Fig. 1. An overview on the development of ghost imaging. The utility of second-order coherence can be traced back to the HBT experiment, with the concept defined by Glauber later. The idea of GI was demonstrated in the 1990s by Klyshko and Shih. The first classical simulation of GI was reported in 2004, based on a pseudo-thermal source. Accompanied by the debates on whether the nature of GI is quantum or classical, in-depth developments emerged in both theory and experiment, with the foundation of GI getting more and more solid, leading to different possible applications. The illumination source used includes matter wave and electromagnetic wave covering from microwave to X-ray. The information dimension was also greatly expanded, covering complex fields, phases, 3D locations, polarizations, spectra, times, etc. All these advances enrich the capability of information acquisition with a single-pixel detector under designed illumination. In addition, great efforts have been made to optimize the imaging process of GI, contributing to applications in optical encryption, remote sensing, microscopy, locating and tracking, etc.

Fig. 2. Typical configuration of GI. Two entangled or correlated beams are used. One beam is illuminated on the surface of the object, with the transmitted or reflected light collected via a bucket detector. The other beam is recorded with an array detector (or scanned). The image of the object is reconstructed via certain algorithms, based on the second-order correlation of the source. The reference beam can also be calculated, under controlled modulation, forming a simplified configuration called computational GI.