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