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arXiv:1608.08692 · Physical Review A

Efficient polarization of high-angular-momentum systems

Simon Rochester, Konrad Szymański, Mark G. Raizen, Szymon Pustelny, Marcis P. Auzinsh, Dmitry Budker

Published Oct 21, 2016• 13 citations
arXivHTMLPDF

Abstract

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 $J$, conventional optical pumping requires about $2J$ spontaneous decays per atom; one of our proposed methods reduces this to about ${log}_{2}2J$, while another of the methods reduces it to about one spontaneous decay, independent of $J$.

DOI
10.1103/physreva.94.043416

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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 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.
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.
Quantum-state preparation has a wide range of applications ranging from quantum optics to quantum metrology to fundamental physics precision measurements. In the context of nuclear $\ensuremath{\beta}$-decay correlation experiments such as the ones using argon, one needs to prepare a source of highly polarized atoms. Creating such a state-prepared sample amounts to generating a so-called stretched state, i.e., a state with the maximal projection of the total angular momentum along the quantization axis. This is typically achieved through optical pumping. Since this technique inherently depends on cycles of absorption followed by spontaneous emission, it may lead to undesirable heating and population loss. It is therefore crucial to devise polarization methods with both optimized efficiency and minimized drawbacks. We propose and compare various schemes, which we have been investigating numerically in the case of argon-35 atoms. We show that polarization degrees as high as 99.9999.99%99.99 can be obtained within less than 140 $\ensuremath{\mu}\mathrm{s}$.
In the optical pumping process, spontaneous emission plays a unique role in redistributing the population of the ground level among the magnetic sub-levels. It is clear that the branching ratio of the relevant transition in the optical pumping must be nearly equal to unity in order to avoid the loss of population involved. Unfortunately, most complicated atoms do not have a simple energy level structure and cannot be polarized using optical pumping. We propose here a new polarizing method of a purely laser-driven process that avoids spontaneous emission and can be used in many heavier atoms. For J = 0 ground-state atoms, a complete population transfer to the meta-stable state can be used as efficient laser isotope separation, whereas a coherently prepared superposition state of the non-magnetic ground state and one of the magnetic states of the meta-stable level can be used as a tool to study the decoherent process. For J of the ground level no less than unity, partly removing the population on one of its sub-magnetic states will enhance the magnetic resonance signal significantly.
The interaction of laser light of arbitrary polarization with systems of high angular momentum is considered. We show that elliptically polarized light creates an anisotropic spatial distribution of atomic and molecular angular momentum which is qualitatively different from the alignment and orientation induced by light of circular or linear polarization. Multilevel coherent population trapping within a manifold of ground-state magnetic sublevels results in a nonclassical behavior of a high-JJJ molecular rotor. The classical approximation for the angular momentum distribution is compared with the exact quantum calculations, and is shown to fail in cases of long interaction times and high intensities of the exciting light. In these limits, the quantum uncertainty defines the spatial width of the angular distribution. The applicability of the classical treatment is analyzed and found to be different in the cases of $\stackrel{\ensuremath{\rightarrow}}{J}J\ensuremath{-}1$ and $\stackrel{\ensuremath{\rightarrow}}{J}J$ transitions. A biaxial spatial orientation with two preferential axes of rotation is experimentally created in sodium atoms via coherent population trapping by elliptically polarized light. A method for producing an arbitrary orientation of atomic angular momentum by magnetic field assisted coherent population trapping is proposed.
The technique of stimulated Raman adiabatic passage has become an established procedure for producing complete population inversion in atoms or molecules via application of Stokes and pump pulses in a counterintuitive sequence. We discuss here some of the new and important phenomena that arise when some of the states involved have nonzero angular momentum, the field polarization directions have no simplifying symmetries, and Zeeman splitting lifts the magnetic sublevel degeneracy. We consider the effects of the linkage structure of the Hamiltonian, the effects of various polarization directions, and the effects of various choices for carrier frequencies, as expressed by detunings from single-photon resonance. In particular, we consider an example with angular momentum J=0\ensuremath{\leftrightarrows}J=1\ensuremath{\leftrightarrows}J=2 in which nine magnetic sublevels occur. We show that under suitable conditions it is possible, simply through the appropriate tuning of pump and Stokes carrier frequencies, to place the entire population into any specified single magnetic sublevel.