The usual method of viewing an object involves photons bouncing directly off the object and then making their way to your eye (or perhaps a photodetector), where an image is formed and stored. Ghost imaging, in contrast, builds an image with photons that have never interacted with the object. Two correlated beams of light are used, one of which interrogates the object while the other heads straight to the detector. If the beams are entangled, a perfect image can be obtained. For stealth imaging, however, thermal (that is, incoherent) light would be preferred-the object would then experience no sign of being under surveillance. With thermal light, an image is assembled by correlating the intensities of signal and reference photons at a pixelated detector and by then subtracting a background.
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