The Nobel Prize in Physics 2022 has been awarded to
- Alain Aspect, Université Paris-Saclay and École Polytechnique, Palaiseau, France,
- John F. Clauser, J. F. Clauser & Associates, Walnut Creek, CA, USA, and
- Anton Zeilinger, University of Vienna, Austria,
for “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science” .
Alain Aspect, John Clauser, and Anton Zeilinger investigated particles that are in entangled states. What happens to one particle in an entangled pair determines what happens to the other. The researchers’ development of experimental tools has laid the foundation for a new era of quantum technology.
Quantum mechanics allow two or more particles to exist in a shared state, regardless of how far apart they are. This is called entanglement, and has been one of the most debated elements of quantum mechanics ever since the theory was formulated. Albert Einstein, for example, called it “spooky action at a distance”. When two particles are in entangled quantum states, someone who measures a property of one particle can immediately determine the result of an equivalent measurement on the other particle, without needing to check.
For a long time, the question was whether this correlation was because the particles in an entangled pair contained hidden variables, instructions that tell them which result they should give in an experiment. In the 1960s, John Stewart Bell developed the mathematical inequality that is named after him. This states that if there are hidden variables, the correlation between the results of a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment will violate Bell’s inequality, thus resulting in a stronger correlation than would otherwise be possible. Experiments have shown that Nature behaves as predicted by quantum mechanics.
John Clauser developed John Bell’s ideas, leading to a practical experiment [2,3]. He built an apparatus that emitted two entangled photons at a time, each towards a filter that tested their polarization. If both the particles in the experiment are sent towards filters that are oriented in the same plane and one slips through, then the other one will also go through. If they are at right angles to each other, one will be stopped while the other will go through. When the filters are set in different directions at skewed angles, then the results can vary: Sometimes both slip through, sometimes just one, and sometimes none.
Quantum mechanics leads to a correlation between measurements. The likelihood of one particle getting through depends on the angle of the filter that tested its partner’s polarization on the opposite side of the experimental setup.
This means that the results of both measurements, at some angles, violate a Bell inequality and have a stronger correlation than they would if the results were governed by hidden variables. When Clauser took measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.
However, some loopholes remained after John Clauser’s experiment due to limitations in the efficiency of producing and capturing particles. Also, the setup was fixed, which allowed doubts such as: What if the experimental setup in some way selected the particles that happened to have a strong correlation, and did not detect the others?
Alain Aspect further developed the setup, using it in a way that closed an important loophole [4–7]. He built a new version of the setup that he refined over several iterations. In his experiment, he could register the photons that passed through the filter, as well as those that did not. This meant more photons were detected and the measurements were better.
In the final variant of his tests, he used a mechanism that switched the direction of the entangled photons after they had been created and emitted from their source. This way, no information about the filters on one side of the experiment could reach the other side and affect the result of the measurement there. With this approach, Aspect closed an important loophole and provided a very clear result: Quantum mechanics is correct and there are no hidden variables.
Using refined tools and long series of experiments, Anton Zeilinger started to use entangled quantum states. Among other things, his research group has demonstrated a phenomenon called quantum teleportation. If the particles in an entangled pair travel in opposite directions and one of them then meets a third particle in such a manner that they become entangled, they then enter a new shared state. The third particle loses its identity, but its original properties have now been transferred to the solo particle from the original pair. This way of transferring an unknown quantum state from one particle to another is called quantum teleportation. This type of experiment was first conducted in 1997 by Anton Zeilinger and his team .
These and similar experiments laid the foundation for the current intense research in quantum information science. Being able to manipulate and manage quantum states and all their layers of properties gives us access to new tools, e.g., for quantum computation, the transfer and storage of quantum information, and algorithms for quantum encryption.
Alain Aspect, born in 1947 in Agen, France, studied physics at the Université d’Orsay, France, and received his Ph.D. in 1983 from Paris-Sud University, Orsay, France. Currently, he is Professor at Université Paris-Saclay and École Polytechnique, Palaiseau, France.
Among many other awards, Alain Aspect received the Holweck Medal in 1991, the Wolf Prize in Physics in 2010 together with John Clauser and Anton Zeilinger, the Albert Einstein Medal in 2021, and the ForMemRS in 2015. He is a member of the Académie des Sciences in France.
John F. Clauser, born in 1942 in Pasadena, CA, USA, studied physics at the California Institute of Technology (Caltech), Pasadena, CA, USA, and received his Ph.D. in 1969 from Columbia University, New York, USA. From 1969 to 1975, he was a Joint Postdoctoral Research Associate at Lawrence Berkeley National Laboratory and University of California, Berkeley, CA, USA, from 1975 to 1986, Research Physicist and Group Leader at Lawrence Livermore National Laboratory, CA, USA, and from 1986 to 1987, Senior Scientist at Science Applications International Corporation, Reston, VA, USA. Since 1988, John Clauser is at J. F. Clauser & Associates, Walnut Creek, CA, USA.
Among many other awards, John Clauser was awarded the Wolf Prize for Physics in 2010, together with Alain Aspect and Anton Zeilinger.
Anton Zeilinger, born in 1945 in Ried im Innkreis, Austria, studied physics and mathematics at the University of Vienna, Austria, and received his Ph.D. in 1971 from the University of Vienna. In 1979, he habilitated at the Vienna University of Technology. After stays at the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, at the Humboldt University of Berlin, Germany, at Merton College, Oxford, UK, and at the Collège de France (Chaire Internationale), Paris, France, he became a full university professor at the University of Innsbruck, Austria, in 1990 and head of the Institute of Experimental Physics. Since 1999, Anton Zeilinger is Professor at University of Vienna, Austria.
Among many other awards, he received the Isaac Newton Medal of the British Institute of Physics (IOP) in 2007 and the Wolf Prize for Physics in 2010, together with Alain Aspect and John Clauser. He served as president of the Austrian Academy of Sciences from 2013 to 2022 and was the sixth Austrian to be inducted into the National Academy of Sciences (NAS).
 NobelPrize.org, The official website of the Nobel Prize
 John F. Clauser, Michael A. Horne, Abner Shimony, Richard A. Holt, Proposed Experiment to Test Local Hidden-Variable Theories, Phys. Rev. Lett. 1969, 23, 880. https://doi.org/10.1103/PhysRevLett.23.880
 Stuart J. Freedman, John F. Clauser, Experimental Test of Local Hidden-Variable Theories, Phys. Rev. Lett. 1972, 28, 938. https://doi.org/10.1103/PhysRevLett.28.938
 Alain Aspect, Proposed experiment to test the nonseparability of quantum mechanics, Phys. Rev. D 1976, 14, 1944. https://doi.org/10.1103/PhysRevD.14.1944
 Alain Aspect, Philippe Grangier, Gérard Roger, Experimental Tests of Realistic Local Theories via Bell’s Theorem, Phys. Rev. Lett. 1981, 47, 460. https://doi.org/10.1103/PhysRevLett.47.460
 Alain Aspect, Philippe Grangier, Gérard Roger, Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell’s Inequalities, Phys. Rev. Lett. 1982, 49, 91. https://doi.org/10.1103/PhysRevLett.49.91
 Alain Aspect, Jean Dalibard, Gérard Roger, Experimental Test of Bell’s Inequalities Using Time-Varying Analyzers, Phys. Rev. Lett. 1982, 49, 1804. https://doi.org/10.1103/PhysRevLett.49.1804
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