Demonstration of interferometric imaging using a path-entangled single photon

Very-long baseline interferometry is one of the major astronomical imaging techniques used for tasks ranging from measuring diameters of stars to imaging black holes at the center of galaxies. In 2011, a new quantum protocol was developed for visible wavelength stellar interferometry where, instead of a high-powered coherent state, a path-entangled single photon is interfered with the light collected by the apertures as a phase reference and the coherence is determined through joint measurements. This eliminates the shot noise due to the common phase reference and, utilizing a quantum network, can reduce the loss associated with routing the collected fields. We demonstrate a proof-of-principle, table-top experiment of this new type of stellar interferometer by interfering a path-entangled single photon generated from parametric down conversion and the light collected from a single spectral-temporal mode, quasi-thermal source representing an astronomical source. The interference signal was used to recover the spatial autocorrelation of double-slit source distributions. We compare this to a theoretical model and see good agreement, allowing further comparison to other weak, non-single-photon, local-oscillator sources such as coherent states.