Optimizing Up-Conversion Single-Photon Detector for Measurements of Femtosecond Time-Bin Pulse and Photon Echo

In a wide range of fields including quantum technology and quantum optics, it is important to measure single-photon-level ultraweak light in the ultrafast range. For measurements with a high signal-to-noise ratio, it is necessary to separate signals from noise in the frequency and/or time domains. However, in conventional electronic single-photon detectors (SPD) such as Si avalanche photodiode (APD) and superconducting nanowire SPD (SNSPD), the dead time and the temporal resolution were limited to 10 ns and 50 ps, respectively. Pulse-pumped frequency up-conversion single-photon detector (UCSPD) is one of the promising candidates to solve these problems. In this method, the signal and pump pulse is incident on a nonlinear crystal, and only the sum-frequency light is extracted by a bandpass filter and detected by a Si APD. By using femtosecond pulses as the pump light, the UCSPD enables dead-time-free measurements with the femtosecond-order temporal resolution. In previous UCSPD research in the ultrafast regime, however, the performance of UCSPD has not been quantitatively evaluated so far [1, 2]. Moreover, ultraweak light measurements from materials using UCSPD have also not been reported so far.<br/>In this study, we developed a UCSPD using commercial PPMgSLT bulk crystals and optimized the experimental conditions such as the crystal length and pump power for measurements of femtosecond single-photon pulses [3]. In addition, we demonstrated the measurements of temporal waveforms of time-bin pulses and photon echo from quantum dots using UCSPD in the femtosecond range.<br/>First, we optimized the performance of the UCSPD. Owing to the group velocity dispersion of the nonlinear crystal, the time delay between the pump and signal pulses is changed as propagating in the PPMgSLT crystal. Therefore, the temporal resolution and up-conversion efficiency strongly depend on the crystal length L. In this study, we estimated the temporal resolution to be 255, 415, 591 fs and the up-conversion efficiencies to be 6.1, 10.1, 10.2 % (the pump power of 300 mW) for L = 1, 2, 3 mm, respectively. By calculating the convolution of the pump and signal waveforms by taking into account the group velocity dispersion, we find that the experimental results of the waveforms and the crystal length dependence of the conversion efficiency can be well explained.<br/>Next, we investigated the dependence of the pump power on the average number of photons at the detection limit. Consequently, the detection limit did not change significantly above 200 mW for any L and reached a minimum value at 300 mW. In this situation, the average number of photons at the detection limit was 8.6, 3.3, 3.1 × 10^-5 /pulse (L = 1, 2, 3 mm, respectively). These results indicate that L = 2 mm and a pump power of 300 mW are the optimal conditions for UCSPD. These results should be important because they provide guidelines for setting conditions for single-photon-level weak light measurements using UCSPD in the femtosecond range.<br/>Finally, as a demonstration for the measurement of emitted light from materials, we measured photon echo light from broadband quantum memory using quantum dots. The stored light was a femtosecond time-bin signal with a pulse interval of 1 ps, which is the shortest time-bin signal measurement ever reported. This technology is expected to enable quantum communication in the femtosecond range, making possible communications that are 1,000 times faster than previous.<br/><br/>Acknowledgments<br/>This work was supported by MEXT Q-LEAP (No. J PMXS0118067395), and CSRN, Keio University. The quantum dot samples were fabricated with the support of the NICT Advanced ICT Device LABO. The authors would like to thank Prof. R. Shimizu from the University of Electro-Communications for the useful discussion.<br/><br/>References<br/>1. O. Kuzucu, et al., Opt. Lett. 33, 2257 (2008).<br/>2. M. Allgaier, et al., Quantum Sci. Technol. 2, 034012 (2016).<br/>3. Y. Kochi, et al., arXiv, 2205.06957 (2022).