  Here we present a high-sensitivity, rapid, and low-cost method for methane
sensing based on a nonlinear interferometer. This method utilizes signal
photons generated by stimulated parametric down-conversion (ST-PDC), enabling
the use of a silicon detector to capture high-precision methane absorption
spectra in the mid-infrared region. By controlling the system loss, we achieve
more significant changes in visibility, thereby increasing sensitivity. The
methane concentration within a gas cell is determined accurately. In addition,
ST-PDC enables long-distance sensing and the capability to measure low ambient
methane concentrations in the real world. A low-cost CMOS camera is employed to
capture spatial interference fringes, ensuring fast and efficient detection.
