Four-Photon Superposition and Its Application to Quantum Imaging

We present a quantum interference phenomenon in which four-photon quantum states generated by two independent sources are used to create a two-photon interference pattern without detecting two of the four photons. In contrast to standard two-photon interference, the interference patterns produced in our case do not depend on the tunable spatially-independent interferometric phase. Furthermore, the patterns contain information of spatially-dependent phases acquired by the undetected photons. This phenomenon paves the path of a conceptually new quantum imaging technique that (1) allows image acquisition without detecting the photons probing an object, and (2) is immune to the time-dependent fluctuations of the interferometric phase. This technique can be used to retrieve object-information at a wavelength for which detectors are not available. It will be useful for imaging with low-intensity light, especially when conventional interferometry-based imaging techniques fail due to uncontrollable phase fluctuations.