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Conference on Lasers and Electro-Optics

Free-space quantum cryptography and satellite key uploading

John Rarity, P.M. Gorman, T.E. Wall, P. R. Tapster

Published Aug 29, 2005• 1 citation
Publisher

Abstract

Summary form only given. The possibility of generating and exchanging keys between secure sites is of interest to various commercial and military programmes. Conventional key exchange methods generally utilise public key methods and rely on computational complexity as proof against tempering and eavesdropping. One area where future proofing against the rapid improvements in computational power is absolutely essential is in the upload of keys to satellites. Here we discuss the first tentative steps to an optically based system for uploading keys to remote platforms using polarisation encoding of weak pulses of light. This method relies on the rules of physics, those of quantum mechanics, to guarantee the absolute security of key generation and exchange and is known as quantum cryptography. We have built a breadboard based free space quantum cryptography system. The transmitter uses an attenuated pulsed visible laser diode (635 nm) at 1O MHz repetition rate. The four polarisations 0, 90/spl deg/, 45/spl deg/, 135/spl deg/ encoding key bits in two non-orthogonal measurement bases are selected by acousto-optic switches and beams recombine in a collimating telescope. In the receiver the collected beam is reduced in diameter and split into two beams by a non-polarising 50/50 beamsplitter. The beams are then recombined in a polarisation beamsplitter before detection in one of two photon-counting detectors.

DOI
10.1109/iqec.2000.907966

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Summary form only given. The possibility of generating and exchanging keys between secure sites is of interest to various commercial and military programmes. Conventional key exchange methods generally utilise public key methods and rely on computational complexity as proof against tempering and eavesdropping. One area where future proofing against the rapid improvements in computational power is absolutely essential is in the upload of keys to satellites. Here we discuss the first tentative steps to an optically based system for uploading keys to remote platforms using polarisation encoding of weak pulses of light. This method relies on the rules of physics, those of quantum mechanics, to guarantee the absolute security of key generation and exchange and is known as quantum cryptography. We have built a breadboard based free space quantum cryptography system. The transmitter uses an attenuated pulsed visible laser diode (635 nm) at 1O MHz repetition rate. The four polarisations 0, 90/spl deg/, 45/spl deg/, 135/spl deg/ encoding key bits in two non-orthogonal measurement bases are selected by acousto-optic switches and beams recombine in a collimating telescope. In the receiver the collected beam is reduced in diameter and split into two beams by a non-polarising 50/50 beamsplitter. The beams are then recombined in a polarisation beamsplitter before detection in one of two photon-counting detectors.
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.
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.
As more aspects of modern society depend on digital communication, we increasingly rely on infrastructure that ensures the privacy and security of this communication. Classically, this has been provided by cryptographic protocols such as public-key encryption, in which secrets called keys are exchanged between different parties to enable secure communication. The rapid development of quantum algorithms which violate the assumptions of these protocols, however, poses a security challenge to modern cryptography. \n \nQuantum resources can also be used to strengthen cryptographic security, particularly the security of key exchange protocols. This approach, QKD, can be implemented by encoding in quantum systems such as single photons sent through free-space or a fiber. Fiber based QKD devices are already commercially available, but are fundamentally limited to distributing keys over a few hundred kilometers. To address this distance limitation, research QKD systems are being developed to exchange keys through free-space to satellites. This work considers practical challenges to building and testing both types of QKD devices. \n \nFirstly, we consider modeling and mission analysis for airborne demonstrations of QKD to stratospheric balloons and aircraft to simulate a satellite. Based on the mission parameters available for both platforms, we found aircraft platforms were more promising for testing prototype QKD satellite systems. We developed a mission planning tool to help design the flight geometries for testing the device. \n \nNext, we developed three new components for a QKD satellite prototype. The requirements for electro-optical devices in orbit are very different from lab environments, mandating new approaches to designing QKD devices. We developed a quad single photon detector package to meet the requirements for free-space links to low earth orbit. Moreover, we designed and built optical systems for analyzing the polarization of photons and an adaptive optics unit to increase the efficiency of collecting the encoded photons. All three devices were tested in conditions that simulated the time and loss of a satellite pass. \n \nFinally, we demonstrated a laser damage attack on a live commercial QKD system. Our attack injected additional optical power into the sender device to modify security-critical components. Specifically, our attack damaged the PIN diodes which monitor the encoded photon number, reducing their sensitivity or completely blinding them. Our damage could compromise the entire key, and was performed during system operation while raising no alarms. \n \nIn summary, this work shows the trade-offs of testing QKD payloads on different airborne platforms, develops components for a satellite QKD payload, and demonstrates a security vulnerability in a commercial QKD system that can fully compromise the key. These results help address practical challenges to building QKD devices, improving the security of modern cryptography.
The secure distribution of the secret random bit sequences known as 'key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is a new technique for secure key distribution with single-photon transmissions: Heisenberg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non- orthogonal photon polarization states to generate shared key material over line-of-sight optical links. Key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. We have developed and tested a free-space quantum key distribution (QKD) system over an outdoor optical path of approximately 1 km at Los Alamos National Laboratory under nighttime conditions. Results show that free-space QKD can provide secure real-time key distribution between parties who have a need to communicate secretly. Finally, we examine the feasibility of surface to satellite QKD.