Long distance free-space quantum key distribution

In the age of information and globalisation, secure communication
as well as the protection of sensitive data against unauthorised
access are of utmost importance. Quantum cryptography currently
provides the only way to exchange a cryptographic key between two
parties in an unconditionally secure fashion. Owing to losses and
noise of today's optical fibre and detector technology, at present
quantum cryptography is limited to distances below a few 100 km. In
principle, larger distances could be subdivided into shorter
segments, but the required quantum repeaters are still beyond
current technology. An alternative approach for bridging larger
distances is a satellite-based system, that would enable secret key
exchange between two arbitrary points on the globe using free-space
optical communication. The aim of the presented experiment was to
investigate the feasibility of satellite-based global quantum key
distribution. In this context, a free-space quantum key
distribution experiment over a real distance of 144 km was
performed. The transmitter and the receiver were situated in 2500 m
altitude on the Canary Islands of La Palma and Tenerife,
respectively. The small and compact transmitter unit generated
attenuated laser pulses, that were sent to the receiver via a 15-cm
optical telescope. The receiver unit for polarisation analysis and
detection of the sent pulses was integrated into an existing mirror
telescope designed for classical optical satellite communications.
To ensure the required stability and efficiency of the optical link
in the presence of atmospheric turbulence, the two telescopes were
equipped with a bi-directional automatic tracking system. Still,
due to stray light and high optical attenuation, secure key
exchange would not be possible using attenuated pulses in
connection with the standard BB84 protocol. The photon number
statistics of attenuated pulses follows a Poissonian distribution.
Hence, by removing a photon from all pulses containing two or more
photons, an eavesdropper could measure its polarisation without
disturbing the polarisation state of the remaining pulse. In this
way, he can gain information about the key without introducing
detectable errors. To protect against such attacks, the presented
experiment employed the recently developed method of using
additional "decoy" states, i.e., the the intensity of the pulses
created by the transmitter were varied in a random manner. By
analysing the detection probabilities of the different pulses
individually, a photon-number-splitting attack can be detected.
Thanks to the decoy-state analysis, the secrecy of the resulting
quantum key could be ensured despite the Poissonian nature of the
emitted pulses. For a channel attenuation as high as 35 dB, a
secret key rate of up to 250 bit/s was achieved. Our outdoor
experiment was carried out under real atmospheric conditions and
with a channel attenuation comparable to an optical link from
ground to a satellite in low earth orbit. Hence, it definitely
shows the feasibility of satellite-based quantum key distribution
using a technologically comparatively simple system.