Interferometric phase measurement with two-photon states

We demonstrate experimentally a phase measurement using general two-photon superposition states in a multiport interferometer shown schematically. A pair of photons, produced in the process of spontaneous parametric down-conversion, is transformed into the desired superposition state using beam splitters BS1 and BS2, provided that no photon escapes through the unmonitored port of BS2. In the signal arm, the photon pair experiences losses introduced by the beam splitter BS3 and monitored by a detector C, and subsequently probes the phase shift phi. The phase information is read out by sending the photons onto the beam splitter BS4, whose output ports are monitored by detectors A and B.