Kento Maeda

Quantum key distribution (QKD) with conventional optics tools is limited to a linear scaling of the repeaterless bound. Recently, twin field (TF) QKD was conjectured to beat the limit by using an untrusted central station conducting a single-photon interference detection. So far, the effort to prove the conjecture was confined to the infinite key limit which neglected the time and cost for monitoring an adversary’s act. Here we propose a variant of TF-type QKD protocol equipped with a novel monitoring scheme and provide a finite-size-key security proof. We show that the protocol beats the linear bound in a reasonable running time of sending 10^12 pulses, which positively solves the conjecture.