Relying in part on techniques used to sharpen up images from ground-based telescopes, researchers in China have implemented a type of secure quantum communications protocol—known as measurement-device-independent quantum key distribution (MDI-QKD)—across a wireless link connecting two users nearly 20 km apart in a Shanghai neighborhood.
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 a 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, the team realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness.
Their experiment takes the first step toward 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. (APS Physics)
The paper has been published in Physical Review Letters.
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