non-locality

Researchers at the University of Tokyo have succeeded for the first time in verifying Einstein’s proposal of the non-locality of a single quantum: the idea that a single photon diffracted by a slit can spread over arbitrarily large distances, but is never detected in two or more places simultaneously.

This elusive phenomenon of non-locality 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. They demonstrate it by splitting a single photon between two laboratories and experimentally testing whether the choice of measurement in one laboratory really causes a change in the local quantum state in the other laboratory.

This enabled the group to successfully verify the non-locality of a single photon with high precision and rigor. The experiment also verifies the entanglement of the split single photon even when one side is untrusted.

M. Fuwa, S. Takeda, M. Zwierz, H. M. Wiseman, and A. Furusawa. Experimental proof of nonlocal wave function collapse for a single particle using homodyne measurements. Nat Commun, 6, 03 2015.