Abstract: Channel loss is one of the most severe limitation to extend the transmission distance of quantum key distribution in practice, given that quantum signals cannot be amplified. Many efforts have been made towards the goal of achieving longer-distance QKD. In long terms, quantum repeater-based quantum network may provide an ultimate solution; In short term, both theoretical and experimental progress are made to distribute secure keys in longer distance in practical situation.
In around 15 years ago, the development of decoy-state method improves the key rate of coherent-state based QKD from scaling quadratically to linearly with the channel transmittance. Recently, the development of twin-field QKD (TF-QKD) protocol further improve the secure key rate to scale with the square root of the channel transmittance with coherent state source. Due to the square root scaling, the secure key rate of TF-QKD can be even higher than the repeaterless QKD rate limit of the Takeoka-Guha-Wilde (TGW) bound and, more tightly, the Pirandola-Laurenza-Ottaviani-Bianchi (PLOB) bound.
Here, we experimentally demonstrate the TF-QKD protocol with a distribution distance of 509 km. Two independent laser sources are used in the experiment, to eliminate the potential Trojan horse attacks; fiber spools under practical situation are used, to simulate field deployed environment; Finite key effects are into consideration, with a failure probability of 1E−10; The experiment result not only shows the possible of long distance TF-QKD distribution, but also presents a higher secure key rate than the repeaterless QKD limit at various distances.