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Physical Review Letters

Atomic velocity selection using stimulated Raman transitions

Mark A. Kasevich, David S. Weiss, Erling Riis, Kathryn A. Moler, Steven Kasapi, Steven Chu

Published May 6, 1991• 305 citations
Publisher

Abstract

Stimulated Raman transitions between the ground-level hyperfine states of atoms have been used to manipulate slowly moving atoms in an atomic fountain. An ensemble of sodium atoms with an inferred velocity spread along one dimension of 270 \ensuremath{\mu}m/sec has been prepared by this technique. We also show that this velocity-selection method is effective in measuring ultracold temperatures of laser-cooled atoms in a regime where traditional ballistic methods fail.

DOI
10.1103/physrevlett.66.2297

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Stimulated Raman transitions between the ground-level hyperfine states of atoms have been used to manipulate slowly moving atoms in an atomic fountain. An ensemble of sodium atoms with an inferred velocity spread along one dimension of 270 \ensuremath{\mu}m/sec has been prepared by this technique. We also show that this velocity-selection method is effective in measuring ultracold temperatures of laser-cooled atoms in a regime where traditional ballistic methods fail.
A technique for cooling atoms in two dimensions to temperatures below the single photon recoil limit k{sub B}T{sub rec} = (hk){sup 2}/2M is demonstrated. This technique combines velocity selection using stimulated Raman transitions and dissipation by spontaneous emission. It is an extension of the one dimensional cooling technique that was demonstrated recently. Experimental results for sodium atoms are presented, and the limits on the minimal cooling temperatures are analyzed. Finally, the possibility of applying the Raman cooling technique to atoms in a dipole trap is discussed.
It is known [1-3], that the samples of atoms with a velocity spread much lower than the recoil velocity uR= ħk/M (k is the wave number of exciting light field, M is the mass of atom) can be obtained by use of velocity selection with stimulated Raman transitions. A theoretical analysis of the velocity selection method is presented in [2,3]. So, it was shown in [2] that the formation of such narrow velocity structures is performed not only by the counterpropagating, but also by co-propagating light waves. This fact allows to form the two-dimensional velocity distribution of the widths δvx,y « uR by the velocity selection technique using laser configuration as in Fig. 1b.
We report the 1D cooling of ^{85}Rb atoms using a velocity-dependent optical force based upon Ramsey matter-wave interferometry. Using stimulated Raman transitions between ground hyperfine states, 12 cycles of the interferometer sequence cool a freely moving atom cloud from 21 to 3 μK. This pulsed analog of continuous-wave Doppler cooling is effective at temperatures down to the recoil limit; with augmentation pulses to increase the interferometer area, it should cool more quickly than conventional methods and be more suitable for species that lack a closed radiative transition.
Sodium atoms prepared in hyperfine ground-state 11 > are driven to the other ground-state hyperfine level 12> when two laser beams nearly resonant with an optical transition have a frequency difference equal to the ground-state hyperfine splitting. When the two laser beams are counterpropagating, conservation of momentum between the atom and the light field requires that the atom acquire a velocity kick ~22 ħk/m during the 11> and 12> transition (~6 cm/sec in our experiment). When the lasers are detuned far from the optical transition, spontaneous emission from the intermediate optical level becomes negligible, and the atom behaves as a two-level system coupled to a resonant rf driving field. We have created an interferometer by applying a π/2-π-π/2 Raman pulse sequence: a first π/2 pulse coherently splits the atomic wave packet by putting it in a superposition of states 11> and 12>, a second π pulse occurring a time Δt later redirects each wave packet and a final π/2 pulse recombines the wave packets at time 2Δt. We have observed wave packet interference for wave packet separations of 2.4 mm and, by using the interferometer as an accelerometer, have measured the atom’s acceleration due to gravity to 3 parts in 106.
We present an elegant application of matter-wave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the one-dimensional velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely expanding 85Rb^{85}\mathrm{Rb}85Rb atoms with temperatures of 34 and $18\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favorably with conventional Doppler and time-of-flight techniques, and it reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matter-wave accelerometry, gravimetry, and rotation sensing.