Fig. 1. Dual-arm HOM interferometer.

A pumped type II BBO crystal is used as a source of SPDC photon pairs, which are separated by a polarizing BS (PBS) before being subject to a differential time delay and recombination on separate input ports (1 and 2) of a fiber-coupled 50:50 BS (HOM BS). Coarse control of the optical delay is achieved by a motorized translation stage (HOM stage, controlling τ_HOM), whereas fine control is achieved using a piezo actuator (controlling τ_sample and representing a transparent sample). From a scan of the HOM dip (bottom left, blue), a peak in the Fisher information (red) is identified to be used in the sensing procedure (green). The difference in temporal delay between the two photons (τ: = τ_sample − τ_HOM) is quantified through MLE. Typical measured photon pair rates were on the order of 30,000 counts s⁻¹ with an estimated loss rate of 87%. SPAD, single-photon avalanche photodiodes.