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Physical Review Letters

Experimental Observation of Optically Trapped Atoms

Steven Chu, John E. Bjorkholm, Arthur Ashkin, A. Cable

Published Jul 21, 1986• 830 citations
Publisher

Abstract

We report the first observation of optically trapped atoms. Sodium atoms cooled below ${10}^{\ensuremath{-}3}$ K in "optical molasses" are captured by a dipole-force optical trap created by a single, strongly focused, Gaussian laser beam tuned several hundred gigahertz below the ${D}_{1}$ resonance transition. We estimate that about 500 atoms are confined in a volume of about ${10}^{3}$ \ensuremath{\mu}${\mathrm{m}}^{3}$ at a density of ${10}^{11}$-${10}^{12}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Trap lifetimes are limited by background pressure to several seconds. The observed trapping behavior is in good quantitative agreement with theoretical expectations.

DOI
10.1103/physrevlett.57.314

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•Jan 1, 2002•7 citations
We report the first observation of optically trapped atoms. Sodium atoms cooled below 10\ensuremath−3{10}^{\ensuremath{-}3}10\ensuremath−3 K in "optical molasses" are captured by a dipole-force optical trap created by a single, strongly focused, Gaussian laser beam tuned several hundred gigahertz below the D1{D}_{1}D1​ resonance transition. We estimate that about 500 atoms are confined in a volume of about 103{10}^{3}103 \ensuremath{\mu}m3{\mathrm{m}}^{3}m3 at a density of 1011{10}^{11}1011-1012{10}^{12}1012 cm\ensuremath−3{\mathrm{cm}}^{\ensuremath{-}3}cm\ensuremath−3. Trap lifetimes are limited by background pressure to several seconds. The observed trapping behavior is in good quantitative agreement with theoretical expectations.
Bose-Einstein condensates of sodium atoms have been confined in an optical dipole trap using a single focused infrared laser beam. This eliminates the restrictions of magnetic traps for further studies of atom lasers and Bose-Einstein condensates. More than $5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ condensed atoms were transferred into the optical trap. Densities of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{cm}}^{\ensuremath{-}3}$ of Bose condensed atoms were obtained, allowing for a measurement of the three-body loss rate constant for sodium condensates as ${K}_{3}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.1(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}{\mathrm{cm}}^{6}{\mathrm{s}}^{\ensuremath{-}1}$. At lower densities, the observed 1/e1/e1/e lifetime was longer than 10 s. Simultaneous confinement of Bose-Einstein condensates in several hyperfine states was demonstrated.
In the experiment, single Rubidium-87 atoms are loaded out of a molasses of Doppler-cooled atoms into the focus of a Gaussian laser beam created by a single mode laser diode with a wavelength of 856 nm. A confocal microscope outside our vacuum chamber is used to focus the trapping beam to a minimal waist of 3.5 /spl mu/m (thus creating a trap depth of 1-2 mK) and to collect the fluorescence light from the center of the dipole trap. The collected fluorescence light is coupled into a single-mode optical fiber for spatial filtering, and the photons are detected with an avalanche photodiode. With this setup the overall detection efficiency of photons emitted from the trap region is about 0.2%. First experimental results show sub-Poissonian occupation statistics of the dipole trap: only single atoms are observed inside our trap, never two ore more. The measured 1/e lifetime of atoms inside the dipole trap in the presence of cooling light is about 1-2 seconds. To prove that we really observe single atoms inside the trap, a Hanbury-Brown-Twiss measurement of the fluorescence light from the trap will be shown. The trapping of single atoms inside our dipole trap marks the first step towards the investigation of the properties of an entangled atom photon state. Progress in preparing and analyzing single atoms in well defined hyperfine ground states will be reported.
A small number of atoms that are confined to dimensions on the order of an optical wavelength are expected to exhibit novel collective behavior, including superradiance, subradiance, and unusual photon statistics and spatial correlations. We have identified a technique by which such a wavelength-sized atomic point sample may be realized in the laboratory. Atoms that are strongly confined in an optical dipole-force potential may be laser-cooled via stimulated and spontaneous Raman transitions between ground state atomic sublevels. Because the ac Stark shift of the two lower state sublevels is the same, the atoms may be cooled and trapped simultaneously. Also, interatomic radiation pressure and collisional effects can be kept to a minimum in a dipole force trap. We are currently pursuing such experiments using laser-cooled Rb atoms. We have successfully loaded laser-cooled Rb into a dipole force trap and have also driven the stimulated Raman transitions between the Rb ground state sublevels. We will report on our experimental progress and discuss their application to fundamental investigations in quantum optics.
In order to confine any object, it is necessary to exchange kinetic for potential energy in the trapping field, and in neutral atom traps, the potential energy must be stored as internal atomic energy. Thus practical traps for ground-state neutral atoms are necessarily very shallow compared with thermal energy because the energy level shifts that result from conveniently sized fields are typically considerably smaller than k B T , even for T = 1 K. Neutral atom trapping therefore depends on substantial cooling of a thermal atomic sample, and is often connected with the cooling process. In most practical cases, atoms are loaded from magneto-optical traps, where they have been efficiently accumulated and cooled to mK temperatures (see Sec. 20.3), or from optical molasses, where they have been optically cooled to μK temperatures (see Sec. 19.4). The small depth of typical neutral atom traps dictates stringent vacuum requirements, because an atom cannot remain trapped after a collision with a thermal energy background gas molecule. Since these atoms are vulnerable targets for thermal energy background gas, the mean free time between collisions must exceed the desired trapping time. The cross-section for destructive collisions is quite large because even a gentle collision (i.e. large impact parameter) can impart enough energy to eject an atom from a trap. At pressure P sufficiently low to be of practical interest, the trapping time is ~(10 -8 / P ) s, where P is in torr. This chapter begins with a discussion of ordinary, single-center traps, but also considers arrays of traps. Such “optical lattices” are playing an increasingly important role in research in atomic physics, so discussion of their properties is an appropriate part of the education of a student of the subject. Dipole force optical traps Single-beam optical traps for two-level atoms The simplest imaginable optical trap consists of a single, strongly focused Gaussian laser beam (see Fig. 20.1) [214, 215] whose intensity at the focus varies transversely with r as where is the beam waist size. Such a trap has a well-studied and important macroscopic classical analog in a phenomenon called optical tweezers (see App. 1.C) [216–218].
We report the confinement and cooling of an optically dense cloud of neutral sodium atoms by radiation pressure. The trapping and damping forces were provided by three retroreflected laser beams propagating along orthogonal axes, with a weak magnetic field used to distinguish between the beams. We have trapped as many as 107{10}^{7}107 atoms for 2 min at densities exceeding 1011{10}^{11}1011 atoms ${\mathrm{cm}}^{\ensuremath{-}3}$. The trap was \ensuremath{\simeq}0.4 K deep and the atoms, once trapped, were cooled to less than a millikelvin and compacted into a region less than 0.5 mm in diameter.
This thesis reports on the design, setup, and operation of a new atom trap apparatus, which is developed to confine few rubidium atoms in ultrahigh vacuum by means of taylored and dynamically variable light potentials (dipole traps), thus making the atoms available for spectroscopy and controlled manipulations. To maintain low background pressure, atoms of a vapour cell are transferred into a cold atomic beam by laser cooling techniques, and accumulated by a magneto-optic trap (MOT) in a separate part of the vacuum system. The laser cooled atoms are then transferred into dipole traps made of focused far-off-resonant laser fields in single or crossed-beam geometry, which are superimposed with the center of the MOT. Gaussian as well as hollow Laguerre-Gaussian (LG01) beam profiles are used with red-detuned and blue-detuned light. Microfabricated dielectric phase objects allow the efficient and robust mode conversion of Gaussian into Laguerre-Gaussian laser beams. Trap geometries can easily be changed due to the highly flexible experimental setup. The dipole trap laser beams have powers up to several hundred milliwatts and are focused to around 10 m. At a detuning of several tens of nanometers from the atomic resonance, the resulting traps are a few millikelvin deep, and the trapped atoms exhibit motional frequencies near 30 kHz, trap light scattering rates of a few hundred photons per second per atom, and storage times of several seconds. The number of dipole-trapped atoms ranges from more than ten thousand to below ten. The dipole-trapped atoms are detected either by a photon counting system with efficient straylight discrimination, or by recapture into the MOT, which is imaged onto a sensitive photodiode and a CCD-camera. Due to the strong AC-Stark shift imposed by the high-intensity trapping light, energy-selective resonant excitation and detection of the stored atoms is possible. The measured energy distribution is consistent with a harmonic potential shape and allows for the determination of temperatures and heating rates. From initial measurements, the thermal energy is found to be about 10 % of the trap depth. Using a crossed-beam geometry with red-detuned laser light, efficient transfer of atoms between the beams is observed. Optimum transfer occurs when the two beams cross at a radial offset, which can qualitatively be understood when the particle energy and geometrical properties of the two-beam trapping potential are considered. Numerical simulations reproduce the general features of the measured transfer efficiency vs. radial beam offset. This experiment is a first step towards dynamically variable potentials. Atoms have been radially confined in a blue-detuned hollow beam, and their decay time has been determined. This configuration is currently extended to a three-dimensionally confining blue-detuned dipole trap. For advanced laser cooling, state manipulation and spectroscopy of trapped atoms, a system of two diode lasers has been set up which are phase-coherently locked with a difference frequency near 6.834 GHz, to drive Raman transitions between the hyperfine-split ground states of 87Rb atoms. Resonances of such a transition have been recorded in a buffer gas loaded rubidium vapour cell with linewidths below 100 Hz.