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.
The nonclassical light sources, such as frequency-time entangled photons, are anticipated to offer significant benefits for emerging quantum optical sensing or spectroscopic measurements and manifest on ultrafast time scales (sub-ps to fs). However, the constrained time resolution (ns to ps) of photon-counting detectors poses challenges in comprehensively characterizing their detailed properties on ultrafast time scales. Therefore, we present a novel asynchronous optical sampling (ASOPS) technique utilizing two-color optical frequency combs to demonstrate highly precise and sensitive ultrafast time-resolved cross-correlation measurements at the single-photon level. By employing photon counting statistics, this method successfully reconstructed the picosecond pulse width cross-correlation waveforms at extremely low power level (<1 photon per pulse), while effectively suppressing the residual temporal jitter between the two combs via optically triggered averaging using asynchronous optical sampling of combs. The use of repetition frequency stabilized distinct-wavelength pulses allowed for the effective suppression of strong background light from the pump through spectral filtering, achieving single-photon sensitivity. Subsequently, we parametrically down converted the frequency doubled light from the Er comb in the nonlinear ppKTP waveguide to generate quantum entangled photons at telecom band. A 9.04% Klyshko efficiency with a photon pair generation rate of 0.98 MHz/mW was obtained using heralding detection. Employing the established ASOPS technique to the generated photon pairs enabled the realization of ultrafast time-resolved and quantum mechanical correlation measurements. This paves the way for a versatile and comprehensive manipulation of quantum-entangled photon pairs in the time-domain, with potential applications in ultrafast optical quantum technology and ultrashort fluorescence measurements.
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