  Quantum key distribution (QKD) allows two distant parties to share encryption
keys with security based on physical laws. Experimentally, it has been
implemented with optical means, achieving key rates of 1.26 Megabit/s over 50
kilometres (km) of standard optical fibre and 1.16 bit/hour over 404 km of
ultralow-loss fibre in a measurement-device-independent configuration.
Increasing the bit rate and range of QKD is a formidable, but important,
challenge. A related target, currently considered unfeasible without quantum
repeaters, is overcoming the fundamental rate-distance limit of point-to-point
QKD. Here we introduce a conceptually new scheme where pairs of
phase-randomised optical fields are first generated at two distant locations
and then combined at a central measuring station. The fields imparted with the
same random phase are "twins" and can be employed to distil a quantum key, as
we prove under an explicit security assumption. The key rate of this Twin-Field
QKD (TF-QKD) shows the same dependence on distance as a quantum repeater,
scaling with the square-root of the channel transmittance, irrespective of whom
is in control of the measuring station. Differently from a quantum repeater,
however, the new scheme is feasible with current technology and presents
manageable levels of noise even on 550 km of standard optical fibre. This is
promising to overcome the QKD rate-distance barrier and to greatly extend the
range of secure quantum communications.
