Li Qian

The twin-field (TF) quantum key distribution (QKD) protocol and its variants are highly attractive because they can beat the well-known fundamental limit of secret key rate for point-to-point (point-to-point bound) QKD without quantum repeaters. In this paper, we perform a proof-of-principle experimental demonstration of TF-QKD based on the protocol proposed by Curty et al., which removes the need for post-selection on the matching of a global phase from the original TF-QKD. Furthermore, we employ a Sagnac loop structure to overcome the major difficulty in the practical implementation of TF-QKD, namely, the need to stabilize the phase of the quantum state over kilometers of fiber. The experimental results show that the secret key rate of TF-QKD at high loss region can surpass the point-to-point bound of QKD with current technology.

Twin-field quantum key distribution (TFQKD) systems have shown great promise for implementing practical long-distance secure quantum communication due to its measurement-device-independent nature and its ability to offer fundamentally superior rate-loss scaling than conventional point-to-point QKD systems. A surge of research has produced many variants of protocols and experimental demonstrations. To make TFQKD more applicable in quantum communication, a study of TFQKD in a networking setting is essential. In this work, we experimentally demonstrate a proof-of-principle Sagnac-interferometer based TFQKD network with three users and one untrusted central node. We show that our network enables users to share secure keys with channel losses up to 58dB, and channel loss asymmetric up to 15dB. In some cases, the secure key rates still beat the rate-loss bounds for conventional point-to-point repeaterless QKD systems. It is to our knowledge the first multi-user-pair TFQKD network demonstration, an important step in advancing quantum communication network technologies.

We identify a time-dependent passive source side-channel in common measurement-device-independent quantum key distribution implementations that rely on Faraday mirrors for stable phase modulation. We model the time-dependence of the side channel and use this information in conjunction with a recently developed numerical security proof technique based on semidefinite programming to quantify the impact on the secure key rate of the protocol. We explore the sensitivity of security to the parameters of the side channel and the choice of model for the signal.