  Quantum communications promise to revolutionise the way information is
exchanged and protected. Unlike their classical counterpart, they are based on
dim optical pulses that cannot be amplified by conventional optical repeaters.
Consequently they are heavily impaired by propagation channel losses, which
confine their transmission rate and range below a theoretical limit known as
repeaterless secret key capacity. Overcoming this limit with today's technology
was believed to be impossible until the recent proposal of a scheme that uses
phase-coherent optical signals and an auxiliary measuring station to distribute
quantum information. Here we experimentally demonstrate such a scheme for the
first time and over significant channel losses, in excess of 90 dB. In the high
loss regime, the resulting secure key rate exceeds the repeaterless secret key
capacity, a result never achieved before. This represents a major step in
promoting quantum communications as a dependable resource in today's world.
