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Three physicists win the Nobel Prize in Physics for their work in entangled photons

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The Royal Swedish Academy of Sciences announced on October 4th that it would award the Nobel Prize in Physics 2022 to Alain Aspect (France), John F. Clauser (United States), and Anton Zeilinger (Austria) for their experiments with entangled photons, where they established the violation of Bell inequalities and pioneered quantum information science.

The work completed by the aforementioned physicists has orbited the investigation of how two particles interact, behaving as a single particle despite the distance separating them. The awarded research builds upon the work of the physicist John Stewart Bell. This phenomenon was deemed ‘spooky action at distance‘ by Albert Einstein, winner of the physics Nobel Prize in 1921.

In 1936, Schrödinger published a two-part article discussing an argument by Einstein, Podolsky, and Rosen. The Einstein-Podolsky-Rosen (EPR) argument was the critique of the Copenhaguen interpretation of quantum mechanics. It expresses the specification of a set of parameters from which the properties of a system can be reconstructed. The properties refer to the position and momenta of the particles. Einstein rejected this view and proposed a series of arguments to show that the quantum state is simply an incomplete characterization of a quantum system and that missing parameters are referred to as ‘hidden variables’, unknown factors that invisibly tie the two outcomes together. Bell inequality states that if there are hidden variables, the correlations between the results of a large number of measurements of entangled particles will never exceed a certain value. However, quantum mechanics predicts that some experiment types will violate Bell’s inequality.

The EPR argument also states that two particles are prepared from a source in a certain pure quantum state of the composite system. After the particles move apart, there are matching correlations between the positions of the two particles and their momenta. The measurement of either position or momentum on a particle allows the prediction of the outcome of the respective measurement on the other particle–call it position or momentum. But both measurements cannot be performed simultaneously. Classical correlations can be explained by a common cause or correlated ‘elements of reality.’

Schrodinger coined the term entanglement to refer to the connection between two systems. “By the interaction, the two representatives [the quantum states] have become entangled.”

The scientist found out that the entanglement between two separating systems would persist only for distances small enough that the time taken by light to travel from one system to the other could be neglected.

According to Bell, the probabilistic quantum mechanical predictions take the form of inequalities that must be satisfied but are violated by correlations calculated from quantum mechanics. 

The three laureates have worked independently, conducting experiments that helped to clarify a fundamental claim concerning the behaviour of tiny particles that interacted in the past and subsequently moved apart. Their experiments proved that connections between quantum particles were not down to local hidden variables. The phenomenon comes from an actual association in which manipulating one quantum object affects another far away. 

Clauser led a practical experiment by developing Bell’s ideas. His results supported quantum mechanics by clearly violating a Bell inequality. Quantum mechanics cannot be replaced by a theory that uses hidden variables.

After some loopholes remained from Clauser’s experiment, Aspect developed a setup and used it in such a way that would close one major loophole. He could switch the measurement settings after an entangled pair had left its source. By doing so, the setting that existed when the particles were emitted did not affect the result.

Anton Zeilinger started using entangled quantum states when employing refined tools and a long series of experiments. His research group demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to another at a distance.

According to quantum mechanics, particles can exist in two or more places at the same time. They do not take on formal properties until they are observed or measured in some way. When the position or spin of one particle is measured, a change can be observed in the pair regardless of the distance it has travelled from its partner.

David Havilman, chair of the Nobel Committee for Physics, declared, “if we have this property of entanglement between the two photons, we can establish a common information between two different observers of these quantum objects.” Such a momentous discovery allows for the furthering of domains such as secret communication, something previously inconceivable in the field of physics. The development of these quantum mechanics experiments helped us to improve areas like secure information transfer -data encryption-, quantum computing and sensing technology. The work of the three scientists has laid the foundations for a new era of quantum technology.

Last year, three physicists won the Nobel Prize for their contributions to understanding complex physical systems. One half jointly to Syukuro Manabe and Klaus Hasselmann “for the physical modelling of Earth”s climate, quantifying variability and reliably predicting global warming” and the other half to Giorgio Parisi “for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.

Alain Aspect, John F. Clauser, and Anton Zeilinger will equally share the prize of 10 million Swedish kronor.

Alain Aspect, age 75, is a professor at the Université Paris-Saclay and École Polytechnique, Palaiseau, in France.

John F. Clauser, age 79, is a research physicist and works in the J. F. Clauser & Associates in California, USA.

Anton Zeilinger, age 77, is a professor at the University of Vienna in Austria.

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