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Physical Review A

Loading and compressing Cs atoms in a very far-off-resonant light trap

D. J. Han, Marshall T. DePue, David S. Weiss

Published Jan 12, 2001• 54 citations
Publisher

Abstract

We describe an experiment in which $3\ifmmode\times\else\texttimes\fi{}{10}^{7} \mathrm{Cs}$ atoms are loaded into a $400 \ensuremath{\mu}\mathrm{m}$ crossed beam far-off-resonant trap (FORT) that is only $2 \ensuremath{\mu}\mathrm{K}$ deep. A high-density sample is prepared in a magneto-optic trap, cooled in a three-dimensional far-off-resonant lattice (FORL), optically pumped into the lowest-energy state, adiabatically released from the FORL, magnetically levitated, and transferred to the final trap with a phase-space density of ${10}^{\ensuremath{-}3}.$ Spontaneous emission in the FORT is negligible, and we have compressed the atoms in the FORT to a spatial density of $2\ifmmode\times\else\texttimes\fi{}{10}^{13} {\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.$ Evaporative cooling under these conditions proceeds rapidly.

DOI
10.1103/physreva.63.023405

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We describe an experiment in which 3\ifmmode×\else\texttimes\fi107Cs3\ifmmode\times\else\texttimes\fi{}{10}^{7} \mathrm{Cs}3\ifmmode×\else\texttimes\fi107Cs atoms are loaded into a 400\ensuremathμm400 \ensuremath{\mu}\mathrm{m}400\ensuremathμm crossed beam far-off-resonant trap (FORT) that is only 2\ensuremathμK2 \ensuremath{\mu}\mathrm{K}2\ensuremathμK deep. A high-density sample is prepared in a magneto-optic trap, cooled in a three-dimensional far-off-resonant lattice (FORL), optically pumped into the lowest-energy state, adiabatically released from the FORL, magnetically levitated, and transferred to the final trap with a phase-space density of 10\ensuremath−3.{10}^{\ensuremath{-}3}.10\ensuremath−3. Spontaneous emission in the FORT is negligible, and we have compressed the atoms in the FORT to a spatial density of 2\ifmmode×\else\texttimes\fi1013atoms/cm3.2\ifmmode\times\else\texttimes\fi{}{10}^{13} {\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}.2\ifmmode×\else\texttimes\fi1013atoms/cm3. Evaporative cooling under these conditions proceeds rapidly.
We report a way to overcome heating due to the reabsorption of scattered light when relatively many atoms are laser cooled at high density. A sample of $8\ifmmode\times\else\texttimes\fi{}{10}^{7}\mathrm{Cs}$ atoms, with $6\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{atoms}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}3},$ is loaded into a deep far-detuned one-dimensional optical lattice, which acts as a large volume three-dimensional (3D) trap. Polarization gradient cooling with a separate set of cooling beams is found to work well in this trap, populating the trap's vibrational ground state with more than half the atoms. After cooling, the lattice is shut off adiabatically, leaving a still dense sample at 1.2 \ensuremath{\mu}K. We propose and demonstrate a generalization to a 3D lattice, which will allow many more atoms to be loaded into conservative light traps, and can greatly increase the initial phase-space density for loading magnetic traps.
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This work deals with the cooling and trapping of single cesium (Cs) atoms in a large-magnetic-gradient magneto-optical trap (MOT) and the confinement of single Cs atoms in a far-off-resonance optical dipole trap (FORT). The experiment setup is based on two large-numerical-aperture lens assemblies which allow us to strongly focus a 1064-nm TEM00-mode Gaussian laser beam to a 1/e2 radius of ~ 2.3 μm to form a microscopic FORT for isolating single atom with environment and to efficiently collect the laser-induced-fluorescence photons emitted by single atoms for detecting and recognizing single atom’s internal state. We have tried both of “bottom-up” and “top-down” loading schemes to confine single atoms in the microscopic FORT. In the “bottom-up” scheme, we have successfully prepared single Cs atoms in the MOT and transferred it into FORT with a probability of almost 100%. In the “top-down” scheme, we have achieved ~ 74% of single atom loading probability in the FORT using light-assisted collisions induced by blue detuning laser and with prepared many Cs atoms in the MOT. The relaxation time in hyperfine level of ground state of trapped single Cs atom is measured to be ~5.4 s. To coherently manipulate atomic quantum bits (qubit) encoded in the clock states (mF = 0 states in Fg = 3 and 4 hyperfine levels) of single Cs atom via the two-photon simulated Raman adiabatic passage (STIRAP), we have prepared two phase-locked laser beams with a frequency difference of ~ 9.192 GHz by optically injecting an 852-nm master laser to lock the +1-order sideband of a 9-GHz current-modulated slave diode laser. The two phase-locked laser beams are used to drive STIRAP process in the Λ-type three-level system consists of Cs |6S1/2 Fg = 4, mF = 0> and |6S1/2 Fg = 3, mF = 0< long-lived clock states and Cs |6S1/2 Fe = 4, mF = +1 > excited state with the single-photon detuning of ~ -20 GHz. Rabi flopping experiments are in progress.
Summary form only given.Laser cooling, where excess kinetic energy is extracted by a photon, is ideally suited to loading atoms into a trap; however, quantization and reabsorption of light, limit the maximum phase-space density attainable. A sequence of laser cooling followed by evaporative cooling, where the excess energy is extracted by escaping atoms, may be used to overcome the laser cooling limit; however, such a scheme dramatically reduces the number of trapped atoms. A remaining challenge is to achieve continuous loading without the phase-space density limit imposed by near-resonant light. We consider continuous evaporative loading of a magnetic trap using a magnetically insensitive buffer gas. The key is to deliver sufficient cold atoms to the trapping region such that the steady-state trap population is large. To achieve this we employ an atomic fountain guided by a far-off resonant laser beam. At the apex of the fountain, the density can be a factor of 30-40 higher than in a magneto-optical trapping region, and importantly, the atoms are spatially separated from the region of resonant laser light. We propose to use this high-density region as a reservoir for continuous evaporative loading of a trap.
In our experiment, a single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) can be efficiently transferred into a 1064-nm far-off-resonance microscopic optical dipole trap (FORT). The efficient transfer of the single atom between the two traps is used to determine the trapping lifetime and the effective temperature of the single atom in FORT. The typical trapping lifetime has been improved from ∼ 6.9 s to ∼ 130 s by decreasing the background pressure from ∼ 1 × 10 −10 Torr to ∼ 2 × 10 −11 Torr and applying one-shot 10-ms laser cooling phase. We also theoretically investigate the dependence of trapping lifetimes of a single atom in a FORT on trap parameters based on the FORT beam’s intensity noise induced heating. Numerical simulations show that the heating depends on the FORT beam’s waist size and the trap depth. The trapping time can be predicted based on effective temperature measurement of a single atom in the FORT and the intensity noise spectra of the FORT beam. These experimental results are found to be in agreement with the predictions of the heating model.
We form a one-dimensional optical lattice for Cs atoms using light tuned a few thousand linewidths below atomic resonance. Atoms are selectively loaded into deeply bound states by adiabatic transfer from a superimposed, near-resonance optical lattice. This yields a mean vibrational excitation n-bar\ensuremath{\approx}0.3 and localization \ensuremath{\Delta}z\ensuremath{\approx}\ensuremath{\lambda}/20. Light scattering subsequently heats the atoms, but the initial rate is only of order ${10}^{\mathrm{\ensuremath{-}}3}$ vibrational quanta per oscillation period. Low vibrational excitation, strong localization, and low heating rates make these atoms good candidates for resolved-sideband Raman cooling.