Nonlinear quantum spectroscopy with parity–time-symmetric integrated circuits

We propose a novel quantum nonlinear interferometer design that incorporates passive PT symmetric coupler sandwiched between two sections where signal-idler photon pairs are generated. The PT-symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates sensing losses in second waveguide exposed to analyte under investigation, whose absorption can be inferred by measuring only signal intensity at shorter wavelength detectors readily available. Remarkably, we identify new phenomenon sharp fringe shift critical loss values, which is distinct from previously studied breaking. discuss how such unconventional properties arising interference provide route enhancing analytes facilitate broadband spectroscopy applications integrated photonic platforms.