Reduction of the two-photon temporal distinguishability for measurement-device-independent quantum key distribution

Measurement-device-independent quantum key distribution (MDI-QKD) has been proven to protect legitimate users from attacks against measurement devices. The MDI-QKD requires that the two photons arriving at the instrument be indistinguishable. Precise time control is required to eliminate the distinguishability due to differences in photon arrival times. In the conventional methods, the time difference between photons is measured at a measuring instrument (Charlie), and a control signal is transmitted to the users (Alice and Bob). However, this method requires a long feedback loop, and the control may become unstable for long-distance transmission. This article proposes a method in which the photon arrival time difference is detected and controlled at Charlie. The reference signal for the time control is generated by an optical frequency comb in synchronization with the quantum signal. Therefore, the quantum signal photons can also be synchronized by synchronizing the reference signal pulses. A proof-of-principle experiment confirmed that the time synchronization accuracy required for protocol execution could be obtained. This proposal simplifies the implementation of the MDI-QKD.