The Physics of Ghost imaging

One of the most surprising consequences of quantum mechanics is the nonlocal correlation of a multi-particle system observable in joint-detection of distant particle-detectors. Ghost imaging is one of such phenomena. Taking a photograph of an object, traditionally, we need to face a camera to the object. But with ghost imaging, we can image the object by pointing a CCD camera towards the light source, rather than towards the object. Ghost imaging is reproduced at quantum level by a non-factorizable point-to-point image-forming correlation between two photons. Two types of ghost imaging have been experimentally demonstrated since 1995. Type-one ghost imaging uses entangled photon pairs as the light source. The nonfactorizable image-forming correlation is the result of a nonlocal constructive-destructive interference among a large number of biphoton amplitudes, a nonclassical entity corresponding to different yet indistinguishable alternative ways for the photon pair to produce a joint-detction event between distant photodetectors. Type-two ghost imaging uses chaotic light. The typetwo non-factorizable image-forming correlation is caused by the superposition between paired two-photon amplitudes, or the symmetrized effective two-photon wavefunction, corresponding to two different yet indistinguishable alternative ways of triggering a join-detection event by two independent photons. The multi-photon interference nature of ghost imaging determines its peculiar features: (1) it is nonlocal; (2) its imaging resolution differs from that of classical; and (3) the type-two ghost image is turbulence-free. Ghost imaging has attracted a great deal of attention, perhaps due to these features for certain applications. Achieving these features, the realization of nonlocal multi-photon interference is a necessary condition. Classical simulations, such as the man-made factorizable speckle-speckle correlation, can never have such features.