Atoms In Light Fields Research Articles

Itô – Stratonovich dilemma in the problem of laser cooling of atoms: limits of applicability of the semiclassical approximation

We report a comparative analysis of the limits of laser cooling of atoms in light fields both on the basis of a numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account quantum recoil effects during the interaction of atoms with field photons, and within the framework of the semiclassical approach using various forms of the Fokker – Plank equation. The analysis allows one to outline the limits of applicability of the semiclassical approach, as well as to choose an unambiguous form for introducing the diffusion contribution to the Fokker – Planck equation in the semiclassical description of atomic kinetics.

Quasirelativistic behavior of cold atoms in light fields

We study the influence of three laser beams on the center-of-mass motion of cold atoms with internal energy levels in a tripod configuration. We show that, as for electrons in graphene, the atomic motion can be equivalent to the dynamics of ultrarelativistic two-component Dirac fermions. We propose and analyze an experimental setup for observing such a quasirelativistic motion of ultracold atoms. We demonstrate that the atoms can experience negative refraction and focusing by Veselago-type lenses. We also show how the chiral nature of the atomic motion manifests itself as an oscillation of the atomic internal state population, which depends strongly on the direction of the center-of-mass motion. For certain directions an atom remains in its initial state, whereas for other directions the populations undergo oscillations between a pair of internal states.

Formation of periodical and quasi-periodical structures of atoms in light fields

This manuscript considers the formation of two-dimensional (2D) atomic structures in light monochromatic coherent beams. The shape of atomic pattern has strong dependence on spatial structures of field invariants such as intensity and polarization ellipticity of total light field. For two types of dipole transitions j→ j (with half-integer j) and j→ j+1, the localization sites are possible in vicinities of circular or linear polarization and minima or maxima of intensity depending on the detuning sign. The topology of the sites varies from points to lines in 2D case in accordance with number and layout of laser beams. The presented results of computer modelling within the bounds of semiclassical approach confirm the connection between the atomic structures and the distribution of field areas with circular polarization and extremums of intensity.

Geometric potentials for subrecoil dynamics

Quantum motion of atoms in light fields is described on the basis of adiabatic internal states. Forces arise due to the spatial variation of these states, which is determined by the electric field polarization. In a dark state, these are the only forces present. They are described by a geometric vector and a scalar potential. We give analytical expressions for the geometric potentials in the dark states of a driven $\stackrel{\ensuremath{\rightarrow}}{j}j\ensuremath{-}1$ transition and the dark state in the $\stackrel{\ensuremath{\rightarrow}}{1}1$ system, for arbitrary electromagnetic fields. For systems with velocity selective trapping states, the scalar geometric potential is inversely proportional to the field intensity squared. When the field has nodes the potential diverges. In one dimension, this constitutes an exact realization of the Kronig-Penney model.

Localization of atoms in light fields: Optical molasses, adiabatic compression and squeezing

We present a theoretical study of the localization1 of atoms with an angular momentumJg=3 toJe=4 transition (e.g., chromium atoms) in quantized optical molasses created by two counterpropagating linearly polarized laser beams. We study the localization as a function of the potential depth, the angle between the polarizations and the interaction time with the molasses in the low-intensity limit, and discuss the possibility of adiabatic compression and squeezing of the atomic distribution.

Selection rules and centre-of-mass motion of ultracold atoms

In recent years much attention has been paid to the quantized evolution of the centre-of-mass momentum and position of ultracold atoms in light fields. We consider the effects resulting from the quantization of the external angular momentum variables. We investigate how spin and orbital angular momentum of light are transferred to internal and external angular momentum of an atom in dipole and quadrupole transitions.

Theory of radiation forces and momenta for mobile atoms in light fields.

A theory for a bound system of charges interacting with electromagnetic fields is developed, with special emphasis on the radiation-pressure effects imposed by the fields on the gross motion of the charges. The theory is particularly simple and transparent for the case of two opposite charges of finite mass M in the electric-dipole approximation. This enables a rigorous description to be given for the quantum electrodynamics of the problem with the various interaction terms affecting the gross motion being easily identified. Here, too, it becomes clear that, even in the dipole approximation, the conventional interaction term should be supplemented by the so-called R\ontgen term. Besides ensuring the consistency of the theory for the overall (charges plus fields) system regarding conservation laws, the R\ontgen term has dynamical consequences. This is established by explicit calculations of the expectation values of the mechanical momentum 〈MR\ifmmode \dot{}\else \.{}\fi{}〉 and the radiation pressure force 〈MR\ifmmode\ddot\else\textasciidieresis\fi{}〉 on a two-level atom in the dipole approximation. Results of calculations are displayed for the case of a single light beam and for counterpropagating beams. The feasibility of experimentally observing the transverse force is explored. Generalizations of the well-known friction force arising in one-dimensional optical molasses are given and discussed.

Cooling atoms by means of laser radiation pressure

The present state of research on the cooling of atomic ensembles by means of laser radiation pressure is discussed. First the basic theory of resonance radiation pressure is reviewed. The effect of recoil on the drift and fluctuations of the momentum of an atom interacting with resonance radiation is examined. There is an analysis of the forces acting on an atom in some simple light fields: a traveling plane wave, a Gaussian beam, and counterpropagating Gaussian beams. The review then focuses on the monochromatization of atoms by resonance radiation pressure and related methods for laser cooling of atoms. The roles played by the radiation pressure force and by momentum diffusion in shaping the narrow velocity distributions of the ensemble of atoms are analyzed. The lowest temperature obtainable in the radiation cooling of atoms is estimated. The basic experimental methods for laser cooling of atomic beams are described. Problems of the three-dimensional cooling of atomic ensembles and problems of localizing cold atoms in light fields, steady-state magnetic fields, and electrostatic fields are discussed. The review concludes with a discussion of some applications of cold atoms in high-precision spectroscopy, frequency standards, and atomic physics.