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Physical Review A

Magnetization and spin-flip dynamics of atoms in optical lattices

Georg A. Raithel, William D. Phillips, S. L. Rolston

Published Oct 1, 1998• 7 citations
Publisher

Abstract

We show, experimentally and using quantum Monte Carlo simulations, that the quasithermal paramagnetic behavior of cesium atoms in a one-dimensional $\mathrm{lin}\mathrm{\ensuremath{\perp}}\mathrm{lin}$ optical lattice is characterized by a spin temperature of twice the zero-field kinetic temperature. The magnetization is maximum and almost perfect when the Zeeman shift of the extreme magnetic sublevels approximately equals half the maximum light shift. The lifetime of magnetization and the laser-cooling time constant are found to be clearly distinct, the magnetization decay time amounting to at least three times the cooling time. Using simulations we find that the magnetic-dipole correlation time is approximately proportional to the angular momentum $F$ of the atoms, while cooling and energy correlation time constants are approximately independent of $F.$ Our results imply that for large $F$ laser cooling within single light-shift potential wells (local cooling) occurs.

DOI
10.1103/physreva.58.r2660

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•Jan 1, 2002•0 citations
We show, experimentally and using quantum Monte Carlo simulations, that the quasithermal paramagnetic behavior of cesium atoms in a one-dimensional lin\ensuremath⊥lin\mathrm{lin}\mathrm{\ensuremath{\perp}}\mathrm{lin}lin\ensuremath⊥lin optical lattice is characterized by a spin temperature of twice the zero-field kinetic temperature. The magnetization is maximum and almost perfect when the Zeeman shift of the extreme magnetic sublevels approximately equals half the maximum light shift. The lifetime of magnetization and the laser-cooling time constant are found to be clearly distinct, the magnetization decay time amounting to at least three times the cooling time. Using simulations we find that the magnetic-dipole correlation time is approximately proportional to the angular momentum FFF of the atoms, while cooling and energy correlation time constants are approximately independent of F.F.F. Our results imply that for large FFF laser cooling within single light-shift potential wells (local cooling) occurs.
We demonstrate that weak magnetic fields can significantly enhance laser cooling and state preparation of Cs atoms in a one-dimensional optical lattice. A field parallel to the lattice axis increases the vibrational ground-state population of the stretched state ∣m=F〉|m=F〉∣m=F〉 to 2828%28. A transverse field reduces the kinetic temperature. Quantum Monte Carlo simulations agree with the experiment, and predict 4545%45 ground-state population for optimal parallel and transverse fields. Our results show that coherent mixing and local energy relaxation play important roles in laser cooling of large-FFF atoms.
A complete laser cooling setup was built, with focus on threedimensional near-resonant optical lattices for cesium. These consist of regularly ordered micropotentials, created by the interference of four laser beams. One key feature of optical lattices is an inherent ”Sisyphus cooling” process. It efficiently extracts kinetic energy from the atoms, leading to equilibrium temperatures of a few µK. The corresponding kinetic energy is lower than the depth of the potential wells, so that atoms can be trapped. We performed detailed studies of the cooling processes in optical lattices by using the time-of-flight and absorption-imaging techniques. We investigated the dependence of the equilibrium temperature on the optical lattice parameters, such as detuning, optical potential and lattice geometry. The presence of neighbouring transitions in the cesium hyperfine level structure was used to break symmetries in order to identify, which role “red” and “blue” transitions play in the cooling. We also examined the limits for the cooling process in optical lattices, and the possible difference in steady-state velocity distributions for different directions. Moreover, in collaboration with ´Ecole Normale Sup´erieure in Paris, numerical simulations were performed in order to get more insight in the cooling dynamics of optical lattices. Optical lattices can keep atoms almost perfectly isolated from the environment and have therefore been suggested as a platform for a host of possible experiments aimed at coherent quantum manipulations, such as spin-squeezing and the implementation of quantum logic-gates. We developed a novel way to trap two different cesium ground states in two distinct, interpenetrating optical lattices, and to change the distance between sites of one lattice relative to sites of the other lattice. This is a first step towards the implementation of quantum simulation schemes in optical lattices.
This dissertation reports on quantum state preparation of cesium atoms in a two-dimensional optical lattice, by resolved-sideband Raman cooling. An optical lattice is a periodic potential produced by the light shift interaction between an atom and light field. Laser cooled atoms can become strongly localized about the bottom of potential wells in an optical lattice, where they occupy a discrete spectrum of bound vibrational energy levels. The distribution over vibrational levels of atoms in the lattice is characterized by the mean vibrational excitation, n . In an optical lattice, absorption and emission of photons from lattice beams causes n to increase in time. This source of heating is always present, but its rate can be greatly reduced in a lattice detuned far from the atomic resonance. Sideband cooling is an efficient means of transferring atoms from higher into lower-lying vibrational levels and, thus, it reduces n for the ensemble. If the sideband cooling rate is much greater than the heating rate, then n approaches zero and virtually all atoms are in the lowest vibrational level in their potential wells. Our sideband cooling scheme involves stimulated Raman transitions between bound states in the potential wells of a pair of magnetic sublevels, followed by optical pumping, for a net loss of one quantum of vibration per cooling cycle. The process accumulates 98% of atoms in the ground vibrational level of a potential well associated with a single Zeeman substate. Each atom in the lattice is then very close to a pure state. For two-dimensional lattice with sideband cooling we find nx≈ny≈0.008 &parl0;16&parr0; . Various issues related to state preparation and sideband cooling are also discussed in the context of a one dimensional lin ⊥ lin optical lattice. These include improvement of laser cooling in a near resonance lattice by application of weak magnetic fields, transfer of atoms from near into far off-resonance lattices, and heating rates in far off-resonance lattices.
We present an experimental investigation of a three-dimensional (3D) optical lattice where the light polarization is linear at the bottom of the potential wells instead of being circular as in previous studies. In the present situation, optical pumping induces frequent transitions between differently light-shifted Zeeman sublevels (jumping regime). However, narrow vibrational lines are still observed as a result from a motional narrowing-like phenomenon taking place in this lattice. The atomic kinetic temperature is investigated and shown to be lower than in a standard 3D lin ⊥ lin optical lattice.
We study the dynamics of the cooling of a gas of caesium atoms in an optical lattice, both experimentally and with 1D full-quantum Monte Carlo simulations. We find that, contrary to the standard interpretation of the Sisyphus model, the cooling process does not work by a continuous decrease of the average kinetic energy of the atoms in the lattice. Instead, we show that the momentum of the atoms follows a bimodal distribution, the atoms being gradually transferred from a hot to a cold mode. We suggest that the cooling mechanism should be depicted in terms of a rate model, describing the transfer between the two modes along with the processes occurring within each mode.
A complete laser cooling setup was built, with focus on threedimensional near-resonant optical lattices for cesium. These consist of regularly ordered micropotentials, created by the interference of four laser beams. One key feature of optical lattices is an inherent ”Sisyphus cooling” process. It efficiently extracts kinetic energy from the atoms, leading to equilibrium temperatures of a few µK. The corresponding kinetic energy is lower than the depth of the potential wells, so that atoms can be trapped.We performed detailed studies of the cooling processes in optical lattices by using the time-of-flight and absorption-imaging techniques. We investigated the dependence of the equilibrium temperature on the optical lattice parameters, such as detuning, optical potential and lattice geometry. The presence of neighbouring transitions in the cesium hyperfine level structure was used to break symmetries in order to identify, which role “red” and “blue” transitions play in the cooling. We also examined the limits for the cooling process in optical lattices, and the possible difference in steady-state velocity distributions for different directions. Moreover, in collaboration with ´Ecole Normale Sup´erieure in Paris, numerical simulations were performed in order to get more insight in the cooling dynamics of optical lattices.Optical lattices can keep atoms almost perfectly isolated from the environment and have therefore been suggested as a platform for a host of possible experiments aimed at coherent quantum manipulations, such as spin-squeezing and the implementation of quantum logic-gates. We developed a novel way to trap two different cesium ground states in two distinct, interpenetrating optical lattices, and to change the distance between sites of one lattice relative to sites of the other lattice. This is a first step towards the implementation of quantum simulation schemes in optical lattices.