Weijun Zhang

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step towards satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

Channel loss is one of the most severe limitation to extend the transmission distance of quan- tum key distribution in practice. The twin-field quantum key distribution can achieve a much longer transmission distance with improving the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. Here we demonstrate the real-optical-fibre experimental results of twin-field quantum key distribution through the sending-or-not-sending protocol, which is fault tolerant to large misalignment error. The phase locking technology developed in the frequency transfer field is adopted to ensure Alice’s and Bob’s source wavelengths are locked to each other. Phase reference pulses are used to monitor the phase difference between the channel. Further with a high performance single photon detector, we obtain the positive key rates for different distances, specifically, the obtained secure key rate at 150 km is higher than that of the measurement device independent QKD.