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Journal of Experimental and Theoretical Physics Letters

Multiplex quantum cryptography based on time coding without interferometers

S. N. Molotkov

Published May 1, 2004• 7 citations
Publisher

Abstract

Three fundamentally new experimental prototypes of optical fiber quantum-cryptography schemes are described. They are appreciably simpler than the available prototypes, contain fewer optical fiber components, and do not require adjustment during key generation. These cryptosystems can naturally be called quantum time-shift cryptography. They realize the B92 protocol and use a pair of nonorthogonal single-photon states as information states. One such scheme does not use fiber-optic Mach-Zehnder interferometers and, therefore, provides natural realization of the multiplex secure-key distribution regime.

DOI
10.1134/1.1776241

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Three fundamentally new experimental prototypes of optical fiber quantum-cryptography schemes are described. They are appreciably simpler than the available prototypes, contain fewer optical fiber components, and do not require adjustment during key generation. These cryptosystems can naturally be called quantum time-shift cryptography. They realize the B92 protocol and use a pair of nonorthogonal single-photon states as information states. One such scheme does not use fiber-optic Mach-Zehnder interferometers and, therefore, provides natural realization of the multiplex secure-key distribution regime.
The prototype of a new quantum cryptosystem is described. Such a cryptosystem is naturally called time-shift quantum cryptography. It realizes all basic quantum-cryptography protocols [BB84, B92, BB84(4+2)] in a common fiber-optic system. This scheme does not involve Mach-Zehnder interferometers, which enables one to naturally realize the multiplex mode of secure key distribution and to naturally integrate this quantum-cryptography scheme into traditional fiber-optic telecommunication systems. The proposed time coding method in quantum cryptography makes it possible to significantly simplify experimental schemes and to exclude the finest fiber-optic part, the interferometer. In essence, the fundamental difference of the time coding method from the phase coding method is that the part using phase relations in superposition between “parts” of a quantum state is eliminated from the phase-coding method, and only the part using the time division principle is retained. The time division principle is common for both methods and is minimally necessary, in contrast to the phasecoding method, which can be excluded altogether. The proposed scheme is briefly compared with the two most developed phase-coding schemes (without self-compensation and with passive self-compensation).
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Quantum cryptography is a new, multidisciplinary technique of distributing information to two or more parties in such a way that can guarantee security against an unauthorized eavesdropping attempt. In the presented paper three schemes of implementation of this goal by means of fiber optics, namely a scheme based on photon polarization, delayed interferometry, and quantum correlations of nonorthogonal states, are discussed. Realization of the proposed schemes is discussed as well as fundamental limits imposed on quantum cryptography by fiber-optics technique.
The problem of unconditional security of quantum cryptography (i.e. the security which is guaranteed by the fundamental laws of nature rather than by technical limitations) is one of the central points in quantum information theory. We propose a relativistic quantum cryptosystem and prove its unconditional security against any eavesdropping attempts. Relativistitic causality arguments allow to demonstrate the security of the system in a simple way. Since the proposed protocol does not empoly collective measurements and quantum codes, the cryptosystem can be experimentally realized with the present state-of-art in fiber optics technologies. The proposed cryptosystem employs only the individual measurements and classical codes and, in addition, the key distribution problem allows to postpone the choice of the state encoding scheme until after the states are already received instead of choosing it before sending the states into the communication channel (i.e. to employ a sort of "antedate" coding).
We discuss possible modulation schemes to implement BB84 and B92 quantum key distribution protocols using frequency-coding method. We derive time evolution of quantum states of light in a Mach-Zehnder interferometer based optical intensity modulator. We then show that phase modulator-phase modulator (PM-PM) combination can be used to implement the B92 protocol and intensity modulator-intensity modulator (IM-IM) combination can be used to implement the BB84 protocol. However, the combinations-PM-IM or IM-PM-cannot be used to implement BB84 and B92 protocols using frequency-coding method. Finally, we discuss the security of frequency-coded implementation of B92 protocol.