Extended Data Fig. 5: Schematic of the 40-MHz transceiver QKD system with multiple lasers.

The transmitter and the receiver are combined in the same terminal, with a channel multiplexing module including a DWDM and two circulators (CIR). They are similar to those of the 625-MHz system. The main difference is that the system operates at a frequency of 40 MHz. The transmitter adopts one laser doide with two different driving signals for decoy-state modulation instead of two, and there is no a FBG for wavelength filtering. The InGaAs/InP detector of the receiver is operated in rectangular-wave gated mode at a frequency of 40 MHz, with a detection efficiency of about 15% at a gate width of 1.6 ns and a coincidence width of 400 ps. Its dark count rate is less than 250 counts per second, and the after pulse probability is less than 1% at a dead time of 5 μs. VOA, variable optical attenuator; LH, laser diode for quantum state |H⟩; LV, laser diode for quantum state |V⟩; LP, laser diode for quantum state |+⟩; LN, laser diode for quantum state |−⟩; DH, detector for quantum state |H⟩; DV, detector for quantum state |V⟩; DP, detector for quantum state |+⟩; DN, detector for quantum state |−⟩.