Quantum key distribution with an efficient countermeasure against correlated intensity fluctuations in optical pulses

TL;DR

This paper identifies a security loophole in high-speed GHz-clock QKD systems caused by inter-pulse intensity correlations and proposes an effective countermeasure that enables secure key distribution over long distances.

Contribution

It reveals a previously overlooked security vulnerability in practical high-speed QKD systems and introduces a simple countermeasure that ensures security without major hardware changes.

Findings

Observed inter-pulse intensity correlation in high-speed QKD

Demonstrated a countermeasure that secures keys over 100 km fiber

Validated the effectiveness of the countermeasure experimentally

Abstract

Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices. However, security loopholes of high-speed systems have not been fully explored yet. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. We experimentally observe the inter-pulse intensity correlation and modulation-pattern dependent intensity deviation in a practical high-speed QKD system. Such correlation violates the assumption of most security theories. We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100 km fiber transmission. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems.