impinges on the high spatial-resolution detector imparts target information—the ghost image—to the cross correlation between the photocurrents obtained from the two detectors, information that is unavailable from either photocurrent alone.

Most ghost-imaging experiments and related theory deal with the transmissive case, in which the bucket detector is placed behind the target, and we image the target’s intensity-transmission profile. In the first ghost imaging experiment, the signal and reference beams were the quantum-mechanically entangled outputs from a spontaneous parametric downconverter (SPDC) [@Pittman1995], which afforded them a phase-sensitive cross correlation stronger than permitted by classical physics [@Erkmen2008Unified]. The system was run at low flux, in which the SPDC’s post-selected output state, within the photodetectors’ response time, could be taken to be a biphoton; the ghost image was then formed by counting coincidences between the bucket and high spatial-resolution detections instead of a photocurrent cross correlation. Subsequently, ghost imaging was done with a high-flux pseudothermal source, i.e., a classical-state source realized by sending a laser beam through a rotating ground-glass diffuser and a 50-50 beam splitter to create identical, spatially-incoherent signal and reference beams with a phase-insensitive cross correlation. The ghost image was then obtained by continuous-time cross correlation of the bucket and 