A method is proposed for increasing the density of cold atoms. The method is based on pulsed laser irradiation of the atoms in a nonuniform magnetic field. The interaction conditions under which the velocity of an atom is damped to a value that depends only on the magnitude of the magnetic field and the position of the atom at the moment it is irradiated by the laser field are found. The atom completely loses all memory of its initial coordinates and velocity. In a three-dimensional interaction geometry an irradiated atomic ensemble transforms into an ensemble contracting toward the origin. The basic physical processes accompanying the compression of atoms are studied.
A method is proposed for increasing the density of cold atoms. The method is based on pulsed laser irradiation of the atoms in a nonuniform magnetic field. The interaction conditions under which the velocity of an atom is damped to a value that depends only on the magnitude of the magnetic field and the position of the atom at the moment it is irradiated by the laser field are found. The atom completely loses all memory of its initial coordinates and velocity. In a three-dimensional interaction geometry an irradiated atomic ensemble transforms into an ensemble contracting toward the origin. The basic physical processes accompanying the compression of atoms are studied.
We consider the propagation of an atomic beam in a quadrupole magnetic field under transverse irradiation by a cooling laser field. The cooling laser field was chosen in the form of a two-dimensional σ + -σ − configuration. We show that the sub-Doppler resonance in the radiation force can be used to reduce the diameter of the atomic beam to a value on the order of 10 mm. We establish that the simultaneous transverse cooling and compression of the atomic beam allow its phase density to be increased to values of the order of 10 −4 –10 −3 . The dipole interaction of an atom with the cooling and compressing laser field in a quadrupole magnetic field is analyzed in terms of a simple (3 + 5)-level model atom.
We analyze the light-induced atom–atom interactions in optically thick atomic clouds and show that, when the laser frequency is on-resonance with the atomic transition, they become attractive. On the basis of this analysis we propose and demonstrate a novel scheme to compress a cold and dense atomic cloud with a short on-resonance laser pulse. The compression force arises from attenuation of the laser light by the atomic cloud. The following free propagation of the atoms shows a lenslike behavior that yields a transient density increase at the focal time, where neither laser nor magnetic field perturbations exist. A cooling pulse, which is applied at the focal time of this lens, restores the initial temperature of atoms, and hence the phase space density is increased. Finally, we adopt our compression scheme to a quasi-steady-state mode by temporally chopping it with the cooling and trapping beams of a magnet-optical trap.
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.
We present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.
The parameters of a dark magnetooptical trap for rubidium atoms are measured. The rubidium atoms captured and cooled in the trap occupy a hyperfine level of the ground electronic state, which does not interact with cooling laser radiation. The pulsed filling of the trap is produced due to desorption of rubidium caused by irradiation by a short (1 ms) light pulse. The trap captures and cools 2.5 × 10 8 rubidium atoms approximately for 0.2 s. The absorption spectra of a weak probe field by cold atoms are obtained, which demonstrate a good spectral resolution of the hyperfine structure in the excited state. This structure is completely hidden by the Doppler broadening in rubidium vapours at room temperature.
Abstract That the laser cooling and trapping of neutral atoms are interrelated has already been said in the introduction to Chapter 5. We shall, therefore, now consider various techniques whereby cold atoms can be trapped. All the known techniques for trapping laser-cooled neutral atoms can be classified according to a few basic methods. These basic methods are optical trapping using the forces of the electric dipole interaction between atoms and laser fields, magnetic trapping based on the use of the forces of the magnetic dipole interaction, mixed magneto-optical trapping using simultaneous interaction between atoms and magnetic and laser fields, and mixed gravitooptical and gravito-magnetic trapping. Traps for neutral atoms can have a wide variety of geometries and dimensions: (a) macroscopic traps with a size of a »λ, (b) microscopic traps with a size of a ≃λ/2, (c) various combinations of macroscopic and microscopic traps (one-, two-, and three-dimensional) in the case of optical lattices, and finally (d) nanoscopic traps with a size of a «λ/2.