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

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. A potential security loophole and its countermeasure have been discovered in practical implementations of high-speed quantum key distribution (QKD). Ken-ichiro Yoshino from NEC corporation and a team of researchers from Japan investigated the intensity fluctuations of optical pulses in a GHz-clocked QKD system, revealing that the limited bandwidth of the ultrafast optical transmitter’s electronics generates deviations from the ideal signal. These perturbations have been shown to carry signatures of previous modulation patterns - effectively introducing correlations between individual pulses. As the strength of QKD relies on its proof-of-principle security, which in many cases is derived under the assumption of independent pulses, these correlations constitute a loophole that might compromise the whole protocol. Fortunately, the researchers developed two countermeasures: pattern sifting and alternate key distillation, which recover security and do not impact performances too severely.