 | Alain Aspect, John Clauser, and Anton Zeilinger are to be awarded the 2022 Nobel Prize in Physics for their work demonstrating the violation of Bell inequalities, research that has paved the way for quantum information science, the Royal Swedish Academy of Sciences announced on Tuesday.
Clauser, Aspect, and Zeilinger designed and performed experiments that tested whether some of the most counterintuitive predictions of quantum mechanics could be explained by intrinsic properties that predetermine the outcomes of possible measurements. In 1972 Clauser (now with J. F. Clauser and Associates in Walnut Creek, California) and Stuart Freedman put such local hidden-variable theories to the test by generating pairs of entangled photons and analyzing the measured polarizations of the particles. By plugging the correlations between measurements into an inequality proposed by theorist John Bell, Clauser found that the results were inconsistent with those local theories. In the early 1980s, Aspect (Université Paris–Saclay and École Polytechnique in France) and his colleagues reinforced Clauser's findings with more robust experimental setups that closed a major loophole.
In addition to performing even more stringent Bell tests, Zeilinger (University of Vienna) is one of the leaders of the burgeoning field of quantum information science. He and his colleagues have demonstrated quantum teleportation, entanglement swapping, and other quantum state manipulation techniques that are essential for the development of quantum computers, communication networks, and cryptography schemes.
The awardees will each receive a third of the 10 million Swedish kronor (roughly $900 000) prize.
A testable paradox
The experiments devised by the laureates have their roots in one of Albert Einstein's famous gedanken experiments. In 1935 Einstein, Boris Podolsky, and Nathan Rosen—collectively known as EPR—described their concerns with the phenomenon of quantum entanglement, in which a single wavefunction describes two particles, no matter the distance between them. As Bell summarizes: "For after observing only one particle the result of subsequently observing the other (possibly at a very remote place) is immediately predictable." One way around such a counterintuitive reality is for a local mechanism to inbue the particles with properties that predetermine the results of future measurements.
The question raised in the EPR paper was largely a philosophical one until Bell, a Northern Irish theoretical physicist, came along. In 1964 he proved a theorem that local hidden variables cannot reproduce all the statistical predictions of quantum mechanics (see the article by Reinhold Bertlmann, Physics Today, July 2015, page 40). Most importantly, he derived an inequality that must be obeyed by any local deterministic theory, and he proposed an experiment—albeit one that would be impractical to perform—in which a violation of the inequality would rule out all those theories.
Tricky experiments
Although Bell's work essentially invited experimentalists to investigate the foundations of quantum mechanics and evaluate Einstein, the topic "was still marginal for most working physicists," says David Kaiser, a physicist and historian of science at MIT. Clauser was one of the few researchers who took on the challenge.
In the late 1960s and early 1970s, Clauser, then working at the University of California, Berkeley, and three colleagues devised a related inequality that was testable with the technology of the time. In 1972, Clauser and Freedman conducted such a test. They built a setup that sent two entangled photons in opposite directions toward a fixed set of polarization filters. Depending on the angles of the filters and the polarization of the photons, the photons either were blocked or passed through and pinged a detector. The measured coincidence rates surpassed the limit that could be explained by local hidden-variable theories. |
 | John Clauser works on his experiment to evaluate Bell inequalities. Credit: Steve Gerber, courtesy of Berkeley Lab |
Clauser's work was a major step in solidifying the foundations of quantum mechanics, but it didn't rule out the possibility of local hidden variables. In particular, the orientation of each polarizer was fixed ahead of time, so in principle, information about the measurements to be performed was available at the time the photons were generated. That limitation is known as the locality loophole.
Aspect and colleagues created improved versions of Clauser's experiment. First, they used improved entangled-photon generators and polarizers, which yielded more precise statistics. And in 1982 Aspect's team addressed the locality loophole by following through on a request from Bell that the settings of the detectors be "changed during the flight of the particles." They added a method to select one of two filters at different angles for each photon within a few nanoseconds, too quick for information about one filter to reach and influence the filter on the other side. All the experiments favored the rejection of local hidden variables.
From foundations to applications
Clauser's and Aspect's experiments may not have directly set off the second quantum revolution, but they got theorists thinking about the practical applications of the information encoded in quantum states, particularly entangled ones. In 1984 Charles Bennett and Gilles Brassard devised their BB84 quantum key distribution scheme, a proposal that made quantum information relevant to the high-stakes, financially lucrative field of cryptography (see the article by Daniel Gottesman and Hoi-Kwong Lo, Physics Today, November 2000, page 22). "Once it was discovered there was some practical applications, man, the stuff just took off and skyrocketed," Clauser recounted in a 2002 oral history.
Another exciting theorized application was quantum teleportation. Although the no-cloning theorem states that a quantum state cannot be copied, Bennett, Brassard, and colleagues showed in 1993 that the state of one particle could be transferred to another using the combination of entanglement and the ability to send classical messages (see the article by Bennett, Physics Today, October 1995, page 24). In 1997 Zeilinger and his team performed the first experimental demonstration of quantum teleportation and then executed a similar operation known as entanglement swapping. Soon after, Zeilinger's group entangled multiple photons in what's now called a GHZ (Greenberger-Horne-Zeilinger) state and showed that it violated a Bell inequality. |
 | A green laser shines from Tenerife to La Palma, located nearly 150 km away. The laser was used in a 2012 demonstration by Anton Zeilinger and colleagues to transmit quantum states between stations on each of the islands. Credit: IQOQI Vienna, Austrian Academy of Sciences |
More recently Zeilinger and his team followed up with their own version of a Bell experiment that was the first to close the major loopholes simultaneously. His and colleagues' 2015 experiment used detectors that clocked photons efficiently enough to ensure that undetected photons couldn't tilt the scales. The detectors were also spaced sufficiently distant to close the locality loophole more definitively than did Aspect's team. (See Physics Today, January 2016, page 14.) Zeilinger, Kaiser, and colleagues also addressed another loophole, dubbed "freedom-of-choice," by using light from stars to determine the measurements to be performed on pairs of entangled photons.
"Einstein, Podolsky, and Rosen highlighted the surprising effects of quantum entanglement; John Bell argued that you couldn't wriggle out of them; and [Aspect, Clauser, and Zeilinger] demonstrated them experimentally," wrote the theoretical physicist Sean Carroll on Twitter in response to the Nobel announcement.
Legacy
Bell's inequality has now been violated in dozens of experiments by many standard deviations each. Today the motivation is not the foundational implications but the practical ones. "There is a direct line between these experiments from 30–50 years ago and the current field of quantum technologies," says Ronald Hanson, who researches quantum technologies at Delft University of Technology and led the first loophole-free Bell test. Entanglement is at the heart of those technologies, including quantum computing and quantum cryptography.
Says Kaiser, "The road has been long, stretching over half a century, but thanks to efforts like theirs, we all live in a world in which foundational aspects of quantum theory can both excite our imaginations and, soon, power next-generation technologies."
Read this article online at physicstoday.org. |
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