Sensitive and fast mid‐infrared (MIR) spectroscopy is highly attractive in a variety of applications including astronomical observation, pharmaceutical synthesis, and environmental monitoring. However, the performance of conventional MIR spectrometers has long been hindered by the limited sensitivity of narrow‐bandgap detectors and/or the deficient brightness of broadband light sources. Here, an ultra‐sensitive and broadband MIR upconversion spectrometer, which integrates a supercontinuum source covering 1.5–4.2 m based on a silicon nitride nanophotonic waveguide, is devised and integrated. High‐efficiency and low‐noise nonlinear frequency upconversion is realized based on coincidence pulsed pumping with spectro‐temporal optimization, which enables leverage of silicon detectors for facilitating MIR single‐photon spectroscopy at 0.2 photons/nm/pulse. Furthermore, the upconversion‐based array spectrometer is manifested with high‐speed spectral acquisition rates beyond 200 kHz, which is about tenfold faster than the state‐of‐the‐art scan rates for FTIR‐based spectrometers at a comparable spectral resolution. The achieved features of broadband spectral coverage, single‐photon sensitivity, and sub‐MHz refreshing rate might open up new possibilities for infrared transient spectral measurements in combustion analysis, high‐throughput sorting, and reaction tracking, among others.

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