Fig. 1. Schematic diagram of the nonstandalone MDI protocol. PM, phase modulator; Laser, pulsed weak-coherent source; BS, beam splitter; SPD, single-photon detector.

Fig. 2. Experimental setup for the nonstandalone MDI-QKD system. Alice and Bob can implement phase-encoding MDI-QKD and generate secure key with Charlie via BB84. Laser, frequency-locked lasers; IM1, intensity modulator as pulse generator; IM2, intensity modulator as decoy state generator; BS, beam splitter; PM, phase modulator; PS, phase shifter; FM, Faraday mirror; EVOA, electronic variable optical attenuator; EPC, electronic polarization controller; Circ, circulator; SPD, single-photon detector. For Alice, Bob, and Charlie, the combination of one BS, one phase controller, and two FMs constitutes their own AFMI; the other PM is used for phase randomization.

Fig. 3. Virtual network topology and link rates of our system.

Fig. 4. Schematic diagram of the nonstandalone MDI protocol with checkpoints.

Table 1. Code Table in MDI Protocol

Table 2. Code Table in BB84 Protocol

Table 3. Experimental Gains and Quantum Bit Error Rates of Our MDI-QKD System

Table 4. Experimental Gains and Quantum Bit Error Rates of Our BB84 QKD Systems