A ghost image violates a Bell inequality

In conventional photography, photons bounce off an object and imprint its shape onto film. In ghost imaging, an object’s shape is revealed after interrogating two light beams, only one of which interacts with the object. Ghost imagers have been practicing their craft for more than 10 years. Some of their schemes are based on photon entanglement, but others use classical light sources. Now Barry Jack of the University of Glasgow and colleagues have reported experiments in which quantum entanglement is manifest; their ghost images violate a generalized Bell inequality—a condition on correlations that can arise classically. The accompanying figure presents one of their runs. The object to be imaged introduces a π phase difference between a disk (blue) and the surroundings (red). A smaller reference object (inset) introduces the phase difference on either side of a diagonal. After bouncing entangled photons, separated in space, off the object and reference, Jack and company measured photon coincidence counts. The black-and-white ghost image shown here maps those counts, with brighter regions corresponding to a greater coincidence rate. The brightest sections appear along those portions of the disk’s bounding circle parallel to the reference bisector. The variation in the coincidence rate along the bounding circle violates the Bell inequality, thus demonstrating the quantum nature of Jack’s system. Evidently, the ghost imaging relies on spooky action at a distance, accepted nowadays but so troubling to Albert Einstein decades ago. (B. Jack et al., Phys. Rev. Lett. 103 , 083602, 2009. http://dx.doi.org/10.1103/PhysRevLett.103.083602 )