Optical protection of alkali-metal atoms from spin relaxation

Abstract

We present an optical technique for suppressing relaxation in alkali-metal spins using a single off-resonant laser beam. The method harnesses a physical mechanism that synchronizes Larmor precession in the two hyperfine manifolds, protecting magnetic coherence from relaxation caused by spin-exchange and other hyperfine-changing collisions. We experimentally demonstrate up to a ninefold reduction in decoherence of warm cesium vapor, achieving simultaneous protection from both spin-exchange relaxation and partial depolarization from coated cell walls. The technique substantially enhances the spin precession quality factor and maintains a stable gyromagnetic ratio independent of spin polarization, even under frequent collisions. These findings offer a pathway for mitigating dominant relaxation channels in alkali-metal-based applications and experiments, particularly in anti-relaxation-coated cells.

Figures (4)

• Figure 1: Relaxation of alkali-metal spins and light-shift compensation.a, Spin levels of cesium atoms (${I=7/2}$) in the electronic ground state. The magnetic levels $\ket{F,M}$ in the $F=4$ ($F=3$) hyperfine manifolds are Zeeman-splitted by $\hbar\omega_{\textrm{a}}$ ($\hbar\omega_{\textrm{b}}$) for $M\leq|F|$, resulting in asynchronous precession of the magnetic moments; spins precess clockwise at $F=4$ (blue) but counter-clockwise at $F=3$ (cyan). b, Spin-exchange collisions between pairs of atoms, or collisions with weakly-depolarizing walls of the enclosure, lead to random changes of the hyperfine manifolds and give rise to spin-relaxation by asynchronous precession. c, An off-resonance circularly-polarized optical beam shifts the magnetic levels within the hyperfine manifolds d, Calculated light-shift (complex) cross-sections $\sigma_{\textrm{a}}$ (cyan) and $\sigma_{\textrm{b}}$ (blue) of the Zeeman-like shifts in the $F=4$ and $F=3$ manifolds respectively, as a function of the optical frequency $\nu$ from the D$_1$ transitions. Black lines denote the absorption cross-section of the four transition lines and purple arrow denotes the range where the levels in $F=3$ manifold are primarily shifted. e, Applied Zeeman-like shifts can correct for the difference in precession frequencies and protect from dephasing by asynchronous precession.
• Figure 2: Synchronization of spin precession using light-shifts. a, Fourier spectrum of the measured spin precession of cesium spins at $B=0.43\,\textrm{mG}$. The protection beam with power $P_{\textrm{op}}$ induces a Zeeman-like field that shifts the precession frequency of the magnetic moments in $F=3$ (i.e. $\omega_{\textrm{b}}$, red dash-dots lines) and, to a lesser extent, in $F=4$ (i.e., $\omega_{\textrm{a}}$, orange dash line). At the resonance condition $\omega_{\textrm{a}}=\omega_{\textrm{b}}$ near $P_{\textrm{op}}=9.7\,\textrm{mW}$ the frequencies synchronize and the spectrum is significantly enhanced. b, Spin precession without the optical protection beam (top) and in the presence of the protection beam (bottom). At synchronized precession, the coherence time is prolonged fivefold.
• Figure 3: Protection from spin relaxation.a, Measured fundamental relaxation rate $\Gamma$ of cesium vapor as a function of the magnetic field $B$ and power $P_{\textrm{op}}$ of the optical protection beam. The relaxation is minimized along the resonance line $\omega_{\textrm{a}}(B,P_{\textrm{op}})=\omega_{\textrm{b}}(B,P_{\textrm{op}})$ (white dashed line), providing up to an eightfold reduction in relaxation compared to precession without light shifts. b, Calculated fundamental relaxation rate using the hyperfine-Bloch model (see text and SI). Increased relaxation is experimentally observed for $\omega_{\textrm{b}}(B,P_{\textrm{op}})=0$ (black dashed line).
• Figure 4: Experimentally measured number of visible precession cycles. The number of visible spin-precession cycles, $Q={\omega}/\Gamma$, is considerably higher along the resonance line (black dashed line) compared with low magnetic fields ($Q\lesssim1$). This significant improvement in $Q$ is a unique feature of the optical protection technique.