Increasing the secure key rate of a quantum-key-distribution system via synchronized detector dead times

Quantum-key-distribution (QKD) protocols that rely on coincidence events registered with two or more detectors are prone to detector saturation effects due to detector dead times. In particular, systems using avalanche photodiodes may incur a reduction in the achievable secret key rate. We demonstrate a detector gating scheme tuned to mitigate these effects but keep the detectors in an otherwise free-running mode. When a first detector is inactive due to a previous detection, other unfavorable detection events in a second detector are blocked, as they would not contribute to the key but just lead to another dead time. We implement this method by synchronizing the detectors’ dead time in our time-bin entanglement QKD system and demonstrate its feasibility for various fiber link distances and detector settings, increasing the sifted key rate by up to 78% with no significant change in the quantum bit error rate.