The physics of ghost imaging: nonlocal interference or local intensity fluctuation correlation?

In this special Volume on Quantum Imaging, Shapiro and Boyd present a review on The Physics of Ghost Imaging. Here I argue that their Review is incomplete, as it overlooks a form of ghost imaging that requires quantum mechanical nonlocal effects for their understanding. From the very beginning, Klyshko brought attention to the nonlocal nature of biphoton interference and biphoton imaging by introducing his “advanced wave” theory [1]. His “advanced wave” propagates backward in time from a photodetector to the source and then propagates forward in time from the source to another distant photodetector. The “advanced wave” connects the object-arm and the image-arm, as well as two independent photodetection events, in his biphoton interference and imaging model. Strekalov et al. and Pittman et al. realized nonlocal biphoton interference [2] and biphoton imaging [3] experimentally. The physics community immediately named these two experiments “ghost interference” and “ghost imaging”. Simultaneously with the experimental realization, we found that Klyshko’s advanced wave theory can be replaced by a standard quantum theory of second-order coherence in which a nonlocal biphoton wavepacket is introduced to connect the two arms and two photodetection events [4]. Soon after that, we realized that nonlocal interference and imaging are not restricted to entangled biphoton systems: it also happens in thermal radiation. Ghost imaging can be observed from thermal light in the Fresnel near-field zone in a lensless configuration [5–7]. Although nonlocality is a feature of quantum mechanics [8], this concept is not part of classical physics. In the classical theory of light, ghost imaging, especially thermal light ghost imaging, is a result of locally measured intensity fluctuation correlations. So this brings us to ask, is ghost imaging a nonlocal interference phenomenon (and hence quantum) or is it a result of local intensity fluctuation