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Europhysics Letters (EPL)

Optical Trapping in a Cesium Cell with Linearly Polarized Light and at Zero Magnetic Field

A Höpe, D. Haubrich, Harald Schadwinkel, Frank Strauch, Dieter Meschede

Published Oct 1, 1994• 7 citations
Publisher

Abstract

We have found that linearly polarized light can be used efficiently for optical trapping of cesium atoms in a magnetic-quadrupole field. The number and density of atoms of the trapped samples are comparable to a standard magneto-optical trap with σ + -σ - polarized light, but the influence of the magnetic-quadrupole strength is strikingly different. When the polarization of counterpropagating light beams is orthogonal, trapping is observed also for zero magnetic field.

DOI
10.1209/0295-5075/28/1/002

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We have found that linearly polarized light can be used efficiently for optical trapping of cesium atoms in a magnetic-quadrupole field. The number and density of atoms of the trapped samples are comparable to a standard magneto-optical trap with σ + -σ - polarized light, but the influence of the magnetic-quadrupole strength is strikingly different. When the polarization of counterpropagating light beams is orthogonal, trapping is observed also for zero magnetic field.
We report on the observation of a new laser trap for neutral atoms. It uses three orthogonal pairs of counterpropagating laser beams having linear polarizations at 45° and a quadrupole magnetic field. 10 8 cesium atoms were thus confined in a 0.15 mm 3 volume at a temperature of 60 μK, a factor of 2 below the Doppler cooling limit. We interpret this trapping as being due to the new magneto-optical force recently observed by Grimm et al. and which is essentially a dipole force rectified by the magnetic field. This trap opens new possibilities for increasing the phase-space density of laser-cooled atoms.
We report our study of the magneto-optical effect in a strictly linear regime on spin-polarized cold cesium atoms. Due to the low intensity and the short illumination period of the probe beam, less than 7.5% of the sample atoms change their states by absorbing probe photons. We produce a medium of atoms at rest in either the 6S1∕2,F=3,mF=06{S}_{1∕2},F=3,{m}_{F}=06S1∕2​,F=3,mF​=0 or 6S1∕2,F=3,mF=36{S}_{1∕2},F=3,{m}_{F}=36S1∕2​,F=3,mF​=3 state by optically pumping atoms trapped in a magneto-optical trap. We use the D1D1D1 resonance with large lower and upper state hyperfine splittings as a probe transition to avoid hyperfine mixing from the Zeeman interaction. Under this idealized situation we measure the Stokes parameters in order to find the polarization rotation and circular dichroism experienced by the probe light. We find that there are qualitative differences between the results for the mF=0{m}_{F}=0mF​=0 and mF=3{m}_{F}=3mF​=3 cases. While dispersion and consequent Faraday rotation play a dominant role when the atoms are in the mF=0{m}_{F}=0mF​=0 state, it is dissipation and circular dichroism that are important when they are in the mF=3{m}_{F}=3mF​=3 state. Similarly, while the size of the Faraday rotation and the circular dichroism for the mF=0{m}_{F}=0mF​=0 case scales linearly with the applied magnetic field, for the mF=3{m}_{F}=3mF​=3 case it is the shift of the probe polarization change versus frequency that is linearly proportional to the magnetic field strength.
Magneto-optical traps (MOTs) are a workhorse technology for neutral-atom quantum sensing, simulation, and computing. Here, we demonstrate a MOT optimized for inertial quantum sensing and precision metrology applications. Our MOT traps 4×1084\times10^84×108 cesium (Cs) atoms in a robust, modular system that achieves stable operation through two design features: (1) we use divergent cooling laser beams to reduce unwanted reflections and (2) optical elements are mounted in a fiber-coupled, compact, and modular cage rigidly attached to the vacuum chamber. Following polarization-gradient cooling (PGC), the system produces atom samples with temperatures below 10 μμμK, similar to those achieved in conventional MOTs that use collimated laser beams. In addition, we observe trapping of 2×1072\times10^72×107 Cs atoms in a non-conventional MOT geometry, where the cooling laser beams are diagonal to the principal axis of the quadrupole magnetic field.
We realized two species of magneto-optical traps (MOTs) by using linearly-polarized cooling beams. A mirror MOT was realized by using pairs of σ-π polarized cooling beams. Since the linearly-polarized field could act as opposite circular polarizations, atoms could be loaded on the mirror MOT with the elimination of one quarter-wave plate (QWP) among the four QWPs. However, for a typical MOT, counter-propagating cooling beams had to have opposite circular polarizations. The number of trapped atoms depended strongly on the polarization and optical alignment of the cooling beams when linear polarization with the elimination of one QWP was used. By using this anomalous alignment, we could increase the fluorescence of the mirror MOT. Furthermore, we were also able to realize a six-beam MOT with a configuration of pairs of orthogonal linearly-polarized cooling beams. The direction of the polarization depended strongly on the magnetic field for trapping.
The magneto-optical trap (MOT) serves as the culmination of optical cooling and trapping, where by applying a linear magnetic field to orthogonal, circularly polarized, retroreflecting laser beams, one can produce a dense cloud of very cold atoms, on the order of μK. This work set out to create a magneto-optical trap using only visible light with the 6s 2S1/2 → 7p 2P◦ 3/2 transition of cesium-133, which is far weaker than the transition typically employed for a cesium-133 MOT. This transition alone has yet to be used in isolation to create a MOT, and the purpose of this work is to demonstrate the feasibility of such a MOT. While the creation of a MOT with the setup produced has thus far been unsuccessful, the components are in place for one to be made as a demonstration of optical cooling and trapping that should be visible to the naked eye. Additionally, an experimental setup was created for high precision measurements of the absolute energies of the 7p 2P◦ 1/2 and 7p 2P◦ 3/2 states. The theory behind the MOT and high precision spectroscopy is expounded upon within.
We have investigated optical capture of neutral cesium atoms by radiation pressure forces in a stiff magnetic-quadrupole field. Permanent magnets were used to generate field gradients ranging from 60 G/cm to more than 300 G/cm. With increasing gradient we observe a step decrease in the equilibrium number of trapped atoms but only a moderate reduction of their density. From the dynamics of the trapped atomic sample we derive temperatures as low as 40 μK or 30% of the Doppler limit even in these strong magnetic-quadrupole fields.