One photon stored in four places at once

Abstract

When light passes through two slits to hit a distant screen, a periodic light pattern emerges that is associated with the interference of the waves emanating from the two sources. Some of quantum physics’ deepest mysteries – or, according to the iconic Richard Feynman, its only mystery – arise when that observation is made with a single particle that, although indivisible, must have passed simultaneously through both slits. Recent advances in the storage of single photons in atomic gases [1] have now enabled a tour-de-force experiment that investigates interference with light stored simultaneously in four spatially distinct atom clouds, as reported on p. xxx of this issue. Chou et al. demonstrate stronger-than-classical correlations (entanglement) in this composite matter-light system, and study how the entanglement gives way to weaker and weaker, and ultimately only classical, correlations.