Differential-phase-shift QKD with practical Mach-Zehnder interferometer

TL;DR

This paper improves the security proof of differential-phase-shift quantum key distribution by accounting for practical imperfections in measurement devices, demonstrating robustness against transmittance fluctuations.

Contribution

It provides a security proof that tolerates transmittance variations in beam splitters, enhancing the practical feasibility of DPS-QKD implementations.

Findings

Key rate degrades only by a factor of 0.57 with ±0.5% transmittance fluctuation

Security proof incorporates realistic device imperfections

Demonstrates feasibility of practical DPS-QKD setups

Abstract

Differential-phase-shift (DPS) quantum key distribution stands as a promising protocol due to its simple implementation, which can be realized with a train of coherent pulses and a passive measurement unit. To implement the DPS protocol, it is crucial to establish security proofs incorporating practical imperfections in users' devices, however, existing security proofs make unrealistic assumptions on the measurement unit using a Mach-Zehnder interferometer. In this paper, we enhance the implementation security of the DPS protocol by incorporating a major imperfection in the measurement unit. Specifically, our proof enables us to use practical beam splitters with a known range of the transmittance rather than the one with exactly $50\%$, as was assumed in the existing security proofs. Our numerical simulations demonstrate that even with fluctuations of $\pm0.5\%$ in the transmittance from the ideal value, the key rate degrades only by a factor of 0.57. This result highlights the feasibility of the DPS protocol with practical measurement setups.