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Journal of Physics Conference Series

Fundamentals of metastability exchange optical pumping in helium

M Batz, Pierre-Jean Nacher, Geneviève Tastevin

Published Jun 1, 2011• 34 citations
Publisher

Abstract

Advances in metastability exchange optical pumping (MEOP) at high laser powers, but also at high gas pressures and high magnetic field strengths, has provided strong motivation for revisiting the understanding of the limitations of this powerful technique. A comprehensive model has been developed for improved description of the combined effects of OP, ME, and relaxation, and of detailed MEOP features observed over the broad range of operating conditions. A brief description is provided, with illustrative comparisons of computed and experimental results. This improved tool is used to explain the excellent photon efficiency of OP obtained at all field strengths. It is combined with an angular momentum budget approach to quantitatively investigate the newly discovered strong OP-enhanced polarisation losses that currently limits MEOP performance.

DOI
10.1088/1742-6596/294/1/012002

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Journal of Physics Conference Series•Jun 1, 2011•67 citations
Advances in metastability exchange optical pumping (MEOP) at high laser powers, but also at high gas pressures and high magnetic field strengths, has provided strong motivation for revisiting the understanding of the limitations of this powerful technique. A comprehensive model has been developed for improved description of the combined effects of OP, ME, and relaxation, and of detailed MEOP features observed over the broad range of operating conditions. A brief description is provided, with illustrative comparisons of computed and experimental results. This improved tool is used to explain the excellent photon efficiency of OP obtained at all field strengths. It is combined with an angular momentum budget approach to quantitatively investigate the newly discovered strong OP-enhanced polarisation losses that currently limits MEOP performance.
At low magnetic field, the efficiency of metastability-exchange optical pumping of helium-3 is known to be optimal for pressures around 1 mbar. We demonstrate on several examples (up to 32 mbar) that operating in a higher magnetic field (up to 0.12 T) can significantly increase the nuclear polarisations achieved at higher pressures. Since polarisation measurements cannot be made with the standard technique, we use a general optical method based on absorption measurements at 1083 nm to measure the polarisation of the atoms in the ground state.
Abstract Achieving long spin coherence times is crucial for quantum precision measurements, and closed-loop control techniques are often employed to accomplish this goal. Here, we demonstrate the impact of closed-loop feedback control on nuclear spin precession in a metastability exchange optical pumping (MEOP)-based polarized 3 He system. We analyze the effects of feedback theoretically and validate our predictions experimentally. With optimized feedback parameters, the spin coherence time T 2 is extended by an order of magnitude. When the feedback strength surpasses a critical threshold, robust maser oscillations are spontaneously excited, demonstrating remarkable resistance to environmental noise and maintaining stable oscillation. This proof-of-principle experiment highlights the viability of MEOP-based 3 He spin oscillators, especially in low-frequency domains. The operational simplicity and easy integration associated with MEOP-based systems make them particularly promising for fast, high-precision magnetic field measurements.
Metastability exchange optical pumping (MEOP) is a widely used technique for producing polarized 3^{3}3He. In connection with an experiment to search for the electric dipole moment of the neutron (nEDM) we have built a MEOP based 3^{3}3He polarization and injection system to prepare 80 % polarized 3^{3}3He at room temperature which will be injected into a ~400 mK measurement cell filled with superfluid 4^{4}4He. We describe the polarization and injection system, which is designed to allow for final concentrations of 10−8−10−1010^{-8}-10^{-10}10−8−10−10 of 80 % polarized 3^{3}3He in the superfluid filled measurement cell. Only ≈0.72%\approx0.72\%≈0.72% polarization loss due to gradients is expected during injection
Metastability-exchange collisions (MECs) lie at the heart of metastability-exchange optical pumping (MEOP) in 3He, enabling the transfer of polarization from the metastable state to the ground state, as well as the optical detection of nuclear magnetic resonance. Leveraging MECs, optically pumped 3He nuclear magnetometers have been developed since the earliest demonstrations of MEOP. However, it also induces an additional frequency shift and relaxation of the nuclear spin precession, thereby limiting the sensitivity of the magnetometer. In this work, we identify a new source of frequency shift and relaxation in the 3He nuclear spin, arising from the light shift. This effect arises from an MEC-mediated interaction between light and the nucleon spin. We develop a theoretical model to describe this light-induced effect and highlight its significance in low magnetic fields. This effect is experimentally demonstrated, and its dependence on various parameters -- including magnetic field strength, light intensity, and wavelength -- is investigated. Our result provides a better understanding of the frequency shift and relaxation of 3He spin precession under MEOP conditions. Moreover, our experiment reveals an MEC-mediated coupling between the 3He nuclear spin and light, which may indicate the feasibility of MEC-assisted optical manipulation of 3He nuclear spins at the quantum level, as proposed in several theoretical schemes.
Highly polarized 3He ensembles prepared by metastability-exchange optical pumping (MEOP) have been widely used in precision measurements and fundamental physics. Metastability-exchange (ME) collisions, serving as the basis of MEOP, are traditionally described in terms of atomic orientation, while the significant contributions of metastable alignment tensor at high polarization remain unexplored. In this work, we develop a linearized model under mean-field approximation to investigate alignment tensor effects in highly polarized 3He , which originate from the metastable F = 3/2 manifold and are revealed through ME-induced relaxation and frequency shift. By means of free-induction-decay (FID) measurements, a pronounced dependence on nuclear polarization is experimentally observed in the response of the ground-state-metastable hybrid 3He ensembles to the external magnetic field. Furthermore, after obtaining the characteristics of tensor-induced phenomena, we demonstrate good agreement between the experiment and the theory. This work advances the understanding of nuclear spin dynamics in highly polarized 3He using MEOP. It further provides applications in systematic error correction of high-accuracy magnetometry, as well as in optimal protocol for the generation of nuclear spin-squeezed states.
The standard model of Spin-Exchange Optical Pumping of 3 He has come under close scrutiny as researchers try to maximize the quantity and polarization of 3 He. This paper will review four important effects that have come to light: 1) pumping with frequency narrowed diode lasers significantly improves the polarization and production rate as compared to off-the-shelf broadband sources 2) Rb-K mixtures again improve the production rate due to reduced collisional loss in K-He collisions, 3) a unexplained 3 He wall relaxation mechanism that scales closely with alkali vapor pressure limits the attainable 3 He polarization to about 80%, 4) a small collisionally-induced breakdown in the selection rules for optical pumping explains why narrowband lasers are so advantageous, and may explain some observed limitations for Rb-K pumping as well.