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arXiv:2301.13121 · Journal of Physics B Atomic Molecular and Optical Physics

Efficient cooling of high-angular-momentum atoms

Logan Edward Hillberry, Dmitry Budker, Simon Rochester, Mark G. Raizen

Published Jul 5, 2023• 2 citations
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Abstract

Abstract We propose a highly efficient and fast method of translational cooling for high-angular-momentum atoms. Optical pumping and stimulated transitions, combined with magnetic forces, can be used to compress phase-space density, and the efficiency of each compression step increases with the angular momentum. Entropy is removed by spontaneously emitted photons, and particle number is conserved. This method may be an attractive alternative to evaporative cooling of atoms and possibly molecules in order to produce quantum degenerate gases.

DOI
10.1088/1361-6455/ace4ad

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Abstract We propose a highly efficient and fast method of translational cooling for high-angular-momentum atoms. Optical pumping and stimulated transitions, combined with magnetic forces, can be used to compress phase-space density, and the efficiency of each compression step increases with the angular momentum. Entropy is removed by spontaneously emitted photons, and particle number is conserved. This method may be an attractive alternative to evaporative cooling of atoms and possibly molecules in order to produce quantum degenerate gases.
We propose an alternative method to laser cooling. Our approach utilizes the extreme brightness of a supersonic atomic beam, and the adiabatic atomic coilgun to slow atoms in the beam or to bring them to rest. We show how internal-state optical pumping and stimulated optical transitions, combined with magnetic forces, can be used to cool the translational motion of atoms. This approach does not rely on momentum transfer from photons to atoms, as in laser cooling. We predict that our method can surpass laser cooling in terms of flux of ultracold atoms and phase-space density, with lower required laser power.
We propose methods of optical pumping that are applicable to open, high-angular-momentum transitions in atoms and molecules, for which conventional optical pumping would lead to significant population loss. Instead of applying circularly polarized cw light, as in conventional optical pumping, we propose to use techniques for coherent population transfer (e.g., adiabatic fast passage) to arrange the atoms so as to increase the entropy removed from the system with each spontaneous decay from the upper state. This minimizes the number of spontaneous-emission events required to produce a stretched state, thus reducing the population loss due to decay to other states. To produce a stretched state in a manifold with angular momentum JJJ, conventional optical pumping requires about 2J2J2J spontaneous decays per atom; one of our proposed methods reduces this to about log22J{log}_{2}2Jlog2​2J, while another of the methods reduces it to about one spontaneous decay, independent of JJJ.
We propose an alternative method to laser cooling. Our approach utilizes the extreme brightness of a supersonic atomic beam, and the adiabatic atomic coilgun to slow atoms in the beam or to bring them to rest. We show how internal-state optical pumping and stimulated optical transitions, combined with magnetic forces, can be used to cool the translational motion of atoms. This approach does not rely on momentum transfer from photons to atoms, as in laser cooling. We predict that our method can surpass laser cooling in terms of flux of ultracold atoms and phase-space density, with lower required laser power. © 2014 Optical Society of America OCIS codes: (020.0020) Atomic and molecular physics; (020.7010) Laser trapping; (020.7490) Zeeman effect; (020.3320) Laser cooling.
We present a novel technique of efficient optical pumping of open, high-angular-momentum systems. The method combines two well-established approaches of population manipulation (conventional optical pumping and coherent population transfer), offering the ability to achieve higher population of a sublevel with the highest or lowest quantum number m (the “end state”) than obtainable with either of the techniques. To accomplish this task, we propose to use coherent-population-transfer technique (e.g., adiabatic fast passage) to arrange the system in such a way that spontaneously emitted photon (conventional optical pumping) carries away more entropy than in conventional schemes. This enables reduction of a number of spontaneous decays Nsd required to pump the system with the total angular momentum J from Nsd = J decays in the conventional scheme to Nsd ≲ log2(2J) decays in the proposed scheme. Since each spontaneous-emission event is potentially burdened with a loss of population (population is transferred to a dark state), this enables increasing population accumulated in the “end state”, which is important for many applications.
Demagnetization cooling utilizes dipolar relaxations that couple the internal degree of freedom (spin) to the external (angular momentum) in order to cool an atomic cloud efficiently. Optical pumping into a dark state constantly recycles the atoms that were thermally excited to higher spin states. The net energy taken away by a single photon is very favorable since the lost energy per atom is the Zeeman energy rather than the recoil energy. As the density of the atomic sample rises the presence of the photons leads to limiting processes. In our previous publication [Volchkov et al. (2014)] we have shown that light-assisted collisions are such an important limiting process. In this paper we suppress light-assisted collisions by detuning the optical pumping light such that the Condon point coincides with the first node of the ground state wave function of two colliding atoms. This leads to an increased cooling efficiency χ ≥ 17 as well as to increased maximum densities of n ≈ 1 · 10(20) m(-3). However, due to the high number of involved molecular states the net suppression is not strong enough to reach quantum degeneracy.
Protocols for attaining quantum degeneracy in atomic gases almost exclusively rely on evaporative cooling, a time-consuming final step associated with substantial atom loss. We demonstrate direct laser cooling of a gas of rubidium-87 (⁸⁷Rb) atoms to quantum degeneracy. The method is fast and induces little atom loss. The atoms are trapped in a two-dimensional optical lattice that enables cycles of compression to increase the density, followed by Raman sideband cooling to decrease the temperature. From a starting number of 2000 atoms, 1400 atoms reach quantum degeneracy in 300 milliseconds, as confirmed by a bimodal velocity distribution. The method should be broadly applicable to many bosonic and fermionic species and to systems where evaporative cooling is not possible.