Empty-Signal Detection: Proof-of-Principle Scheme for Arbitrarily Long-Distance Quantum Communication

Abstract: Secure quantum communication, particularly quantum key distribution (QKD), has remained a central research focus for more than four decades and continues to attract significant attention in recent years. However, due to the unavoidable dark counts in single-photon detectors (SPDs), it has long been constrained by the rapid increase of the quantum bit error rate (QBER) induced by empty signals at the receiver, which prevents it from extending to arbitrarily long distances. To address this challenge, this paper introduces an empty-signal detection (ESD) strategy for quantum communication receivers, which integrates state preparation, controlled gates, and multi-copy analysis into a single block. The core idea is to encode photon-existence information onto an auxiliary degree of freedom (DOF) that is independent of the message-encoding DOF, allowing the receiver, through controlled gates on the auxiliary DOFs and subsequent measurements on the auxiliary copies, to verify the non-emptiness of the received signal without disturbing the transmitted message. In this way, empty signals can be excluded from further communication processing, thereby resolving the QBER explosion problem caused by empty signals. Moreover, the proposed design relies solely on practical, even commercially available, devices, and can be realized under current technological conditions. Therefore, this study provides a framework for achieving arbitrarily long-distance quantum communication by allowing a lossy channel to be asymptotically regarded as lossless in QBER estimations, and offers a proof-of-principle demonstration that secure quantum communications can be extended to arbitrary distances under current technology.