Kui-Xing Yang

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.

Satellite-based quantum communication is a promising approach for realizing globalscale quantum networks. However, due to atmospheric turbulence, achieving a highly efficient and stable spatial single-mode receiver, which is very important for daylight free-space quantum key distribution and more complex quantum information tasks involving quantum interference, is difficult. Here, we develop a spatial single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent algorithm (M-SPGD) and conduct a field test of its performance over an 8 km urban terrestrial free-space channel. Our experimental results demonstrate the AO technology based on M-SPGD has a great boosting for single-mode receiver and is useful for large-scale quantum communication.