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

Simulation of ground-satellite quantum key exchange using a dedicated atmospheric free-space testbed

Paul J. Edwards, Peter Lynam, Colin Cochran, Adrian Blake

Published Feb 3, 2004• 4 citations
Publisher

Abstract

The University of Canberra and its research collaborators have established a dedicated free-space laser communications test-bed between a national telecommunications facility and four selected monitoring sites at distances ranging from 4 to 47 km. This test-bed is currently being used to simulate and characterize the free-space quantum channel along which secure quantum key exchanges could take place between earth stations and low earth orbit satellites, the latter acting as global key couriers. We present the results of these trials conducted under a variety of atmospheric conditions. We use the results to estimate error rates and secure key exchange rates for ground/satellite key exchange systems employing weak Poissonian pulses with more stringent security requirements than are sometimes quoted.

DOI
10.1117/12.505350

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The University of Canberra and its research collaborators have established a dedicated free-space laser communications test-bed between a national telecommunications facility and four selected monitoring sites at distances ranging from 4 to 47 km. This test-bed is currently being used to simulate and characterize the free-space quantum channel along which secure quantum key exchanges could take place between earth stations and low earth orbit satellites, the latter acting as global key couriers. We present the results of these trials conducted under a variety of atmospheric conditions. We use the results to estimate error rates and secure key exchange rates for ground/satellite key exchange systems employing weak Poissonian pulses with more stringent security requirements than are sometimes quoted.
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
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.
Summary form only given. Quantum cryptographic systems employ secret keys exchanged between two parties in the form of a stream of polarisation- or phase-encoded single photons. Under ideal conditions the absolute security of the quantum key is guaranteed by the laws of quantum physics, since any attempt at eavesdropping will introduce gross bit errors which will be quickly detected by the legitimate parties to the key exchange. Secure quantum key distribution (QKD) it currently limited to distances of several tens of kilometres. It has been suggested that key distribution over global distances will require the use of free-space optical intersatellite (O-ISL) and ground to satellite links. Free-space terrestrial paths provide a natural propagation medium in which to investigate the viability of ground to satellite single photon QKD links. We describe the use of an optical free-space range based on a satellite laser ranging observatory instrumented for this purpose.
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.
Extending quantum communication to space environments would enable us to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales. Here, we report on the first experimental study of the conditions for the implementation of the single-photon exchange between a satellite and an Earth-based station. We built an experiment that mimics a single photon source on a satellite, exploiting the telescope at the Matera Laser Ranging Observatory of the Italian Space Agency to detect the transmitted photons. Weak laser pulses, emitted by the ground-based station, are directed toward a satellite equipped with cube-corner retroreflectors. These reflect a small portion of the pulse, with an average of less-than-one photon per pulse directed to our receiver, as required for faint-pulse quantum communication. We were able to detect returns from satellite Ajisai, a low-Earth orbit geodetic satellite, whose orbit has a perigee height of 1485 km.