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Physical Review A

Precision polarization measurements of atoms in a far-off-resonance optical dipole trap

Fengzhou Fang, David J. Vieira, Xinxin Zhao

Published Jan 31, 2011• 12 citations
Publisher

Abstract

Precision measurement of atomic and nuclear polarization is an essential step for beta-asymmetry measurement of radioactive atoms. In this paper, we report the polarization measurement of Rb atoms in an yttrium-aluminum-garnet (YAG) far-off-resonance optical dipole trap. We have prepared a cold cloud of polarized Rb atoms in the YAG dipole trap by optical pumping and achieved an initial nuclear polarization of up to 97.2(5)$%$. The initial atom distribution in different Zeeman levels is measured by using a combination of microwave excitation, laser pushing, and atomic retrap techniques. The nuclear-spin polarization is further purified to 99.2(2)$%$ in 10 s and maintained above 99$%$ because the two-body collision loss rate between atoms in mixed spin states is greater than the one-body trap loss rate. Systematic effects on the nuclear polarization, including the off-resonance Raman scattering, magnetic field gradient, and background gas collisions, are discussed.

DOI
10.1103/physreva.83.013416

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European Quantum Electronics Conference (EQEC)•Jan 1, 2003•0 citations
Precision measurement of atomic and nuclear polarization is an essential step for beta-asymmetry measurement of radioactive atoms. In this paper, we report the polarization measurement of Rb atoms in an yttrium-aluminum-garnet (YAG) far-off-resonance optical dipole trap. We have prepared a cold cloud of polarized Rb atoms in the YAG dipole trap by optical pumping and achieved an initial nuclear polarization of up to 97.2(5)%. The initial atom distribution in different Zeeman levels is measured by using a combination of microwave excitation, laser pushing, and atomic retrap techniques. The nuclear-spin polarization is further purified to 99.2(2)% in 10 s and maintained above 99% because the two-body collision loss rate between atoms in mixed spin states is greater than the one-body trap loss rate. Systematic effects on the nuclear polarization, including the off-resonance Raman scattering, magnetic field gradient, and background gas collisions, are discussed.
Optical trapping of radioactive atoms has a great potential in precision measurements for testing fundamental physics such as electric-dipole moment, atomic parity nonconservation, and parity-violating $\ensuremath{\beta}$-decay correlation coefficients. One challenge that remains is to polarize the atoms to a high degree, and to measure the polarization of the sample and its evolution over time. In this paper we report on the polarization study of Rb atoms in a yttrium-aluminum-garnet (YAG)-laser optical dipole trap using both Faraday rotation polarimetry and resolved Zeeman spectroscopy techniques. We have prepared a cold cloud of polarized atoms and observed that its spin relaxation due to light scattering is suppressed in the YAG dipole trap. The spin polarization is further purified and maintained when the two-body collision loss rate between atoms in mixed spin states is greater than the one-body trap loss. These advancements are an important step toward a new generation of precision measurement with polarized trapped atoms.
Optical trapping of selected species of radioactive atoms has great potential in precision measurements for testing fundamental physics such as the electric dipole moment, atomic parity nonconservation, and parity-violating $\ensuremath{\beta}$-decay correlation coefficients. We report on the trapping of 104{10}^{4}104 radioactive 82Rb^{82}\mathrm{Rb}82Rb atoms (t1∕2=75s)({t}_{1∕2}=75\phantom{\rule{0.3em}{0ex}}\mathrm{s})(t1∕2​=75s) with a trap lifetime of $\ensuremath{\sim}55\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ in an optical dipole trap. Transfer efficiency from the magneto-optical trap is $\ensuremath{\sim}14%$. We further report evidence of spontaneous spin polarization of the atoms in optical dipole trap loading. These advancements are an important step toward a new generation of precision nuclear-spin--$\ensuremath{\beta}$-emission direction correlation measurements with polarized 82Rb^{82}\mathrm{Rb}82Rb atoms.
The behavior of atoms in an optical far-off resonance trap is found to depend strongly on the trap polarization. Large loss rates occur unless the polarization is perfectly linear or perfectly circular, in which case exponential lifetimes up to 10 s are observed. Through optimization of trap loading, samples of $4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms are obtained in a circularly polarized trap. Spin polarization of 98(1)98(1)%98(1) is measured. This trap should prove useful for precision measurements such as $\ensuremath{\beta}$-decay asymmetry and should make rf cooling possible in optical traps.
The efficient coupling of an optical trap to a mass separator is being developed to undertake high-precision electroweak interaction measurements. The use of an ion implantation and subsequent heated foil release method is being pursued as a suitable way of introducing radioactive samples into the ultra high vacuum region of the optical trap without gas loading. The batchwise trapping and cooling of one million {sup 82}Rb atoms is envisioned with overall efficiencies of 1% or more. The cloud of cold radioactive atoms ({approximately}1 mm in diameter) will than be spin polarized and transferred to a second optical trap, repolarized, and loaded in a magnetic trap in which a beta-asymmetry measurement will be performed. The latest developments towards undertaking this experiment will be presented.
We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100% and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of 4.2+/-0.7 s has been measured. Possible applications are briefly discussed.
A cloud of /sup 87/Rb atoms in the |f, m/sub f/>= |1, -1> ground state was evaporatively cooled in a Ioffe-Pritchard trap close to or below the BEC transition temperature. The atoms were then transferred into an optical dipole trap made of an Yb:YAG laser operating at a wavelength of 1030 nm. After turning off the magnetic trap, an RF pulse transferred the atoms to the |1,1> Zeeman state. Subsequently, a homogeneous dc magnetic field was applied. The field range was scanned between 0.5 and 1260 Gauss. Monitoring the atom loss, more than 40 resonances, are observed including the predicted ones. The additional resonances are due to closed channels with rotational quantum number l = 2. The measured resonance positions were included in a fit to obtain an improved set of parameters for the rubidium interatomic potential. Although all resonances are narrow, the magnetic field control should be accurate enough to resolve changes in the elastic scattering length on the broadest resonance at 1007 Gauss.