experiments with entangled photons establishing the violation of bell inequalities

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Alain Aspect, John Clauser and Anton Zeilinger win the 2022 Nobel Prize for Physics

Alain Aspect, John Clauser and Anton Zeilinger have won the 2022 Nobel Prize for Physics. The trio won “for their experiments with entangled photons, establishing the violation of Bell’s inequalities and pioneering quantum information science”.

The prize will be presented in Stockholm in December and is worth 10 million kronor ($900,000). It will be shared equally between the winners.

Working independently, the three laureates did key experiments that established the quantum property of entanglement. This is a curious effect whereby two or more particles display much stronger correlations than are possible in classical physics. Entanglement plays an important role in quantum computers, which in principle could outperform conventional computers at some tasks.

Bell’s inequality

All three of the experiments measured violations of Bell’s inequality, which places a limit on the correlations that can be observed in a classical system. Such violations are an important prediction of quantum theory.

The first experiment was done in 1972 at the University of California at Berkeley by Clauser, who measured the correlations between the polarizations of pairs of photons that were created in an atomic transition. He showed that Bell’s inequality was violated – which meant that the photon pairs were entangled.

However, there were several shortcomings or “loopholes” in this experiment, making it inconclusive. It is possible, for example, that the photons detected were not a fair sample of all photons emitted by the source – which is the detection loophole.  It is also possible that some aspects of the experiment that are thought to be independent were somehow causally connected – which is the locality loophole.

Ten years later, in 1982, Aspect and colleagues at the Université Paris-Sud in Orsay, France, improved on Clauser’s experiment by using a two-channel detection scheme. This avoided making assumptions about the photons that were detected. They also varied the orientation of the polarizing filters during their measurements. Again, they found that Bell’s inequality was violated.

Third loophole

The locality loophole was closed in 1998 by Zeilinger and colleagues at the University of Innsbruck in Austria. They used two fully independent quantum random-number generators to set the directions of the photon measurements. As a result, the direction along which the polarization of each photon was measured was decided at the last instant, such that no signal travelling slower than the speed of light would be able to transfer information to the other side before that photon was registered.

As well as confirming a fundamental prediction of quantum mechanics, the three experiments laid the groundwork for the development of modern quantum technologies.

Speaking at the press conference when the prize was announced, Zeilinger said he was “very surprised” to receive a call from the Nobel committee. “This prize is an encouragement to young people and the prize would not be possible without more than 100 young people who have worked with me over the years. I alone could not have achieved this.”

Zeilinger also said he hoped the prize would encourage young researchers.

“My advice to young people would be do what you find interesting and don’t care too much about possible applications. On the other hand, this recognition is very important for the future development of possible applications. I am curious what we will see in the next 10–20 years.”

A profound impact

Sheila Rowan , president of the Institute of Physics, which publishes Physics World , congratulated the trio on their “well-deserved” recognition. “This is an area of physics with ongoing, profound impact, at a fundamental level to help understand the world around us and being explored for use in highly novel technologies for sensing and communication today,” she added.

Quantum physicist Artur Ekert from the University of Oxford says that while he is “happy” to see the field and the trio being recognized with this year’s Nobel,  he adds that it is a “pity” that John Bell, who formulated the inequalities, missed out given that he died in 1990 and Nobel prizes are not awarded posthumously.

Ekert adds that the advent of quantum cryptography has provided an additional motivation to push the Bell inequality experiments to their limits. “From the foundations of science perspective, I think the Bell inequality experiments simply had to be done — they refute a certain world view and so they are important,” adds Ekert. “Fixing all the loopholes in such experiments is another story. This is probably more important for the quantum cryptography perspective as if we want to use Bell inequalities to detect eavesdropping we have to close the loopholes.”

Indeed, congratulations also came from those who are trying to use the work of Aspect, Clauser and Zeilinger for practical applications. In a joint statement, Ilyas Khan and Tony Uttley, chief executive and president, respectively, of the quantum technology firm Quantinuum , noted they were thrilled” by the announcement.

“This recognition of the power of quantum information systems is timely on many counts, but above all is a wonderful acknowledgement of the fact that experimental advances underpin the quantum technologies revolution that we are embarking upon.”

A life in science

Aspect was born in Agen, France, on 15 June 1947. He passed the “agrégation” – the national French exam – in physics in 1969 and received his Master’s degree from the Université d’Orsay two years later. He then embarked on a PhD at Orsay, working on experimental tests of Bell’s inequalities, which he completed in 1983.

Following a lectureship at the Ecole Normale Supérieure de Cachan, which Aspect held while he was doing his PhD, in 1985 he worked at Collège de France in Paris. In 1992 he then moved to the Laboratoire Charles Fabry de l‟Institut d’Optique at the Université Paris-Saclay.

Clauser was born in Pasadena, California, on 1 December 1942. He received his Bachelor’s degree in physics from California Institute of Technology in 1964 and a Master’s in physics two years later. In 1969 he received a PhD in physics from Columbia University.

From 1969 to 1975 Clauser was a researcher at Lawrence Berkeley National Laboratory and from 1975 to 1986 worked at the Lawrence Livermore National Laboratory. Following a stint as a senior scientist at the US firm Science Applications International Corporation, in 1990 he moved to the University of California, Berkeley until 1997 where he then focused on his research and consultancy firm J F Clauser & Associates.

Anton Zeilinger: a quantum pioneer

Zeilinger was born in Ried im Innkreis, Austria, on 20 May 1945. In 1963 he began studying physics and mathematics at the University of Vienna and in 1971 completed his PhD in atomic physics. He then worked at the Atomic Institute in Vienna until 1983 before heading to the Vienna University of Technology.

In 1990 Zeilinger moved to the University of Innsbruck and in 1999 worked at the University of Vienna where he also became director of the Vienna-based Institute for Quantum Optics and Quantum Information from 2004 to 2013. In 2013 he served as president of the Austrian Academy of Sciences, a position he held until this year.

• Anton Zeilinger has written several articles for Physics World : “ A quantum renaissance ” by Markus Aspelmeyer and Anton Zeilinger; “ Probing the limits of the quantum world ” by Markus Arndt, Anton Zeilinger and Klaus Hornberger; and “ Fundamentals of quantum information ” by Anton Zeilinger.

Physics World ‘s Nobel prize coverage is supported by Oxford Instruments Nanoscience , a leading supplier of research tools for the development of quantum technologies, advanced materials and nanoscale devices. Visit nanoscience.oxinst.com to find out more.

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Quantum Physics

Title: experimental tests of bell's inequalities: a first-hand account by alain aspect.

Abstract: On 04 October 2022, the Royal Swedish Academy of Sciences announced that the Nobel Prize for Physics of 2022 was awarded jointly to Alain Aspect, John Clauser, and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science". What follows is an interview of Alain Aspect, conducted by Bill Phillips and Jean Dalibard, during the summer of 2022, and completed not long before the announcement of the Nobel Prize. The subject matter is essentially that for which the Nobel Prize was awarded.

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October 4, 2022

Explorers of Quantum Entanglement Win 2022 Nobel Prize in Physics

Alain Aspect, John F. Clauser and Anton Zeilinger won the 2022 Nobel Prize in Physics for their work using entangled photons to test the quantum foundations of reality

By Lee Billings

vanbeets/Getty Images (medal)

This year’s Nobel Prize in Physics was awarded in equal parts to Alain Aspect of the University of Paris-Saclay, John F. Clauser of J. F. Clauser & Associates, and Anton Zeilinger of the University of Vienna, for their pathfinding work in quantum mechanics and quantum information science.

Working independently, each of the three researchers forged new experiments demonstrating and investigating quantum entanglement, the curious phenomenon in which two or more particles exist in a so-called entangled state. In this bizarre situation, an action taken on one of the particles can instantaneously ripple through the entire entangled assemblage, predicting the other particles’ behavior, even if they are far apart. If an observer determines the state of one such particle, its entangled counterparts will instantly reflect that state—whether they are in the same room as the observer or in a galaxy on the opposite side of the universe. Although this phenomenon has become an essential aspect of modern quantum technologies, it is so counterintuitive and seemingly impossible that Albert Einstein once famously derided it as “spooky action at a distance.”

The scientists’ work has collectively addressed core enigmas that were deeply investigated in the 1960s by Northern Irish physicist John Stewart Bell, who sought to understand what entanglement’s “spookiness” implies about the fundamental nature of reality. Could it be, as Einstein believed, that all objects—whether planets or particles—possess fundamental properties that can be discerned through precise observation? If so, quantum weirdness would only be an illusion arising from the failure of quantum mechanics to account for as-yet-undiscovered “hidden variables” inherent to the particles that populate the subatomic world. By preordaining the states of widely separated entangled particles before those particles were measured, hidden variables would neatly explain how they managed to reflect each other’s state without information traveling between them faster than the speed of light—a profound violation of one of the most fundamental tenets of physics. Einstein’s idea flew in the face of an alternative preferred by Niels Bohr, Erwin Schrödinger and other physicists, who rejected the notion of such hidden variables. Reality, they said, is inherently fuzzy, and particles only gain certain specific characteristics upon being measured.

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Expanding on a concept proposed some three decades prior by Einstein and his physicist colleagues Boris Podolsky and Nathan Rosen, in 1964 Bell showed that if hidden variables existed, their presence could be experimentally inferred through careful measurements of multiple pairs of entangled particles. When, for instance, such particles’ spins are measured along a varying directional axis (that is, not only “up” or “down” but also various states in between), certain correlations between the collective paired spins should be much stronger if they are dictated purely by quantum mechanics and correspondingly weaker if they are influenced by hidden variables. Performing this experiment could thus confirm that reality, at its heart, is purely quantum mechanical or instead reveal the presence of deeper, more fundamental layers of physical complexity. Such “Bell tests,” however, contained multiple potential loopholes that could confound their results. And across decades, legions of researchers ardently worked to close them.

Credit: Matthew Twombly; Source: Modified from TU Delft—A Loophole-Free Bell Test (based on text by Michel van Baal and graphics by Scixel). TU Delft, 2015

In 1969 Clauser was the first to conceive a practical Bell test, which involved measuring quantum entanglement by determining the polarization of entangled photon pairs launched in opposite directions. He then performed the experiment in 1972 alongside the late Stuart Freedman, who was then a graduate student, confirming that the photons acted in concert despite their physical separation. Clauser’s work indicated that hidden variables could not explain entanglement’s effects, suggesting that quantum theory remained intact as an essentially complete description of reality.

But the loopholes remained. And about 10 years later, after refining Clauser’s experiment, Aspect and his collaborators closed one of them by developing a way to switch the directions of entangled photon pairs in billionths of a second—after the photons had left their source but before they arrived at a detector. This ensured that the measurement settings that existed when the photons were emitted could not affect the ultimate result, greatly reinforcing the notion that hidden variables do not exist. Whatever fundamentally occurs during the measurement of entangled particles (still a topic of intense debate among scientists), Aspect’s work showed it does so within the confines of existing quantum theory. The last major loophole for Bell tests was closed in 2015  through the work of four different research groups.

Zeilinger and his colleagues greatly expanded the use and study of entangled quantum states. In 1997 they were one of two research groups to independently demonstrate a phenomenon called quantum teleportation, which uses entanglement to allow quantum states to be moved from one particle to another across arbitrary distances. (The other team was headed by Italian physicist Francesco De Martini.) Zeilinger’s group went on to achieve several more “firsts” in quantum information science. Today quantum teleportation has become central for nascent efforts to build a globe-spanning “ quantum Internet .” And Zeilinger collaborated on work that successfully used the technique to create China’s Micius spacecraft, the first quantum communications satellite .

The Nobel Prize in Physics 2022 was awarded jointly to Alain Aspect, John F. Clauser and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” Credit: Niklas Elmehed © Nobel Prize Outreach

During the prize’s announcement, Eva Olsson, a member of the Nobel Committee for Physics, said Aspect’s, Clauser’s and Zeilinger’s work has “opened doors to another world, and it has also shaken the very foundations of how we interpret measurements.”

Collectively, the trio’s research has contributed greatly to quantum information science , the scientific field underpinning the ongoing race to develop practical devices and techniques that leverage quantum principles to achieve breakthroughs in computing, communications and cryptography.

“The work of Aspect, Clauser and Zeilinger provides the practical theoretical method and conclusive experimental measurements that underline the distinction between the quantum and classical worlds, showing that quantum objects can be related through entanglement in a way that is not possible with classical objects,” says Andrew Cleland, a quantum physicist at the University of Chicago. “Their work forms the fundamental basis for quantum computation and quantum communication.”

John Preskill, a leading quantum information scientist at the California Institute of Technology, notes that many mature technologies are already “quantum” in some sense: lasers, magnetic resonance imaging machines and multibillion-transistor computer chips all rely on quantum mechanics unfolding on subatomic scales. “But those technologies,” he says, “have only scratched the surface of how quantum theory has modified our view of what’s possible in the universe.”

And, Preskill adds, “the burgeoning investments in quantum technologies now occurring all over the world are building on scientific foundations which flow from the pioneering work of Bell, Clauser, Aspect and Zeilinger.”

In a press conference shortly after the announcement, Zeilinger expressed shock at being awarded the prize and acknowledged the influence of the late Helmut Rauch, who had been his academic adviser, as well as the “more than 100 young people who worked with me over the years and made this all possible.” Despite the myriad emerging applications for quantum entanglement, Zeilinger said that what most inspires him remains the mystery it still poses about the nature of reality. “Some of the fundamental questions—the very question ‘What does this really mean?’—in a basic way, are still unanswered in my eyes,” he said. “And that is an avenue for new research.”