  Quantum key distribution (QKD) based on coherent states is well known for its
implementation simplicity, but it suffers from loss-dependent attacks based on
optimal unambiguous state discrimination. Crucially, previous research has
suggested that coherent-state QKD is limited to short distances, typically
below 100 km assuming standard optical fiber loss and system parameters. In
this work, we propose a six-coherent-state phase-encoding QKD protocol that is
able to tolerate the total loss of up to 38 dB assuming realistic system
parameters, and up to 56 dB loss assuming zero noise. The security of the
protocol is calculated using a recently developed security proof technique
based on semi-definite programming, which assumes only the inner-product
information of the encoded coherent states, the expected statistics, and that
the measurement is basis-independent. Our results thus suggest that
coherent-state QKD could be a promising candidate for high-speed
provably-secure QKD.
