Single-photon-level femtosecond time-resolved measurement by asynchronous optical sampling with dual-wavelength comb

Single-photon time-resolved measurements are of great importance in broad application fields, such as ultrafast phenomena, sensing, and quantum information science. Single-photon detectors have limited, temporal resolution, hence, there is need for novel approaches. In this study, we developed an asynchronous optical sampling technique for single-photon time-resolved cross-correlation measurements using a dual-wavelength comb. Employing slightly different repetition frequencies, high-speed and high-time resolution detection was achieved without the need for a mechanical delay stage. Using distinct-color combs for the signal and pump pulses, highly sensitive detection was achieved by efficiently suppressing the strong background caused by the high-power pump. Furthermore, we experimentally demonstrated femtosecond time-resolved measurements at the single-photon level. The signal and pump pulses were derived from the Er and Yb fiber combs. The center wavelengths of the comb were 1560 and 1050 nm, and their repetition frequencies were 107 and 750 MHz. Signal pulses were attenuated to the single-photon level, and the pump pulses were amplified to 1.3 W. The high power and high repetition frequency of the pump enabled highly efficient nonlinear time gating. Temporal characteristics of a weak signal pulse is obtained by photon counting of the generated sum frequency light of the signal and pump using a nonlinear crystal. We obtained the temporal profiles of the single-photon Er comb pulses as a cross-correlation waveform with a half-width of 173 fs and measured the higher-order chirp of a single-photon femtosecond pulse. The developed technique is promising for single-photon-level ultrafast optical applications.