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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Free-space secure key exchange from 1 m to 1000 km

M. S. Godfrey, A. M. Lynch, J. L. Duligall, William J. Munro, K. J. Harrison, John Rarity

Published Sep 28, 2006• 3 citations
Publisher

Abstract

Coding data bits in the phase or polarisation state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fibre and free-space quantum cryptography apparatus used for secure exchange of keys. To date free space key exchange has aimed at long range with 144km range achieved and future experiments aiming to extend this range to 1000km exchanging keys with low earth orbit satellites. At the other end of the spectrum we are developing low cost hand held systems. These systems could be an effective way for the user to generate a store of secrets shared with a central repository. These secrets can then be used up to protect a wide variety of sensitive classical communications. Examples include on-line PIN protection for consumer transactions and password protection in secure access schemes.

DOI
10.1117/12.690521

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Coding data bits in the phase or polarisation state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fibre and free-space quantum cryptography apparatus used for secure exchange of keys. To date free space key exchange has aimed at long range with 144km range achieved and future experiments aiming to extend this range to 1000km exchanging keys with low earth orbit satellites. At the other end of the spectrum we are developing low cost hand held systems. These systems could be an effective way for the user to generate a store of secrets shared with a central repository. These secrets can then be used up to protect a wide variety of sensitive classical communications. Examples include on-line PIN protection for consumer transactions and password protection in secure access schemes.
Coding data bits in the phase or polarization state of light allows us to exploit the wave particle duality for novel communication protocols. Using this principle the first practical quantum communication systems have been built. These are the fiber and free-space quantum cryptography apparatus used for secure exchange of keys. Beyond this state of the art, various quantum communication schemes are being studied including entangled state key exchange quantum dense coding, state teleportation, and entanglement swapping. The feasibility, advantages and disadvantages of space based realisations of these novel schemes are discussed.
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.
We have designed and built a free space quantum cryptography system. It operates using weak laser pulses with polarization modulation by acousto-optic switching. The system also incorporates full key sifting and error correction between computers connected by a serial port. We have used this system to exchange keys over ranges up to 1.9km with absolute security. The system is robust against transmission (and diffraction) losses up to 20 dB. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.
Quantum key distribution is now a mature quantum communication protocol which allows the verifiably secure sharing of encryption keys between two communicating parties. It seeks to address potential vulnerabilities of data transmission and storage, offering a realistic possibility to share encryption keys which are robust to eavesdropping attacks and future-proof against hacking. Fibre-optic implementations of quantum key distribution currently have a limited practical transmission distance, of less than 400 km, making commercial applications limited. Quantum-specific amplifier/repeater technology is not yet mature enough to increase the transmission distance to achieve global capabilities. Optical fiber is also impractical and expensive for applications where a remote area or moving platform are involved. In recent years, long-distance free-space quantum communications using low-Earth orbit satellites has seen an increase in interest from the academic community as well as from industrial organisations and national research institutes. The source of this new interest was a series of proof-of-principle demonstrations of satellite-based quantum key distribution in 2017. The use of free-space channels implementing airborne or satellite platforms also opens a range of new applications for quantum communications, as they allow coverage of remote areas, moving platforms, and also avoid the requirement of spooling fibre through volatile regions. This presentation will give a general introduction to satellite-based quantum communications, an overview of the field, and discuss future endeavours. The talk will also include an overview of our research into novel photonic technology, such as the use of detector array technology, for the optical ground station receiver.