .
P. Moskal et al. , “Non-maximal entanglement of photons from positron-electron annihilation demonstrated using a novel plastic PET scanner,” arXiv: 2407.08574 ; “Positronium image of the human brain in vivo,” Sci. Adv. 10 , eadp2840 (2024) .
G. Romanchek et al. , “Application of quantum entanglement induced polarization for dual-positron and prompt gamma imaging,” Bio-Algorithms Med-Syst. 19 , 9 (2023) .
D. Kim et al. , “Background reduction in PET by double Compton scattering of quantum entangled annihilation photons,” J. Instrum. 18 , P07007 (2023) .

About the Author

Paweł Moskal is an inventor of cost-effective positron emission tomography (PET) based on plastic scintillators and a method of positronium imaging. He conceived and headed a medical experiment demonstrating the first positronium images of the human brain in vivo . He is a professor of physics and the head of the Cluster of Nuclear Physics Departments at Jagiellonian University, Poland. He has supervised 32 PhD students, coauthored 42 patents, and published more than 450 scientific articles. He founded and leads the J-PET group, constructing the first total-body PET system from plastic scintillators with the capability of multiphoton imaging, including positronium and entanglement imaging.

First Detailed Study of the Quantum Decoherence of Entangled Gamma Photons

Julien Bordes, James R. Brown, Daniel P. Watts, Mikhail Bashkanov, Kieran Gibson, Ruth Newton, and Nicholas Zachariou

Phys. Rev. Lett. 133 , 132502 (2024)

Published September 25, 2024

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1st-ever observation of 'spooky action' between quarks is highest-energy quantum entanglement ever detected

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Physicists at the world's largest atom smasher have observed two quarks in a state of quantum entanglement for the first time.

The observation, made at the Large Hadron Collider (LHC) at CERN, near Geneva, revealed a top quark — the heaviest fundamental particle — quantumly linked to its antimatter counterpart in the highest-energy detection of entanglement ever made. The researchers published their findings Sept. 18 in the journal Nature .

The ATLAS experiment (A Toroidal LHC Apparatus) is the largest detector at the LHC, and picks out the tiny subatomic particles created after beams of particles crash into each other at near light speeds.

"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," Andreas Hoecker , a spokesperson for the ATLAS experiment, said in an email statement. "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."

Particles that are entangled have their properties connected to each other, so that a change to one instantaneously causes a change to another, even if they are separated by vast distances. Albert Einstein famously dismissed the idea as "spooky action at a distance," but later experiments proved that the bizarre, locality-breaking effect is indeed real.

Related: Heaviest antimatter particle ever discovered could hold secrets to our universe's origins

But there are many aspects of entanglement that remain unexplored, and the one between quarks is one of them. This is because the subatomic particles cannot exist on their own, instead fusing together into various particle "recipes" called hadrons. Mixtures of three quarks are called baryons — such as the proton and the neutron — and combinations of quarks and their antimatter opposites are called mesons.

When individual quarks are ripped from hadrons, the energy used to extract them makes them immediately unstable, and they decay into branching jets of smaller particles in a process known as hadronization.

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COMMENTS

First Experimental Proof That Quantum Entanglement Is Real
John Clauser, a Caltech alumnus and Nobel laureate, was the first to test the foundations of quantum mechanics and entanglement using Bell's theorem and the CHSH inequality. He performed four experiments in the 1960s and 70s, confirming that entanglement is real and incompatible with locality and causality.
Physics
Learn how the 2022 Nobel Prize in Physics honors research on quantum entanglement, a quantum relationship between two particles that can exist over long distances. Discover how Bell tests, entanglement swapping, and other techniques have confirmed quantum mechanics and challenged classical theories.
LHC experiments at CERN observe quantum entanglement at the highest
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 ...
Proving that Quantum Entanglement is Real
John Clauser was the first to prove experimentally that quantum entanglement is real and incompatible with locality and causality. He shares his story of facing skepticism and resistance from other physicists, and his motivation to test the foundations of quantum mechanics.
Experiments on 'entangled' quantum particles won the physics Nobel Prize
Three physicists won the physics Nobel Prize for testing and using the strange feature of quantum physics called entanglement. Entanglement is when two particles are perfectly linked, even when far apart, and can be exploited for secure communication and quantum computing.
PDF How entanglement has become a powerful tool
Entanglement is a quantum phenomenon that allows two or more particles to be in a shared state, regardless of their distance. Learn how Alain Aspect, John Clauser and Anton Zeilinger have explored and exploited entanglement for quantum technology.
MIT researchers use quantum computing to observe entanglement
This experiment performed on the Sycamore quantum processor device at Google opens the doors to future experiments with quantum computers to probe ideas from string theory and gravitational physics. ... "This message was scrambled as it entered the system and, through entanglement, unscrambled on the other side." ...
'Spooky' quantum-entanglement experiments win physics Nobel
Alain Aspect, John Clauser and Anton Zeilinger win the prize for their pioneering research on quantum entanglement, a phenomenon that connects particles of light in a spooky way. Their experiments ...
Entanglement certification from theory to experiment
Quantum entanglement rose to prominence as the central feature of the famous thought experiment by Einstein, Podolsky and Rosen 1. Initially disregarded as a mathematical artefact showcasing the ...
Exploring large-scale entanglement in quantum simulation
Entanglement is a distinguishing feature of quantum many-body systems, and uncovering the entanglement structure for large particle numbers in quantum simulation experiments is a fundamental ...
Proving that quantum entanglement is real: Researcher answers questions
John Clauser was the first to prove experimentally that two widely separated particles can be entangled, disproving Einstein's locality and causality. He shares his story of testing quantum ...
Light from ancient quasars helps confirm quantum entanglement
Results are among the strongest evidence yet for "spooky action at a distance." Last year, physicists at MIT, the University of Vienna, and elsewhere provided strong support for quantum entanglement, the seemingly far-out idea that two particles, no matter how distant from each other in space and time, can be inextricably linked, in a way that defies the rules of classical physics.
ATLAS achieves highest-energy detection of quantum entanglement
The ATLAS Collaboration reports the first-ever observation of entanglement between a pair of quarks and the highest-energy measurement of entanglement. They use the angular distribution of the top quark decay products to calculate the degree of entanglement and test quantum mechanics at the LHC.
What is quantum entanglement? A physicist explains Einstein's 'spooky
Learn what quantum entanglement is, how it challenges Einstein's relativity and how it was proven by experiments. Discover the Nobel Prize winners who contributed to the understanding of this ...
Advances in high-dimensional quantum entanglement
Entanglement swapping has become an important fundamental concept, with applications such as overcoming long distances in quantum networks 235,236 or in fundamental experiments regarding ...
LHC experiments at CERN observe quantum entanglement at the highest
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 ...
What is quantum entanglement? A physicist explains the science of
Quantum entanglement is a phenomenon where two particles are linked in such a way that measuring one affects the other instantly, regardless of distance. Learn how this concept challenges our ...
Light from ancient quasars helps confirm quantum entanglement
Physicists at MIT and elsewhere used light from distant quasars, emitted billions of years ago, to decide how to measure entangled photons. They found strong evidence for quantum correlations that defy classical physics and constrained the freedom-of-choice loophole.
Quantum entanglement
Quantum entanglement is the phenomenon of particles being correlated in such a way that measuring one affects the other, even if they are far apart. Learn about the history, experiments, and applications of entanglement, and how it challenges classical physics.
Proving that Quantum Entanglement is Real
Clauser went on to perform three more experiments testing the foundations of quantum mechanics and entanglement, with each new experiment confirming and extending his results. The Freedman-Clauser experiment was the first test of the CHSH inequality. It has now been tested experimentally hundreds of times at laboratories around the world to ...
Explain it in 60 Seconds: Quantum entanglement
The CMS and ATLAS experiments recently detected quantum entanglement between top quarks in high-energy collisions at the LHC. What does that mean? Quantum entanglement links subatomic particles in a way that appears to defy logic. When scientists measure one of a pair of entangled particles, the ...
PDF What is quantum entanglement? A physicist explains the science of
satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement. The experiments of the 2022 Nobel laureates, particularly those of ...
Learning interpretable representations of entanglement in quantum
Quantum physics experiments produce interesting phenomena such as interference or entanglement, which are the core properties of numerous future quantum technologies. The complex relationship ...
Positron Emission Tomography Could Be Aided by Entanglement
But in 2023, an experiment indicated that entanglement persisted after one photon from an entangled pair scattered . This unexpected observation was confirmed by independent experiments for scattering angles up to 50°, with a hint of entanglement loss noted only at 50° . Bordes and colleagues extended these studies to a scattering angle of 70 ...
1st-ever observation of 'spooky action' between quarks is highest
"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," Patricia McBride, a spokesperson for the CMS experiment, said in the statement.

First Experimental Proof That Quantum Entanglement Is Real

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

What made you want to carry through with the experiments anyway?

What did you expect to find when you did the experiments?

Can you help us understand exactly what your experiments showed?

What are some of the important technological applications of your work?

Can you tell us more about your family’s strong connection with Caltech?

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Related Articles

Proving that Quantum Entanglement is Real

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

What made you want to carry through with the experiments anyway?

What did you expect to find when you did the experiments?

Can you help us understand exactly what your experiments showed?

What are some of the important technological applications of your work?

Can you tell us more about your family's strong connection with Caltech?

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