Light-induced Frequency Shift and Relaxation of Ground-State 3He via Metastability-Exchange Collisions

TL;DR

This work reveals a light-induced MEC-mediated interaction that shifts and relaxes the ground-state $^3$He spin under MEOP, supported by a theoretical model that treats a vector light shift on the metastable state and MECs coupling to the ground state. By adiabatically eliminating the metastable manifold, the authors derive a ground-state evolution with a complex parameter $\beta$, where the imaginary part drives a frequency shift and the real part modifies relaxation. Experiment at low fields confirms the predicted effects, showing that pump light can significantly alter Larmor frequencies and decoherence times, with the magnitude controlled by light intensity and detuning; importantly, the light shift can be exploited to mitigate MEC-induced shifts, enabling more precise magnetometry and offering a route to MEC-assisted optical control of nuclear spins.

Abstract

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.

Figures (5)

• Figure 1: The MECs induced frequency shift and relaxation of the nuclear spin precession at different strengths of magnetic field calculated from Eq. (\ref{['beta']}). The solid and dashed lines represent the results with and without the light-shift field, respectively.
• Figure 2: (a) Experimental setup of the MEOP-based $^3$He polarization system. The setup consists of three main components: (i) a guiding magnetic field generated by a Merritt coil system housed within a five-layer $\mu$-metal magnetic shield, (ii) an optical pumping system comprising a laser with approximately 2 W output power, along with polarization control and beam expansion optics, and (iii) a signal detection system employing a weak ($\sim$mW), linearly polarized probe laser propagating vertically, combined with a homodyne detection scheme. The inset plot shows a typical FID signal demodulated using a lock-in amplifier. LP: Linear Polarizer; QWP: Quarter Wave Plate; BT: Beam Trap; BD: Balanced Detector; PEM: Photoelastic Modulator; WP: Wollaston Prism. (b) Hyperfine structures of the atomic states of $^3$He involved in the optical pumping. The metastable state and ground state interact through the MECs. The red double arrow represents the optical pump light, and the wavy line indicates the decay of the excited state. The light shift alters the Zeeman sublevels in the metastable state, and through the MECs, the ground state is effected as well.
• Figure 3: The Larmor precession frequency versus the strength of the radio-frequency discharge at different powers of the pump laser. The solid lines are the linear fits. The data marked in green circles, yellow triangles, and blue squares are experimental results. The black star marks the intersection of these three curves.
• Figure 4: The transverse relaxation rate versus the strength of the guiding magnetic field with optical pumping on and off, respectively. The yellow diamonds and green squares represent experimental results, while the solid and dashed lines represent theoretical values derived from Eq. (\ref{['beta']}).
• Figure 5: The frequency shift and relaxation induced by the pump light depend on both its wavelength and intensity. Experimental results are shown as green squares and yellow diamonds, while the corresponding theoretical predictions from Eq. (\ref{['beta']}) are represented by the solid lines. In the left panel, the laser detuning is defined relative to the $C_8$ transition line, with the laser power set to approximately 1 W. In the right panel, the laser wavelength is red-detuned by approximately 1 GHz from the $C_8$ transition.