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arXiv:1006.5618 · Physical Review A

Efficient two-dimensional subrecoil Raman cooling of atoms in a tripod configuration

Vladimir S. Ivanov, Yu. Rozhdestvensky, Kalle‐Antti Suominen

Published Feb 7, 2011• 6 citations
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Abstract

We present an efficient method for subrecoil cooling of neutral atoms by applying Raman cooling in two dimensions to a four-level tripod system. The atoms can be cooled simultaneously in two directions using only three laser beams. We describe the cooling process with a simple model showing that the momentum distribution can be rapidly narrowed to velocity spread down to $0.1{v}_{\mathrm{rec}}$, corresponding to effective temperature equal to $0.01{T}_{\mathrm{rec}}$. This method opens new possibilities for cooling of neutral atoms.

DOI
10.1103/physreva.83.023407

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Robust two-dimensional subrecoil Raman cooling by adiabatic transfer in a tripod atomic system

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We demonstrate two-dimensional robust Raman cooling in a four-level tripod system, in which velocity selective population transfer is achieved by a stimulated Raman adiabatic passage. In contrast to basic two-dimensional Raman cooling with square-envelope pulses [Phys. Rev. A 83, 023407 (2011)], the technique presented here allows for a wide variation in the pulse duration and amplitude once the adiabaticity criterion is satisfied. An efficient population transfer together with attainment of a narrow spread of the resonant-velocity group leads to the narrowing of the velocity-distribution spread down to $0.1{v}_{\mathrm{rec}}$, corresponding to an effective temperature equal to $0.01{T}_{\mathrm{rec}}$. This robust method opens additional possibilities for cooling of neutral atoms.

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We present an efficient method for subrecoil cooling of neutral atoms by applying Raman cooling in two dimensions to a four-level tripod system. The atoms can be cooled simultaneously in two directions using only three laser beams. We describe the cooling process with a simple model showing that the momentum distribution can be rapidly narrowed to velocity spread down to 0.1vrec0.1{v}_{\mathrm{rec}}0.1vrec​, corresponding to effective temperature equal to 0.01Trec0.01{T}_{\mathrm{rec}}0.01Trec​. This method opens new possibilities for cooling of neutral atoms.
We demonstrate two-dimensional robust Raman cooling in a four-level tripod system, in which velocity selective population transfer is achieved by a stimulated Raman adiabatic passage. In contrast to basic two-dimensional Raman cooling with square-envelope pulses [Phys. Rev. A 83, 023407 (2011)], the technique presented here allows for a wide variation in the pulse duration and amplitude once the adiabaticity criterion is satisfied. An efficient population transfer together with attainment of a narrow spread of the resonant-velocity group leads to the narrowing of the velocity-distribution spread down to 0.1vrec0.1{v}_{\mathrm{rec}}0.1vrec​, corresponding to an effective temperature equal to 0.01Trec0.01{T}_{\mathrm{rec}}0.01Trec​. This robust method opens additional possibilities for cooling of neutral atoms.
Both atom localization and Raman cooling, considered in the thesis, reflect recent progress in the area of all-optical methods. We focus on twodimensional (2D) case, using a four-level tripod-type atomic scheme for atom localization within the optical half-wavelength as well as for efficient subrecoil Raman cooling. In the first part, we discuss the principles of 1D atom localization, accompanying by an example of the measurement of a spontaneously-emitted photon. Modifying this example, one archives sub-wavelength localization of a three-level -type atom, measuring the population in its upper state. We go further and obtain 2D sub-wavelength localization for a four-level tripod-type atom. The upper-state population is classified according to the spatial distribution, which in turn forms such structures as spikes, craters and waves. The second part of the thesis is devoted to Raman cooling. The cooling process is controlled by a sequence of velocity-selective transfers from one to another ground state. So far, 1D deep subrecoil cooling has been carried out with the sequence of square or Blackman pulses, applied to -type atoms. In turn, we discuss the transfer of atoms by stimulated Raman adiabatic passage (STIRAP), which provides robustness against the pulse duration if the cooling time is not in any critical role. A tripod-type atomic scheme is used for the purpose of 2D Raman cooling, allowing one to increase the efficiency and simplify the realization of the cooling.
Subrecoil Raman laser cooling of lt;suplt;suplt;supgt;6lt;/suplt;/suplt;/supgt;Li atoms
We realize an efficient one-dimensional Raman cooling of 6Li atoms using 2S→2P Raman transition pulses and 2S→3P ultraviolet (UV) optical repumping. In this cooling scheme, we directly cool an initial cold atomic sample of 350 μK released from a standard magneto-optical trap down to a velocity spread of 0.8 recoil velocity, corresponding to an effective temperature of 4.5 μK, an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling. This technique may provide an ideal initial sub-recoil atomic sample for the application of the large-momentum-transfer in the recoil-sensitive lithium atom interferometers.
We report the implementation of a two-dimensional Raman cooling scheme using sequential excitations along the orthogonal axes. Using square pulses, we have cooled a cloud of ultracold cesium atoms down to an rms velocity spread of 0.39(5) recoil velocities, corresponding to an effective transverse temperature of 30nK30\phantom{\rule{0.3em}{0ex}}\mathrm{nK}30nK (0.15Trec)(0.15{T}_{\mathrm{rec}})(0.15Trec​). This technique can be useful to improve cold-atom atomic clocks and is particularly relevant for clocks in microgravity.
We use velocity-selective Raman pulses generated by two diode lasers to cool cesium atoms in 1 dimension to an r.m.s. of 1.2 mm/s. This is about 1/3 of the single-photon recoil velocity v rec = ℏk/M , where ℏk is the photon momentum and M the atom's mass. The corresponding effective temperature is a factor of 9 below the single-photon recoil temperature given by k B T rec /2 = 1/2 Mv rec 2 . Because of the high cesium mass, this temperature is only (23 ± 5) nanokelvin, the lowest 1D kinetic temperature reported to date.
We report the first observation of subrecoil laser cooling in two dimensions by velocity selective coherent population trapping. We have compressed the momentum distribution of a cloud of trapped metastable helium atoms along two orthogonal axes to $\ensuremath{\delta}p=\frac{\ensuremath{\hbar}k}{4}$, where $\ensuremath{\hbar}k$ is the photon momentum. The corresponding temperature, 16 times below the single-photon recoil temperature, is 250 nK. Atoms are pumped into a nonabsorbing state exhibiting four well-resolved momentum peaks with subrecoil widths. The momentum space density in the peaks is significantly greater than that in the uncooled distribution.