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Quantum key distribution (QKD) offers an unconditionally secure means ofcommunication based on the laws of quantum mechanics. Currently, a majorchallenge is to achieve a QKD system with a 40 dB channel loss, which isrequired if we are to realize global scale QKD networks using communicationsatellites. Here we report the first QKD experiment in which secure keys weredistributed over 42 dB channel loss and 200 km of optical fibre. We employedthe differential phase shift quantum key distribution (DPS-QKD) protocolimplemented with a 10-GHz clock frequency, and superconducting single photondetectors (SSPD) based on NbN nanowire. The SSPD offers a very low dark countrate (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 andthus distribute secure keys over channel loss of 42 dB. In addition, weachieved a 17 kbit/s secure key rate over 105 km of optical fibre, which is twoorders of magnitude higher than the previous record, and a 12.1 bit/s securekey rate over 200 km of optical fibre, which is the longest terrestrial QKD yetdemonstrated. The keys generated in our experiment are secure against bothgeneral collective attacks on individual photons and a specific collectiveattack on multi-photons, known as a sequential unambiguous state discrimination(USD) attack.

Quantum key distribution over 40 dB channel loss using superconducting single photon detectors