Extended Data Fig. 4: Schematic of the 625-MHz QKD system.

On Alice’s side, four signal lasers (S) and four decoy lasers (D) are combined via polarization-maintaining filter couplers (PMFCs), and then combined again into a single PMFC (with 45° difference for fibre-axis inputs), before outputting through a single-mode fibre coupler (SMFC). One beam is for testing; another goes to the optical circulator associated with a 10G fibre Bragg grating filter (FBG) before monitoring and attenuating with two cascaded MEMS attenuators. The synchronization laser (SYN) goes through a dense wavelength-division multiplexing (DWDM) and then a MEMS attenuator, before splitting for monitoring and outputting to the communication channel. On Bob’s side, the signal light enters first a DWDM and then a SMFC, with one set of electrically driven polarization controllers (EPCs) consisting of three components in every path, before single-mode polarizing beam splitters (SMPBSs) detect the four different signal states. The synchonization laser also propagates via a DWDM and is detected by a PIN photodetector. PMPBS, polarization-maintaining polarizing beam splitter; Com, common port for the coarse wavelength-division multiplexing (CWDM) device; Pass, pass port for the CWDM device; Ref, reflect port for the CWDM device; H, quantum state |H⟩; V, quantum state |V⟩; P and +, quantum state |+⟩; N and −, quantum state |−⟩.