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

Practical aspects of quantum crypotography

Hugo Zbinden, G. Ribordy, Nicolas Gisin

Published Feb 1, 1998• 0 citations
Publisher

Abstract

We discuss three problems of practical quantum cryptography: continuous alignment, nose of the photon counters and eavesdropping. We present a new self-balanced interferometric setup using Faraday mirrors. This phase- coding setup needs no alignment of the interferometer nor polarization control, and therefore considerably facilitates the experiment. Moreover it features excellent fringe visibility. The importance of the detector noise is illustrated and means of reducing it are presented. Maximal distances and bit rates achievable with present day technologies are evaluated. Finally, practical eavesdropping strategies taking advantages of the optical fiber that could open a gate into the transmitter's receiver's offices are discussed.

DOI
10.1117/12.300904

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We discuss three problems of practical quantum cryptography: continuous alignment, nose of the photon counters and eavesdropping. We present a new self-balanced interferometric setup using Faraday mirrors. This phase- coding setup needs no alignment of the interferometer nor polarization control, and therefore considerably facilitates the experiment. Moreover it features excellent fringe visibility. The importance of the detector noise is illustrated and means of reducing it are presented. Maximal distances and bit rates achievable with present day technologies are evaluated. Finally, practical eavesdropping strategies taking advantages of the optical fiber that could open a gate into the transmitter's receiver's offices are discussed.
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We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. Use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23km-long interferometer, based on installed telecom fibers. In so-called private-key cryptographic systems, secure transmission through unprotected channels rely on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob. Quantum cryptography (QC) relies on the properties of quantum mechanics to obtain a provably secure key distribution [1, 2, 3, 4]. Most exisiting implementations rely on either the polarization [2, 5, 6] or the phase [7, 8, 9, 10] of very weak pulses of light as information carrier. To date, the longest transmission spans were obtained in optical fibers, at a wavelength of 1300 nm [6, 9, 10]. The main difficulty with polarization-based systems is the need to keep stable polarizations over distances of tens of kilometers, in standard telecom cables. Indeed, due to the birefringence of the fibers and the effect of the environment, the output polarization fluctuates randomly. Recent experiments [6]
Work presents the brief review of the contemporary state of quantum cryptography science. Main principles of the quantum cryptography protocol development are illustrated by giving the example of BB84 protocol. Examples of experimental setups for the quantum key distribution through the atmosphere and fiber quantum channel are described in this work. The work presents results of experiments conducted on the developed setup based on single photon polarization coding. Problem of single photon detection in the visible and near IR area is considered. Work describes developed fiber based quantum cryptography working with the telecom wavelength 1555 nm. With the use of this experimental setup using the 25 km single mode telecom fiber (1555 nm wavelength) between sender and receiver we achieved the 450 bit/s quantum key generation with the use of 5 MHz pulse repetition rate, ~0.2 photon/pulse. Given the achieved photodetector parameters an average quantum bit error rate is less than 3.7%.
A new scheme of experimental quantum cryptography on the nonorthogonal states is described. Nonorthogonality is achieved by the time shift of states in different messages. For this scheme to be efficient, it is sufficient to balance the arms of an interferometer at the receiver and transmitter ends to an accuracy of 1–2 cm. This is a fundamental advantage of this scheme over, e.g., the most developed cryptosystem based on phase coding, where the maximum transmission distance has been achieved. In the latter system, the arms of the interferometer need to be balanced with an accuracy of fractions of a micron for a distance of several tens of kilometers.