  Quantum key distribution (QKD) offers an unconditionally secure means of
communication based on the laws of quantum mechanics. Currently, a major
challenge is to achieve a QKD system with a 40 dB channel loss, which is
required if we are to realize global scale QKD networks using communication
satellites. Here we report the first QKD experiment in which secure keys were
distributed over 42 dB channel loss and 200 km of optical fibre. We employed
the differential phase shift quantum key distribution (DPS-QKD) protocol
implemented with a 10-GHz clock frequency, and superconducting single photon
detectors (SSPD) based on NbN nanowire. The SSPD offers a very low dark count
rate (a few Hz) and small timing jitter (60 ps full width at half maximum).
These characteristics allowed us to construct a 10-GHz clock QKD system and
thus distribute secure keys over channel loss of 42 dB. In addition, we
achieved a 17 kbit/s secure key rate over 105 km of optical fibre, which is two
orders of magnitude higher than the previous record, and a 12.1 bit/s secure
key rate over 200 km of optical fibre, which is the longest terrestrial QKD yet
demonstrated. The keys generated in our experiment are secure against both
general collective attacks on individual photons and a specific collective
attack on multi-photons, known as a sequential unambiguous state discrimination
(USD) attack.
