Photon-pairing-enhanced distributed spectroscopy sensing over 400 km optical fiber

Single-photon detection enables sensitive measurements at ultra-low photon fluxes, supporting long-range sensing and imaging tasks. Photon-counting schemes reconstruct spectra by coherently accumulating interferograms, but phase fluctuations between interfering beams pose challenges for distributed spectroscopy. Here, we introduce a photon-pairing scheme that exploits second-order correlations rather than relying on first-order interference, thereby removing the need for phase alignment of interferograms while enabling long integration times. Our approach enhances the signal-to-noise ratio (SNR) of reconstructed spectra, with SNR increasing linearly with photon count rate, in contrast to the square-root dependence of photon-counting techniques. We accurately reconstruct intensity spectra at an average receiving power of only 5.5 × 10−15 W per comb line over a 400 km fiber link in a distributed configuration, achieving an SNR of over 23.5 dB and demonstrating strong resilience to fiber-induced fluctuations. Our photon-pairing scheme holds strong potential for long-haul dual-comb spectroscopy and opens new avenues for frequency metrology, remote sensing, and quantum networking. Photon-pairing spectroscopy overcomes phase fluctuations to enable sensitive spectral measurements over 400 km of fibre. The study achieves spectral reconstruction at an ultra-low power of 5.5 × 10-15 W per comb line, advancing long-range sensing.