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Bulletin of the Russian Academy of Sciences Physics

Choice of a modulating sequence for code separation of channels in the optical memory based on photon echo

Anatolii N. Leukhin, L. G. Kornilova, A. Yu. Tyukaev

Published Jan 1, 2008• 0 citations
Publisher

Abstract

The possibility of applying of phase-coded sequences in optical-memory systems based on photon echo is considered. Comparative analysis of the characteristics of the efficiency of using noise-like phase-coded signals as reference and reading excitation pulses is performed. Choice of the optimal code sequence is substantiated.
DOI
10.3103/s1062873808010164

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The theory of multi-channel optical memory based on photon echo is developed. It is shown that under long-lived photon echo regime the writing and reading of information with code division is possible using phase modulation of reference and reading pulses. A simple method for construction of a system of noise-like signals, which is based on the segmentation of Frank sequence is proposed. It is shown that in comparison to the system of random biphase signals this system leads to the efficient decreasing of mutual influence of channels and increasing of random/noise ratio under reading of information.
The excitation of the primary and stimulated photon echoes by pulses modulated in phase with Barker codes is modeled. The shape of the complex envelopes of the primary and stimulated echoes is found by solving the optical Bloch equations. Linear and nonlinear excitation modes are considered. In linear modes, it is possible to realize a time delay and form the correlation function of modulated pulses. The results obtained can be used in modeling and analyzing the corresponding algorithms of the photon echo excitation and in calculating the characteristics of signal processors based on the phenomena of photon and spin echoes.
We report the possibility of controlling the distribution of quantum bits within an inhomogeneously broadened line of a resonant transition in recording and transforming information in optical echo-processors. We consider the efficiency of realisation of the elementary logic XOR gate based on a two-pulse excitation of a resonant medium with phase memory. The encoded information is incorporated into the temporal shape of laser pulses in the form of amplitude modulation of an ‘echelon’ of present (‘1’) and absent (‘0’) pulse-codes for obtaining more efficient logic elements that reduce the noise in a quantum communication channel.
An investigation is made of the efficiency of repeated excitation of a sample by laser pulse pairs when data are stored by the photon echo method. Fluctuations of the difference between the pulse phases in different pairs, resulting from instability of the optical path length in the delay line, are shown to prevent cophasal superposition of a set of level-population gratings. The possibility of weakening the negative influence of this effect on the intensity of the read echo signal is considered.
The noise characteristics of the photon echo memory have been investigated. The photon echo memory has the ability to store many bits of information in a diffraction-limited spot, thereby dramatically increasing the storage density. The temporal Fourier transform of the input data sequence is written into the inhomogeneous absorption profile of the recording medium. Data are encoded by means of temporal modulation onto the waveform of a finite duration data beam. Individual bits are not localized to a specific spectral channel; instead, they are stored throughout a region of spectral-addressing space, In order to store and recall the input data accurately, the Fourier transform of the input data sequence must be narrower than the inhomogeneous bandwidth. In the photon echo memory mechanism, there are several factors affecting the system bit error rate such as finite-width write/read pulses, echo efficiency, shot noise, thermal noise, etc. The accuracy of the echo output depends on those system factors. In this paper we formulate a simple model of the photon echo system, and by analyzing this model we derive the relationship between the characteristics of the echo output signal and several factors such as the bandwidth of the system, echo efficiency, atom excited state population fluctuations, shot noise, and thermal noise.