  Quantum mechanics allows the distribution of intrinsically secure encryption
keys by optical means. Twin-field quantum key distribution is the most
promising technique for its implementation on long-distance fibers, but
requires stabilizing the optical length of the communication channels between
parties. In proof-of-principle experiments based on spooled fibers, this was
achieved by interleaving the quantum communication with periodical adjustment
frames. In this approach, longer duty cycles for the key streaming come at the
cost of a looser control of channel length, and a successful key-transfer using
this technique in a real world remains a significant challenge. Using
interferometry techniques derived from frequency metrology, we developed a
solution for the simultaneous key streaming and channel length control, and
demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique
reduces the quantum-bit-error-rate contributed by channel length variations to
<1%, representing an effective solution for real-world quantum communications.
