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Einstein’s spooky action examined after 100 years

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Posted April 23, 2015
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Researchers at the University of Tokyo have succeeded for the first time in verifing Einstein’s proposal of the nonlocality of a single quantum (photon) (the idea that a single photon diffracted by a slit can spread out infinitely, but is never detected in two or more places simultaneously).

(Left) Diagram representing a nonlocality of a single quantum (photon) spread out in two distant locations and measured; the measurement in one location steers the photon in the other. (Right) Experimental setup used to rigorously measure quantum nonlocality. Image credit: Maria Fuwa.

(Left) Diagram representing a nonlocality of a single quantum (photon) spread out in two distant locations and measured; the measurement in one location steers the photon in the other. (Right) Experimental setup used to rigorously measure quantum nonlocality. Image credit: Maria Fuwa.

This elusive phenomenon of nonlocality in quantum mechanics, which has been termed “spooky action at a distance,” spurred a hundred years’ debate among physicists with Einstein’s proposal in 1909. Ever since, physicists have been making zealous efforts towards rigorous confirmation by highly efficient measurement devices. However, detection methods used so far have been for detecting photons as particles. In addition to low detection efficiency, since these methods can only detect the presence or absence of photons, it was theoretically impossible to rigorously verify Einstein’s proposal.

Graduate School of Engineering Professor Akira Furusawa, doctoral student Maria Fuwa and their collaborators utilized the wave-like degree of a photon as an electromagnetic wave and used a homodyne measurement technique to measure the photon amplitude and phase with high efficiency. This enabled the group to successfully verify the nonlocality of a single photon with high precision and rigor.

This is not only a huge achievement in the field of fundamental physics, but also opens way to a novel scheme to realize quantum cryptography and quantum computers using both the particle and wave nature of photons.

Source: University of Tokyo

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