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Fortschritte der Physik

Two‐dimensional Bose‐Einstein condensates in a CO2‐laser optical lattice

Giovanni Cennini, Carsten Geckeler, Gunnar Ritt, Tobias Salger, Martin Weitz

Published Aug 1, 2006• 5 citations
Publisher

Abstract

Abstract We report on experiments with all‐optically produced Bose‐Einstein condensates in a mesoscopic optical lattice. By retroreflecting the radiation of a CO2‐laser operating near 10.6 μm, a lattice with spacing between sites of λ /2 ≃ 5.3 μm is realized, for which atom tunneling is negligible. Once cooled to quantum degeneracy by direct evaporative cooling in the lattice, the properties of the trapped microclouds can be studied with far field interference experiments. The microscopic atom clouds are shown to be in the two‐dimensional regime for small atom filling. The realized system has extremely long coherence times, and holds fascinating prospects for both quantum logic and the study of novel collective phenomena.

DOI
10.1002/prop.200610306

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•Jan 1, 1999•0 citations
Abstract We report on experiments with all‐optically produced Bose‐Einstein condensates in a mesoscopic optical lattice. By retroreflecting the radiation of a CO2‐laser operating near 10.6 μm, a lattice with spacing between sites of λ /2 ≃ 5.3 μm is realized, for which atom tunneling is negligible. Once cooled to quantum degeneracy by direct evaporative cooling in the lattice, the properties of the trapped microclouds can be studied with far field interference experiments. The microscopic atom clouds are shown to be in the two‐dimensional regime for small atom filling. The realized system has extremely long coherence times, and holds fascinating prospects for both quantum logic and the study of novel collective phenomena.
The properties of Bose–Einstein condensates in an external potential produced by a laser light field are discussed. Considered are the condensates embedded in periodic lattices produced by standing laser waves and the condensates confined within 'optical traps' near the focus of a traveling wave. Observations of the Mott transition in a periodic lattice are described, as are experiments on atoms near the Feshbach resonance and experiments on the condensates in a double potential well, which permit investigating the tunnel dynamics of condensate phases.
Summary form only given. Standing wave laser fields produce a periodic potential for atoms, which can trap cold atoms in an ordered crystal-like structure to form an optical lattice. Optical lattices have been used as model systems in studies of quantum dynamical effects and quantum state control. More recently, it has been suggested that such lattices would provide an attractive system for performing quantum logic experiments, as efficient quantum error correction schemes can be implemented due to the inherent possibility of parallel operation. On the other hand, the possibility to selectively address and manipulate single qubits is essential in the operation of quantum logic systems. This is difficult to achieve in conventional optical lattices, where the spatial period is of order half the wavelength of an optical absorption line. We report on the optical imaging of rubidium atoms trapped in the antinodes of an extremely far detuned optical lattice formed by retroreflecting the light of a CO/sub 2/-laser operating near 10.6 /spl mu/m. To our knowledge, this represents the first observation of the individual sites of an optical lattice with a periodicity near half the trapping wavelength. Besides the unusually large lattice constant (5.3 /spl mu/m), intriguing properties of this type of lattice also include extremely long coherence times, as the expected average photon scattering time is over 10 minutes.
In this paper we first focus on the methods developed to control the position and the velocity of atoms, taking advantage of the radiative forces exerted on atoms placed in a laser beam. Temperatures in the range of μK can be reached for dilute atomic clouds trapped under vacuum in a very small region of space. The application to fountain clocks based on cold cesium atoms is presented. We then describe the characterization and the main features of Bose‐Einstein condensates, a new state of matter of purely quantum origin, which can be obtained by subsequent evaporative cooling. The methods in use for cooling molecules are considered, in particular the collision processes or the photoassociation of cold atoms. The possibility of changing interactions between ultracold particles is also explained and photoassociation is illustrated by the recent experiments of our group dealing with metastable helium atoms.
We study the dynamics of Bose–Einstein condensates in accelerated two-dimensional (2D) optical square lattices by numerically solving the Gross–Pitaevskii equation. We consider the regime with negligible mean-field interactions and examine in detail the pulses of atom clouds ejected from the condensate due to Landau–Zener tunnelling. The pulses exhibit patterned structures that can be understood from the momentum–space dynamics of the condensate. Apart from conceiving the realization of a pulsed 2D atom laser, we demonstrate that, by exploring the band structure of the lattice, the Landau–Zener tunnelling and Bragg reflection of the condensate inside the optical lattice can provide a means of manipulation of coherent atom waves.
Bose-Einstein condensation in a dilute vapor of magnetically trapped Rb was achieved, and the condensed atoms were used to observe de Broglie-wave interference due to atom tunneling between the barriers of an array of traps. The condensed atoms were loaded into a vertically-oriented one-dimensional optical lattice formed by a retro-reflected laser beam, creating the trap array. In the limit of negligible interactions between atoms, the energy difference between adjacent array sites was determined by the gravitational potential energy shift between the sites. This energy difference initiated an oscillating atom mass current similar to an electric current in the ac Josephson effect for two coupled superconductors. In the optical lattice, de Broglie-wave interference occurred between atoms in localized lattice states, and the interference was observed as a train of falling pulses of atoms. In a separate experiment, using a dilute vapor of lithium, enhanced loading of a Li magneto-optic trap with spectrally-broadened cooling and trapping light was demonstrated. By placing the Li oven nozzle near the trap, the Li atoms could be efficiently captured from an atomic beam without using a pre-cooling stage. The trapped atoms were transferred into a three-dimensional optical lattice, formed by four intersecting and interfering beams of far-off-resonant laser light, and adiabatic cooling and velocity selection in three dimensions was demonstrated.