Abstract: Quantum key distribution (QKD) allows two distant users to establish a common secret string of bits by sending photons across a communication line, often an optical fibre. The photons, however, are scattered by the propagation medium and have only a small probability of reaching the end of the line, which limits the QKD key rate and transmission range. A rigorous theorem limits to 1.44η the number of secure bits delivered by a point-to-point QKD line, with η the channel transmission probability. This limit, known as the ‘repeaterless secret key capacity’ (or PLOB bound), can be overcome only by QKD systems with a secret key rate (SKR) over distance scaling that improves upon the linear regime.
Quantum repeaters are considered to be the most promising solution to this problem, but are currently still under development and cannot yet support stable and reliable long-distance operation. ‘Twin-Field’ (TF) QKD protocol offers an alternative way to extend the current QKD range, featuring a SKR that scales proportionally to the square root of the channel loss, similarly to a QKD system equipped with a single quantum repeater. Remarkably, TF-QKD is feasible with current technology.
In the work presented here, we develop an experimental setup that allows us to establish the first fibre-based secure quantum communication beyond the 600km and 100dB barriers. This result is enabled by our dual-band phase stabilisation system (based on wavelength division multiplexing) which improves both the maximum communication distance and SKR of TF-QKD experimental implementations. We believe this stabilisation technique will be relevant to other quantum communications applications.