Simultaneous frequency stabilization of two 1550-nm lasers at single-photon level

The twin-field quantum key distribution (TF-QKD) requires that two lasers hundreds of kilometers apart must have the same frequency to achieve the high single-photon interference visibility. This means that both lasers need to be stabilized to the same frequency reference. A simple and robust system is presented for simultaneously stabilizing two 1550-nm lasers at the single-photon level. By utilizing the single-photon multi-frequency modulation technology, both the fiber laser and distributed feedback (DFB) laser are stabilized to the π-phase shifted fiber Bragg grating at the same time. The frequency fluctuations of fiber laser and DFB laser are bounded within 1.39 MHz and 1.53 MHz over 2000 s, respectively. These two frequency-stabilized 1550-nm lasers could be used for TF-QKD.