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Soviet Journal of Quantum Electronics

Excitation of coherent atomic states by strong magnetic field pulses

G. V. Skrotskiǐ, G. I. Solomakho

Published Feb 28, 1975• 0 citations
Publisher

Abstract

An analysis is made of the action of short magnetic field pulses, sufficiently strong to break the hyperfine coupling, on an ensemble of noninteracting atoms. This action alters the polarized states of individual atoms and changes the polarization of the whole ensemble. The ensemble assumes coherent states which correspond to the appearance of nonzero nondiagonal elements in the density matrix. This is manifested by the modulation of resonance fluorescence and may be observed experimentally.

DOI
10.1070/qe1975v005n02abeh010927

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An analysis is made of the action of short magnetic field pulses, sufficiently strong to break the hyperfine coupling, on an ensemble of noninteracting atoms. This action alters the polarized states of individual atoms and changes the polarization of the whole ensemble. The ensemble assumes coherent states which correspond to the appearance of nonzero nondiagonal elements in the density matrix. This is manifested by the modulation of resonance fluorescence and may be observed experimentally.
It is shown that a perturbation of an optically oriented ensemble of atoms by a nonadiabatically applied external magnetic field makes the density matrix of the ground state generally nondiagonal. The appearance of nonzero nondiagonal elements of the density matrix (i.e., the appearance of coherence) between different Zeeman states is manifested not only by the appearance of transverse components of the magnetization but also by the excitation of nonequilibrium higher-order multipole moments. In particular, if the initial state is oriented, a transverse magnetic field pulse produces an oriented nonequilibrium state. A discussion is given of the possibility of optical detection of nonequilibrium states produced in this way and of their relaxation.
Specific features of fluorescence of dense and cold nondegenerate atomic ensembles in an external constant magnetic field are analyzed theoretically. The angular distribution, polarization properties, as well as the spectral composition of fluorescence radiation are calculated. The time variation of these characteristics after the end of the excitation pulse is analyzed. The dependence of the properties of secondary radiation on the duration and carrier frequency of the pulse is investigated. It is shown that, for dense clouds in which the free path length of quasiresonance photons is commensurate with the interatomic distance, the magnetic field significantly modifies all the observable properties of the radiation. Under these conditions, the trapping time may increase by tens of times. Magnetic field enhances the effect of quantum beats observed on time scales commensurate with the lifetime of the excited states of atoms. For individual polarization channels, this field also intensifies the phenomenon of coherent backscattering (CBS). The phenomena found are explained by the effect of magnetic field on the character of resonance dipole–dipole interaction and, as a result, on the specific features of collective phenomena in dense atomic ensembles.
We propose to control light trapping in a large ensemble of cold atoms by an external, static magnetic field. For an appropriate choice of frequency and polarization of the exciting pulse, the field is expected to speed up the fluorescence of a dilute atomic system. In a dense ensemble, the field does not affect the early-time superradiant signal but amplifies intensity oscillations at intermediate times and induces a very slow, nonexponential long-time decay. The slowing down of fluorescence is due to the excitation of spatially localized collective atomic states that appear only under a strong magnetic field and have exponentially long lifetimes. Our results therefore pave a way towards experimental observation of the disorder-induced localization of light in cold atomic systems.
A treatment of resonance fluorescence is given in which the properties of the incident and scattered light are expressed in terms of coherency matrices. Calculations are made for two successive scattering processes corresponding to the experiments described in an accompanying paper. For the first process the scattering atoms are subjected to a magnetic field consisting of parallel static and oscillating components. The fluorescent light is frequency-modulated and may also be polarization-modulated. This light excites fluorescence in a second cell in which the atoms are subjected to a static magnetic field. The equations show under what conditions the second fluorescence is intensity-modulated and describe the resonances in the amplitude of modulation which occur when the modulation frequency matches a Zeeman interval of the excited atoms.