Ghost Imaging & Quantum Correlations

Ghost Imaging: Spontaneous parametric down-conversion (SPDC) produces entangled photon pairs — signal and idler — with correlated momenta (positions). The signal photon passes through an object detected only by a bucket (single-pixel) detector. The idler photon goes to a spatially-resolving detector that never sees the object. By post-selecting on coincidences (both detectors fire simultaneously), the spatial correlations of the entangled pairs reconstruct an image of the object. The reference arm alone shows only the beam profile; the coincidence image reveals the object's transmission function. Classical thermal light can also produce ghost images via HBT correlations, but quantum entanglement enables higher visibility and sub-shot-noise imaging.