  Quantum key distribution endows people with information-theoretical security
in communications. Twin-field quantum key distribution (TF-QKD) has attracted
considerable attention because of its outstanding key rates over long
distances. Recently, several demonstrations of TF-QKD have been realized.
Nevertheless, those experiments are implemented in the laboratory, remaining a
critical question about whether the TF-QKD is feasible in real-world
circumstances. Here, by adopting the sending-or-not-sending twin-field QKD
(SNS-TF-QKD) with the method of actively odd parity pairing (AOPP), we
demonstrate a field-test QKD over 428~km deployed commercial fiber and two
users are physically separated by about 300~km in a straight line. To this end,
we explicitly measure the relevant properties of the deployed fiber and develop
a carefully designed system with high stability. The secure key rate we
achieved breaks the absolute key rate limit of repeater-less QKD. The result
provides a new distance record for the field test of both TF-QKD and all types
of fiber-based QKD systems. Our work bridges the gap of QKD between laboratory
demonstrations and practical applications, and paves the way for intercity QKD
network with high-speed and measurement-device-independent security.
