Quantum key distribution with setting-choice-independently correlated light sources

Despite the enormous theoretical and experimental progress made so far in quantum key distribution (QKD), the security of most existing practical QKD systems is not rigorously established yet. A critical obstacle is that almost all existing security proofs make ideal assumptions on the QKD devices. Problematically, such assumptions are hard to satisfy in the experiments, and therefore it is not obvious how to apply such security proofs to practical QKD systems. Fortunately, any imperfections and security-loopholes in the measurement devices can be perfectly closed by measurement-device-independent QKD (MDI-QKD), and thus we only need to consider how to secure the source devices. Among imperfections in the source devices, correlations between the sending pulses and modulation fluctuations are one of the principal problems, which unfortunately most of the existing security proofs do not consider. In this paper, we take into account these imperfections and enhance the implementation security of QKD. Specifically, we consider a setting-choice-independent correlation (SCIC) framework in which the sending pulses can present arbitrary correlations but they are independent of the previous setting choices such as the bit, the basis and the intensity settings. Within the framework of SCIC, we consider the dominant fluctuations of the sending states, such as the relative phases and the intensities, and provide a self-contained information-theoretic security proof for the loss-tolerant QKD protocol in the finite-key regime. We demonstrate the feasibility of secure quantum communication, and thus our work constitutes a crucial step towards guaranteeing the security of practical QKD systems. A rigorous study on source device security brings practical quantum key distribution (QKD) a step closer to information theoretic security. Existing studies on the security of QKD focus on potential security breaches from imperfect measurement devices, but have overlooked loopholes associated to source imperfections. To tackle this problem, Akihiro Mizutani and co-workers in Japan, Spain and Canada consider the security of an imperfect quantum source that sends pulses with arbitrary correlations, and fluctuations in phase and intensity. They numerically prove that secure quantum communications is feasible provided that these correlations are independent of the choices made for bit, basis and intensity. Their information theoretic security proof with setting-choice-independent correlations in the source is based on practically viable, loss-tolerant QKD in the finite-key regime. The team is confident that their findings will help realize secure quantum communication with practical source devices.