Measurement device-independent quantum key distribution with passive, time-dependent source side-channels

TL;DR

This paper investigates a time-dependent side-channel in polarization-based MDI-QKD sources, quantifies its impact on key rates, and proposes mitigation strategies, enhancing the security analysis of practical quantum communication systems.

Contribution

It identifies a specific source side-channel in MDI-QKD, applies a numerical proof technique to quantify its effect, and compares protocol robustness, providing broadly applicable mitigation strategies.

Findings

Side-channel significantly affects key rate in practical scenarios

BB84 protocol is more robust than three-state protocol against this side-channel

Mitigation strategies can effectively reduce information leakage

Abstract

While measurement-device-independent (MDI) quantum key distribution (QKD) allows two trusted parties to establish a shared secret key from a distance without needing to trust a central detection node, their quantum sources must be well-characterized, with side-channels at the source posing the greatest loophole to the protocol's security. In this paper, we identify a time-dependent side-channel in a common polarization-based QKD source that employs a Faraday mirror for phase stabilization. We apply the recently developed numerical proof technique from [Phys. Rev. A 99, 062332 (2019)] to quantify the sensitivity of the secret key rate to the quantum optical model for the side-channel, and to develop strategies to mitigate the information leakage. In particular, we find that the MDI three-state and BB84 protocols, while yielding the same key rate under ideal conditions, have diverging results in the presence of a side-channel, with BB84 proving more advantageous. While we consider only a representative case example, we expect the strategies developed and key rate analysis method to be broadly applicable to other leaky sources.