  Twin-field (TF) quantum key distribution (QKD) has rapidly risen as the most
viable solution to long-distance secure fibre communication thanks to its
fundamentally repeater-like rate-loss scaling. However, its implementation
complexity, if not successfully addressed, could impede or even prevent its
advance into real-world. To satisfy its requirement for twin-field coherence,
all present setups adopted essentially a gigantic, resource-inefficient
interferometer structure that lacks scalability that mature QKD systems provide
with simplex quantum links. Here we introduce a novel technique that can
stabilise an open channel without using a closed interferometer and has general
applicability to phase-sensitive quantum communications. Using locally
generated frequency combs to establish mutual coherence, we develop a simple
and versatile TF-QKD setup that does not need service fibre and can operate
over links of 100 km asymmetry. We confirm the setup's repeater-like behaviour
and obtain a finite-size rate of 0.32 bit/s at a distance of 615.6 km.
