  Twin-field (TF) quantum key distribution (QKD) could fundamentally alter the
rate-distance relationship of QKD, offering the scaling of a single-node
quantum repeater. Although recent experiments have demonstrated the potential
of TF-QKD, formidable challenges remain for its real world use. In particular,
new methods are needed to extend both the distance beyond 500 km and key rates
above current milli-bit per second values. Previous demonstrations have
required intense stabilisation signals at the same wavelength as the quantum
channel, thereby unavoidably generating noise due to Rayleigh scattering that
limits the distance and bit rate. Here, we introduce a novel dual band
stabilisation scheme based on wavelength division multiplexing that allows us
to circumvent past limitations. An intense stabilisation signal that is
spectrally isolated from the quantum channel is used to reduce the phase drift
by three orders of magnitude, while a second, much weaker reference at the
quantum wavelength locks the channel phase to a predetermined value. With this
strategy, we realise a low noise implementation suitable for all the variants
of TF-QKD protocols proposed so far and capable of generating real strings of
bits for the first time. The setup provides repeater-like key rates over record
communication distances of 555 km and 605 km in the finite-size and asymptotic
regimes, respectively, and increases the secure key rate at long distance by
two orders of magnitude to values of practical significance.
