Phase-Reference-Free Experiment of Measurement-Device-Independent Quantum Key Distribution

Measurement-device-independent quantum key distribution (MDI QKD) is a substantial step toward practical information-theoretic security for key sharing between remote legitimate users (Alice and Bob). As with other standard device-dependent quantum key distribution protocols, such as BB84, MDI QKD assumes that the reference frames have been shared between Alice and Bob. In practice, a nontrivial alignment procedure is often necessary, which requires system resources and may significantly reduce the secure key generation rate. Here, we propose a phase-coding reference-frame-independent MDI QKD scheme that requires no phase alignment between the interferometers of two distant legitimate parties. As a demonstration, a proof-of-principle experiment using Faraday-Michelson interferometers is presented. The experimental system worked at 1 MHz, and an average secure key rate of 8.309 bps was obtained at a fiber length of 20 km between Alice and Bob. The system can maintain a positive key generation rate without phase compensation under normal conditions. The results exhibit the feasibility of our system for use in mature MDI QKD devices and its value for network scenarios.

Figure 1

Diagram of our RFI MDI QKD system. Laser, continuous-wave laser; PC, polarization controller; EPC, electronic polarization controller; IM, intensity modulator for wave chopping; VOA, variable optical attenuator; BS, beam splitter; PM, phase modulator; FM, Faraday mirror; FMI: Faraday-Michelson interferometer; FTR, band pass filter; DET, detector.

Figure 2

$C$ value and secure key rate over time.