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arXiv:1905.12402 · Communications Physics

Transverse optical pumping of spin states

Or Katz, Ofer Firstenberg

Published Jun 19, 2019• 6 citations
arXivHTMLPDF

Abstract

Abstract Optical pumping is an efficient method for initializing and maintaining atomic spin ensembles in a well-defined quantum spin state. Standard optical pumping methods orient the spins by transferring photonic angular momentum to spin polarization. Generally the spins are oriented along the propagation direction of the light due to selection rules of the dipole interaction. Here we present and experimentally demonstrate that by modulating the light polarization, angular momentum perpendicular to the optical axis can be transferred efficiently to cesium vapor. The transverse pumping scheme employs transversely oriented dark states, allowing for control of the trajectory of the spins on the Bloch sphere. This new mechanism is suitable and potentially beneficial for diverse applications, particularly in quantum metrology.

DOI
10.1038/s42005-019-0170-4

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Abstract Optical pumping is an efficient method for initializing and maintaining atomic spin ensembles in a well-defined quantum spin state. Standard optical pumping methods orient the spins by transferring photonic angular momentum to spin polarization. Generally the spins are oriented along the propagation direction of the light due to selection rules of the dipole interaction. Here we present and experimentally demonstrate that by modulating the light polarization, angular momentum perpendicular to the optical axis can be transferred efficiently to cesium vapor. The transverse pumping scheme employs transversely oriented dark states, allowing for control of the trajectory of the spins on the Bloch sphere. This new mechanism is suitable and potentially beneficial for diverse applications, particularly in quantum metrology.
A new optical pumping scheme utilizing selective reflection is proposed and studied for the first time. This method enables us to observe the spin-polarization of atoms even at the high density, at which the pumping beam cannot be transmitted through a sample cell. Theoretical analysis shows that the light reflectivity at the glass-vapor interface is strongly dependent on the direction of the macroscopic magnetic moment of optical pumped atoms. Two types of demonstrating experiments are carried out with Cs and Rb vapors at the temperature ∼ 200°C. In the transverse pumping experiment with a modulated laser beam, asymmetry of the magnetic resonance signal with respect to the inversion of the magnetic field direction is observed as predicted by the theory. In the longitudinal pumping experiment, the transfer of a large amount of the magnetic moment by spin-exchange collisions is directly observed.
Abstract A century ago Einstein introduced the notion of the linear momentum of a photon, h k, and analyzed its transfer to an atom (and back) in the establishment of thermodynamic equilibrium between black-body radiation and matter. In addition to a linear momentum, the photon also possesses an angular momentum, ħ, directed parallel (or antiparallel) to its propagation direction (the OZ axis in Fig. 4.1), depending on its polarization direction (left- or right-handed). This angular momentum is due to the spin of the photon, equal to 1. Half a century later, Kastler (1950) considered the use of the transfer of the angular momentum of a polarized photon to the angular momentum of an atom as a method of optical orientation of atoms. In essence, this was the first method to control atomic motion by means of low-intensity light, developed even before the advent of the laser; it which found widespread application in investigating and then using spin polarization effects in atomic physics. But even after lasers had made their appearance, the method of optical orientation of atoms (or pumping of certain magnetic sublevels in atoms) proved very important, specifically in the laser cooling of atoms (Chapter 5).
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.
The effect of optically pumped atomic spin orientation on the atom–dimer equilibrium in cesium vapor has been observed. Such a shift in equilibrium was predicted over two decades ago and is due to the redistribution of spin-determined bonding and antibonding collision trajectories. Orientation of the total spin angular momentum of 6 2S1/2 ground state Cs atoms in the vapor phase was produced by optical pumping with a tunable cw dye laser. A decrease in the equilibrium molecular density resulted and was measured using laser induced fluorescence from the B 1Πu state. The shift in equilibrium is a function of the degree of atomic orientation and could be observed in the temperature range 383–421 K for cesium vapor in a glass cell which also contained a He–N2 buffer gas mixture with a composition ratio of 9:1.
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 $\ensuremath{\mu}\mathrm{s}$.