Real-time sensing of static displacement and vibrations using HOM interference based quantum sensor

Hong-Ou-Mandel (HOM) interference, bunching of two indistinguishable photons on a balanced beam-splitter, has emerged as a promising tool for quantum sensing. The interference dip-width, thus the spectral-bandwidth of interfering pair-photons, highly influences the resolution of HOM-based sensors. Typically, the pair-photons bandwidth, generated through parametric down-conversion, is increased using bulky and expensive ultrafast lasers, limiting their use outside the lab. Here we show the generation of pair-photons with flexible spectral-bandwidth even using single-frequency, continuous-wave diode laser enabling high-precision, real-time sensing. Using 1-mm-long periodically-poled KTP crystal, we produced egenerate, high-brightness, paired-photons with spectral-bandwidth of 163.42±1.68 nm resulting in a HOM-dip width of 4.01±0.04 µm to measure a displacement of 60 nm, and vibration mplitude of 205 ± 0.75 nm with increment (resolution) of ∼80 nm, and frequency of 8 Hz. Deployment of Fisher-information and maximum likelihood estimator enables optical delay easurement as small as 4.97 nm with precision (Cram´er-Rao bound) and accuracy of 0.89 and 0.54 nm, respectively. The 17× enhancement of Fisher-information for the use of 1 mm crystal over 30 mm empowers the HOMbased sensor achieving any arbitrary precision (say ∼5 nm) in small number of iterations (∼3300) and time (19 minutes); establishing it’s capability for eal-time, precision-augmented, in-field quantum sensing applications.