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arXiv:0812.3630 · Physical Review A

Two-dimensional quantum gas in a hybrid surface trap

Jonathon I. Gillen, Waseem Bakr, Amy Peng, P. Unterwaditzer, Simon Fölling, Markus Greiner

Published Aug 14, 2009• 36 citations
arXivHTMLPDF

Abstract

We demonstrate the realization of a two-dimensional (2D) quantum gas in a smooth optical surface trap. Using a combination of evanescent wave, standing wave, and magnetic potentials, we create a long-lived quantum gas deep in the 2D regime at a distance of a few microns from a glass surface. To realize a system suitable for many-body quantum simulation, we introduce methods such as broadband ``white'' light to create evanescent and standing waves to realize a smooth potential with a trap frequency aspect ratio of 300:1:1. We are able to detect phase fluctuations and vortices, and we demonstrate cooling to degeneracy and low disorder in the 2D configuration.

DOI
10.1103/physreva.80.021602

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We demonstrate the realization of a two-dimensional (2D) quantum gas in a smooth optical surface trap. Using a combination of evanescent wave, standing wave, and magnetic potentials, we create a long-lived quantum gas deep in the 2D regime at a distance of a few microns from a glass surface. To realize a system suitable for many-body quantum simulation, we introduce methods such as broadband ``white'' light to create evanescent and standing waves to realize a smooth potential with a trap frequency aspect ratio of 300:1:1. We are able to detect phase fluctuations and vortices, and we demonstrate cooling to degeneracy and low disorder in the 2D configuration.
A dense gas of cesium atoms at the crossover to two dimensions is prepared in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20,000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system provides atoms at a mean distance from the surface as low as 1 microm, and offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.
Two-dimensional systems are of great interest in physics as the celebrated quantum-Hall effects show, which are based on a 2D electron gas. Neutral systems involve thin He films and spin polarized atomic hydrogen in a cryogenic environment, where the successful creation of a 2D quasi-condensate was recently reported. Over the last years, with laser cooling and trapping techniques at hand, several schemes for the preparation of laser cooled atomic 2D gases have been proposed. Low dimensional gases are discussed as keys to achieve high density samples of purely optically cooled atoms, since the reabsorption of scattered photons is greatly suppressed compared to the situation in 3D. On the other hand, by structuring 2D waveguide light fields laterally, very versatile "integrated atom optical" elements can be realized. We have prepared a quasi-2D gas of argon atoms confined in a planar matter waveguide less than 1 /spl mu/m away from a reflecting surface. We trap metastable Ar/sup +/ atoms in the ls/sub 5/(J=2) state in a magneto-optical trap. Upon release, they fall towards a gold coated prism surface where an evanescent light field is generated by resonant excitation of surface plasmons in the gold layer. This light field is blue detuned from the 1s/sub 5//spl hArr/2p/sub 9/(J=3) transition and hence creates a repulsive potential which slows down the falling atoms.
Quantum gas microscopy has enabled the study on intriguing properties of ultracold atoms in optical lattices. It provides the cutting-edge technology for manipulating quantum many-body systems. In such experiments, atoms have to be prepared into a two-dimensional (2D) system for being resolved by microscopes with limited depth of focus. Here we report an experiment on slicing a single layer of the atoms trapped in a few layers of pancake-shaped optical traps to create a 2D system. This technique is implemented with a microwave “knife”, i.e., a microwave field with a frequency defined by the resonant condition with the Zeeman-shifted atomic levels related to a gradient magnetic field. It is crucial to keep a stable preparation of the desired layer to create the 2D quantum gas for future experimental applications. To achieve this, the most important point is to provide a gradient magnetic field with low noises and slow drift in combination with a properly optimized microwave pulse. Monitoring the electric current source and the environmental magnetic field, we applied an actively stabilizing circuit and realized a field drift of 0.042(3) mG/hour. This guarantees creating the single layer of atoms with an efficiency of 99.92(3)% while atoms are hardly seen in other layers within 48 hours, satisfying future experimental demands on studying quantum many-body physics.
We prepare a quasi-2D gas of argon atoms confined in a planar matter waveguide in close vicinity ( $<1\ensuremath{\mu}\mathrm{m}$) of a reflecting surface. The (anti-)nodes of a far off resonant standing light wave in front of the surface provide a confining potential for the atomic motion normal to the surface. A cloud of cold atoms that approaches the surface is compressed by deceleration in an evanescent field and loaded predominantly into one single potential well close to the surface by optical pumping. The spatial density achieved is more than 100 times higher than the density of the approaching atoms.
We propose a novel scheme to form a 2D surface array of optical micro-traps of cold atoms by using two sets of far red-detuned evanescent wave interference and a blue-detuned evanescent wave. The optical potentials of the 2D surface array of optical micro-traps are high enough to trap cold atoms released from the standard magneto-optical trap，and 87Rb atoms trapped in the array of optical micro-traps can be cooled to 2.56μK with efficient intensity-gradient Sisyphus cooling. Our study shows that the proposed 2D surface array of optical micro-traps can be used in atomic physics，atomic optics, quantum optics and so on.
Summary form only given. Our experiment aims at investigating a two-dimensional degenerate quantum gas of cesium atoms. For this purpose, we exploit a double evanescent-wave trap (DEW trap) consisting of a repulsive and an attractive evanescent wave and providing a highly anisotropic dipole potential. Efficient loading is accomplished by a multi-stage sequence. Cesium atoms are laser cooled in a MOT and then transferred into a gravito-optical dipole trap that serves as a surface reservoir. In the next stage, the phase-space density is enhanced by creating a dimple in the potential with a far-detuned, attractive laser beam that is focused to the center of the reservoir. Finally, we transfer the atoms into the DEW trap. The vertical dipole potential provides a well close to the surface, and depends on the power of the attractive evanescent wave. Ramping down the power of the attractive evanescent wave leads to efficient evaporation. A situation at the crossover to two-dimensionality is reached when the thermal energy of k/sub B/ /spl times/ 100 nK equals the level spacing h/spl nu/; thus the vertical ground state is populated by over 60 percent. Since the final phase-space density is as high as 0.1, the creation of a degenerate two-dimensional quantum gas of Cs comes into experimental reach. In present experiments we polarize the atoms into the F = 3, m/sub F/ = 3 state. As a consequence of the resonant interaction properties of cesium, the scattering length can be tuned in a wide range by applying a magnetic field. We use the tunability of interactions to optimize the evaporation efficiency on the way to a two-dimensional degenerate quantum gas of cesium.