Measurement-Device-Independent Quantum Key Distribution over 200 km

Measurement-device-independent quantum key distribution (MDIQKD) protocol is immune to all attacks on detection and guarantees the information-theoretical security even with imperfect single-photon detectors. Recently, several proof-of-principle demonstrations of MDIQKD have been achieved. Those experiments, although novel, are implemented through limited distance with a key rate less than $0.1\mathrm{bit}/\mathrm{s}$. Here, by developing a 75 MHz clock rate fully automatic and highly stable system and superconducting nanowire single-photon detectors with detection efficiencies of more than 40%, we extend the secure transmission distance of MDIQKD to 200 km and achieve a secure key rate 3 orders of magnitude higher. These results pave the way towards a quantum network with measurement-device-independent security.

Figure 1

(a) Schematic layout of our MDIQKD setup. Alice’s (Bob’s) signal laser pulses (1550 nm) are modulated into three decoy-state intensities by AM1. An AMZI, an AM2–4, and one PM are used to encode qubits. Charlie’s setup consists of a polarization stabilization system and a BSM system. The polarization stabilization system in each link includes an EPC, a PBS, and a SPAPD. The BSM system includes an interference BS and two SNSPDs. (b) Time calibration system. Two SynLs (1570 nm) are adopted, with the 500 kHz shared time reference generated from a crystal oscillator circuit (COC) and with the time delayed by a programmable delay chip (PDC). Alice (Bob) receives the SynL pulses with a PD and then regenerates a system clock of 75 MHz. WDM: wavelength division multiplexer, ConSys: control system. (c) Phase stabilization system. Circ: circulator, PC: polarization controller, PS: phase shifter.

Figure 2

Secure key rates of experiments in the laboratory as well as the simulation results. The four dots correspond to the experimental results with the fiber transmitting loss of 9.9 dB (50 km), 19.9 dB (100 km), 29.8 dB (150 km), and 39.6 dB (200 km). The solid curve shows the result calculated by simulating the vacuum + weak decoy-state scheme with the experimental parameters. The dashed curve represents the optimal result with infinite number of decoy states.