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

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