Stability of coherent states of a Bose gas of two-level atoms in resonant laser field at zero temperature
- PDF / 231,585 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 20 Downloads / 166 Views
LECULES, OPTICS
Stability of Coherent States of a Bose Gas of Two-Level Atoms in Resonant Laser Field at Zero Temperature L. A. Maksimov and A. V. Paraskevov Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia e-mail: [email protected] Received June 30, 2005
Abstract—It is shown that a two-level atom Bose gas in a strong resonant laser field at zero temperature is a mixture of two condensates with a certain density ratio. The stability criteria for stationary states of such a system relative to the increase in the amplitudes of quasi-Bogolyubov elementary excitations of the Bose gas are formulated. It is shown that, in addition to conventional acoustic mode, a mode gap exists with a gap proportional to the laser field amplitude. Under certain conditions, deviations from ideality of the gas may lead to instability and decomposition of the condensate. PACS numbers: 03.75Kk, 03.75.Mn DOI: 10.1134/S1063776106010079
1. INTRODUCTION
the atoms to a dipole-excited state so that each excited atom moves with the same velocity determined by the recoil momentum. If the laser field intensity is so high that spontaneous decay of excited atoms can be ignored as compared to induced emission, a macroscopically large number of dipole-excited atoms moving at the same velocity are formed over a time shorter than that in which excited atoms collide with quasiparticles of the condensate of nonexcited atoms, and a second Bose condensate is formed. We will not describe the formation of this condensate and assume that the state with two condensates has already been formed with the condition that the natural linewidth of the dipole transition is γ ≈ 0 (the criterion of smallness of γ will be given in Section 2). Finally, to simplify analysis of the effect to the maximal possible extent, we assume that the atomic system is spatially homogeneous.
After experimental implementation of the Bose– Einstein condensation of a gas [1] and, subsequently, a binary mixture of Bose gases [2] in magnetic traps at ultralow temperatures, an appropriate theoretical description of the properties of such system is undoubtedly required. This is confirmed by a large number of theoretical publications devoted to classification [3] of all states of a Bose mixture (in the Thomas–Fermi approximation) and its dynamics (long-wave collective excitations [4], metastable states [5], and spatial phase separation of mixture components [6]). In most publications, the mixture is described by two Gross–Pitaevskii equations and the difference between the mixture components is not considered. The representation of mixture components as atoms in the ground and excited states was used in studies devoted to scattering [7] and absorption [8] of laser radiation by a Bose condensate. However, the existence of the stationary coherent state of the interacting system “mixture + laser field” was not considered. The present work is devoted to theoretical analysis of this question. The equilibrium ratio of the densities of the condensates is
Data Loading...
