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Journal of the Optical Society of America B

Differential-phase-shift quantum key distribution with intense decoy pulses

Kyo Inoue, Toshimori Honjo

Published Aug 7, 2025• 1 citation
Publisher

Abstract

In quantum key distribution (QKD), eavesdropping is generally detected based on bit errors resulting from eavesdropper attacks. This study proposes a QKD scheme for detecting eavesdropping based on the photon-detection rate instead of the bit error rate. The differential-phase-shift (DPS) QKD protocol, which transmits lasting weak coherent pulses, is modified such that decoy pulses with a large mean photon number are randomly inserted into a signal pulse sequence. With this modification, eavesdropping can be detected by monitoring the photon-detection rates at the time slots around the decoy pulses. The key-creation performance of the modified DPS-QKD system is evaluated, and the results indicate that the QKD distance is increased with the modification.

DOI
10.1364/josab.564939

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In quantum key distribution (QKD), eavesdropping is generally detected based on bit errors resulting from eavesdropper attacks. This study proposes a QKD scheme for detecting eavesdropping based on the photon-detection rate instead of the bit error rate. The differential-phase-shift (DPS) QKD protocol, which transmits lasting weak coherent pulses, is modified such that decoy pulses with a large mean photon number are randomly inserted into a signal pulse sequence. With this modification, eavesdropping can be detected by monitoring the photon-detection rates at the time slots around the decoy pulses. The key-creation performance of the modified DPS-QKD system is evaluated, and the results indicate that the QKD distance is increased with the modification.
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A photon-number-splitting (PNS) attack against differential-phase-shift (DPS) quantum key distribution (QKD) is described. In the conventional 1984 Bennett-Brassard protocol, using weak laser light, the PNS attacks, which involve installing a lossless transmission line and blocking pulses from which extra photons cannot be picked up, impose a limit on the transmission distance. In contrast, use of a coherent pulse train in DPS QKD prevents the PNS attack and removes the distance limitation imposed by it. We carried out a DPS QKD experiment that simulated the situation where some pulses are blocked. The result showed that extra bit errors are induced in an eavesdropped condition, indicating the robustness of DPS QKD against PNS attacks.
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Differential-phase-shift (DPS) quantum key distribution (QKD) is a type of QKD protocol. It is a unique QKD scheme wherein a lasting coherent pulse sequence containing numerous photons is transmitted as an information carrier instead of qubits in traditional QKD protocols. Regarding the security of DPS-QKD, a general individual attack (GIA), which first conducts a beam-splitting attack and then entangles probe states with each photon in a pulse sequence after beam splitting, has been presented and is usually assumed to evaluate the DPS-QKD system performance. Thus, this study reconsidered the GIA scheme, examined its feasibility, and consequently modified it to render it feasible. A formula for the secret key creation rate, assuming a modified GIA, was formulated. Calculations using the presented and conventional formulas indicated that while the key creation rates in systems assuming conventional and modified GIAs were approximately the same, the modified GIA resulted in a higher key creation rate in short- and medium-distance systems to a certain extent.