  Recent years have seen the development of quantum sensing concepts utilizing
nonlinear interferometers based on correlated photon pairs generated by
spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency
degeneracy allows a 'division of labor' between the mid-infrared photon for
strongest sample interaction and the correlated near-infrared photon for
low-noise detection. The small number of photons provided by SPDC and the
resulting inferior signal-to-noise ratio are, however, a limiting factor
preventing wide applicability of the novel sensing concept. Here, we
demonstrate a nonlinear interferometer based on pump-enhanced SPDC with
strongly improved emission rates, but maintaining broadband, spontaneous
emission. For validation of the concept, we demonstrate high-resolution
mid-infrared spectroscopy with near-infrared detection, showcasing the improved
accuracy. Although the number of mid-infrared photons is about five orders of
magnitude smaller than in classical spectrometers, the sensitivity of the
quantum spectrometer becomes comparable, marking an essential step toward
real-world applications.
