Far-off Resonance Research Articles

Quantum chaos in atom optics

I shall discuss the quantum and classical dynamics of a class of nonlinear Hamiltonian systems. The discussion will be restricted to systems with one degree of freedom. Such systems cannot exhibit chaos, unless the Hamiltonians are time dependent. Thus we shall consider systems with a potential function that has a higher than quadratic dependence on the position and, furthermore, we shall allow the potential function to be a periodic function of time. This is the simplest class of Hamiltonian system that can exhibit chaotic dynamics. I shall show how such systems can be realized in atom optics, where very cord atoms interact with optical dipole potentials of a far-off resonance laser. Such systems are ideal for quantum chaos studies as (i) the energy of the atom is small and action scales are of the order of Planck's constant, (ii) the systems are almost perfectly isolated from the decohering effects of the environment and (iii) optical methods enable exquisite time dependent control of the mechanical potentials seen by the atoms.

Transferring laser-cooled atoms to a spatially separatedmagnetic trap using a far-off resonance optical guide

The size of alkali vapour Bose-Einstein condensates is determined by thenumber of atoms which can be collected in a magneto-optical trap. Thisnumber is limited by processes involving near-resonant light. A possibletechnique to exceed the number limit is to load a conservative trap whichis spatially separated from the laser cooling region. Here we report on anexperiment where cold atoms are transferred from a magneto-optical trapto a spatially separated magnetic trap using an optically guided atomicfountain. This technique offers efficient transfer without requiring anynear-resonant light. The extension to multiple and continuous loading isdiscussed, and we conclude that loading a spatially separatedconservative trap offers a promising route towards steady-stateBose-Einstein condensation.

Cold-atom accumulation using an optical trap door

Experiments are performed on loading cold atoms into a far-off resonance optical dipole trap that is spatially separated from the laser-cooling region. The atoms are delivered using a far-off resonant optical dipole guide and enter the trap through a switchable blue-detuned light sheet or ``trap door.'' This scheme provides a means to transfer atoms from a dissipative to a non-dissipative trap without a significant loss of phase-space density. We study the dynamics of atoms within the trap and measure their lifetime. Preliminary results on multiple loading are presented. As the technique does not rely on near-resonant light, the atom number limit characteristic of laser-cooling experiments can, in principle, be exceeded.

Optical-dipole trapping of Sr atoms at a high phase-space density

Employing a far-off resonance optical-dipole trap (FORT), we attained a phase-space density exceeding 0.1, or an order to quantum degeneracy. Strontium atoms were magneto-optically cooled and trapped using the spin-forbidden ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ transition and then compressed into a FORT that was designed to allow simultaneous Doppler cooling. We discussed that the phase-space density was finally limited by the light-assisted collisions occurring in the optical cooling.

Trapping of neutral molecules in a single state by a combination of electrostatic and dynamic fields

In this paper, we show that it is possible to optically trap, in two or three dimensions, a molecule oriented in space in a single | JKM〉 rotational state of its ground electronic motion. It is readily achieved by shining a far-off resonance laser of moderate intensity on the molecule in a weak, homogenous, orienting electric field E → 0 preceded by an electrostatic hexapole field. The harmonic trapping force depends upon, among other things, the orientation of the laser with respect to E → 0 and the state | JKM〉 of the molecule, in addition to molecular dynamical polarizability.

Spin-Polarized Atoms in a Circularly Polarized Optical Dipole Trap

The behavior of atoms in an optical far-off resonance trap is found to depend strongly on the trap polarization. Large loss rates occur unless the polarization is perfectly linear or perfectly circular, in which case exponential lifetimes up to 10 s are observed. Through optimization of trap loading, samples of $4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms are obtained in a circularly polarized trap. Spin polarization of $98(1)%$ is measured. This trap should prove useful for precision measurements such as $\ensuremath{\beta}$-decay asymmetry and should make rf cooling possible in optical traps.

Decoherence and fidelity in ion traps with fluctuating trap parameters

We consider two different kinds of fluctuations in an ion trap potential: external fluctuating electrical fields, which cause statistical movement (``wobbling'') of the ion relative to the center of the trap, and fluctuations of the spring constant, which are due to fluctuations of the ac-component of the potential applied in the Paul trap for ions. We write down master equations for both cases and, averaging out the noise, obtain expressions for the heating of the ion. We compare our results to previous results for far-off resonance optical traps and heating in ion traps. The effect of fluctuating external electrical fields for a quantum gate operation (controlled-NOT) is determined and the fidelity for that operation derived.

Laser-noise-induced heating in far-off resonance optical traps

Using a simple model, we calculate the heating rates arising from laser intensity noise and beam-pointing fluctuations in far-off resonance optical traps. Intensity noise causes exponential heating, while beam-pointing noise causes heating at a constant rate. The achievement of heating time constants well beyond 10 sec imposes stringent requirements on the laser noise power spectra. Noise spectra are measured for a commercial argon-ion laser to illustrate the expected time scales.

Atom trapping in deeply bound states of a far-off-resonance optical lattice

The ac Stark shift ~light shift! arising in laser interference patterns can be used to create stable periodic potentials for neutral atoms. Under appropriate conditions these ‘‘optical lattices’’ will laser cool and trap atoms in individual optical potential wells, with center-of-mass motion in the quantum regime. Experiments have explored a number of such lattice configurations in one, two, and three dimensions, and detailed insight into the dynamics of cooling and trapping has been gained through probe absorption and fluorescence spectroscopy @1#. The possibility of atomic confinement deep in the Lamb-Dicke regime suggests that it is worthwhile to pursue schemes for resolved-sideband Raman cooling @2# and quantum-state preparation @3#, as recently demonstrated for trapped ions. In a standard optical lattice formed by near-resonance light, control of the center-of-mass motion is limited by rapid laser cooling and heating processes that occur at a rate determined by photon scattering. These dissipative processes are readily avoided when the lattice is formed by intense light tuned far from atomic transition. Such far-off-resonance lattices have been used extensively in atom optics as diffraction gratings and lenses @4# and as model systems in which to study quantum chaos @5# and quantum transport @6#. When far-off-resonance optical lattices are used to trap atoms, however, the absence of built-in laser cooling makes it difficult to obtain vibrational excitation and confinement comparable to the near-resonance case. Accordingly, experiments on faroff-resonance lattices have so far achieved low vibrational excitation only by allowing the majority of vibrationally excited atoms to escape @7#. We demonstrate here a loading scheme, in which cesium atoms are first cooled and trapped in a near-resonance lattice, and then adiabatically transferred to a superimposed far-off resonance lattice. Immediately following transfer we achieve trapping parameters comparable to the near-resonance case, with a mean vibrational excitation as low as n ¯ ’0.3, and a typical rms position spread of Dz’l/20. Based on the lattice parameters we calculate an off-resonance photon scattering rate of order 10 3 s 21 .W e

Photoassociation spectrum of ultracold Rb atoms.

We study the photoassociative collisional loss of laser-cooled Rb atoms from a far-off resonance optical dipole force atom trap. We obtain a well-resolved photoassociation spectrum from 50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to 980 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ below the first excited dissociation limit of the ${\mathrm{Rb}}_{2}$ molecule. Two vibrational series associated with excited ${\mathrm{Rb}}_{2}$ $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$ states are clearly visible. Oscillations in the associated Franck-Condon factors reflect the structure of the triplet ground state wave function. Our results clearly demonstrate the potential of photoassociation spectroscopy as a new probe of molecular structure.

Raman ring resonator stability in the adiabatic approximation

Stability in a Raman ring resonator is studied by using the adiabatic approximation. The analysis is based on far-off resonance Raman scattering in hydrogen. A medium-power approximation is employed, which is good for intensities less than 30 MW/cm2. The resulting differential equations retain their standard low-power Raman gain dependance in addition to an intensity-dependent phase. The steady-state intensity input–output behavior, as well as the linear stability analysis, is accomplished analytically without invoking the mean-field approximation. Feedback is applied to the Stokes beam, a gain situation, or to the depleted pump beam. The Stokes frequency is assumed to be perfectly tuned to the atomic and cavity resonances. It is shown that both situations are multistable and that the power-dependent phase largely determines the stability characteristics. Furthermore, we show that the negative slope branches can be stable when feedback is applied to the pump if the output pump intensity is decreasing with increasing input pump intensity.

Bound-level "multiplets" in photoelectron spectra of above-threshold ionization

Evidence has been found for peak-splitting in photoelectron spectra of above-threshold ionization. This evidence was obtained by analyzing numerically the wave functions computed for a model atom undergoing ionization by an intense laser beam. The multiplets can be explained as replicas of far-off resonance bound-state levels that are transiently and adiabatically populated by a laser pulse and then ionized. The heights of the multiplet peaks relative to the true multiphoton peaks depend on the laser pulse shape.