Twin-field quantum key distribution elevates the secure key rate from a linear to a square-root dependence on channel loss and preserves the measurement-device-independent security. This protocol is uniquely positioned to enable global-scale quantum networks, even under extreme channel loss. Although fibre-based twin-field quantum key distribution implementations have advanced rapidly since its proposal, free-space realizations have remained elusive due to atmospheric-turbulence-induced phase distortions. Here we report an experimental demonstration of free-space twin-field quantum key distribution over 14.2-km urban atmospheric channels, surpassing the effective atmospheric thickness—a critical threshold for satellite compatibility. We achieve a secret key rate exceeding the repeaterless capacity bound, a milestone for practical quantum communication. Our approach eliminates the need for an auxiliary channel to stabilize a closed interferometer, instead leveraging open-channel time and phase control of optical pulses. This work represents a pivotal advance towards satellite-based global quantum networks, combining high-speed key distribution with inherent resistance to real-world channel fluctuations.

Source:https://www.nature.com/articles/s41566-026-01910-6