  The basic principle of quantum mechanics guarantee the unconditional security
of quantum key distribution (QKD) at the cost of inability of amplification of
quantum state. As a result, despite remarkable progress in worldwide
metropolitan QKD networks over the past decades, long haul fiber QKD network
without trustful relay has not been achieved yet. Here, through
sending-or-not-sending (SNS) protocol, we complete a twin field QKD (TF-QKD)
and distribute secure keys without any trusted repeater over a 511 km long haul
fiber trunk linking two distant metropolitans. Our secure key rate is around 3
orders of magnitudes greater than what is expected if the previous QKD field
test system over the same length were applied. The efficient quantum-state
transmission and stable single-photon interference over such a long distance
deployed fiber paves the way to large-scale fiber quantum networks.
