Ji-Gang Ren

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.

Hong-Ou-Mandel interference is essentially a quantum mechanical phenomenon, which is a key part in quantum communication protocols and linear optical quantum computation, especially for measurement-device-independent quantum key distribution. The main challenge of the two-photon interference is to keep the indistinguishability of the two photons. This requirement becomes much more crucial on a large scale and even in free space, where both fluctuations and photon loss overwhelm the fragile interference phenomenon. Here we report a field test of the Hong-Ou-Mandel interference with one arm being a 220 m free-space path. Our experiment is a step towards long distance free-space measurement-device-independent quantum key distribution.