Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution Over 509 km

TL;DR

This paper demonstrates a secure twin-field quantum key distribution over 509 km using independent lasers, surpassing traditional limits and enabling practical long-distance quantum communication.

Contribution

It introduces an experimental implementation of TF QKD with independent lasers over record distances, achieving higher secure key rates than previous methods.

Findings

Secure key rates over 5 times higher than the repeaterless QKD limit.

Successful long-distance distribution over 509 km fiber.

Use of independent lasers with remote frequency locking.

Abstract

Twin field quantum key distribution promises high key rates at long distance to beat the rate distance limit. Here, applying the sending or not sending TF QKD protocol, we experimentally demonstrate a secure key distribution breaking the absolute key rate limit of repeaterless QKD over 509 km, 408 km ultra-low loss optical fibre and 350 km standard optical fibre. Two independent lasers are used as the source with remote frequency locking technique over 500 km fiber distance; Practical optical fibers are used as the optical path with appropriate noise filtering; And finite key effects are considered in the key rate analysis. The secure key rates obtained at different distances are more than 5 times higher than the conditional limit of repeaterless QKD, a bound value assuming the same detection loss in the comparison. The achieved secure key rate is also higher than that a traditional QKD protocol running with a perfect repeaterless QKD device and even if an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve high secure key rate at long distribution distance, and hence practically useful for field implementation of intercity QKD.