Broadband photon-counting dual-comb spectroscopy with attowatt sensitivity over turbulent optical paths.

Photon-counting dual-comb spectroscopy (DCS) opens new possibilities for deploying DCS in scenarios previously constrained by limited detection sensitivity. However, inevitable optical path fluctuations hinder its practical implementation. Here, we propose a method to ensure the long-term stability of photon-counting DCS, overcoming turbulent optical paths, achieving attowatt-level detection sensitivity and quick acquisition times. Using a compact all-fiber dual-comb system, we achieve 20 nm broadband DCS of HCN across the C-band with an average detected power of only 4 attowatts per-comb line. Despite significant vibrations throughout measurements, the spectra maintain comb-line resolution and shot-noise-limited signal-to-noise ratios. Additionally, the system demonstrates successful deployment in open-path measurements, overcoming 93 dB attenuation. Our approach enables remote sensing of CO, HO, and HDO over a continuous 20-h observation period. This method highlights the potential for applications in fields such as metrology, quantum physics, and atmospheric sensing, especially in turbulent environments like open air or water, within a field-deployable system.