Synchronous Spin-Exchange Optical Pumping

We demonstrate a new approach to precision NMR with hyperpolarized gases designed to mitigate NMR shifts due to the alkali spin-exchange field. The NMR bias field is implemented as a sequence of alkali (Rb) $2\pi$ pulses, allowing the Rb polarization to be optically pumped transverse to the bias field. When the Rb polarization is modulated at the noble-gas (Xe) NMR resonance, spin-exchange collisions buildup a precessing transverse Xe polarization. We study and mitigate novel NMR broadening effects due to the oscillating spin-exchange field. Spin-exchange frequency shifts are suppressed $2500\times$, and Rb magnetometer gain measurements project photon shot-noise limited NMR frequency uncertainties below $10\mathrm{nHz}/\sqrt{\mathrm{Hz}}$.

Figure 1

Essential components of synchronous spin-exchange optical pumping. An NMR bias field ${\mathit{B}}_1(t)$ is applied as a sequence of short alkali $2\pi$ pulses, allowing the atoms to be optically pumped with resonant light perpendicular to ${\mathit{B}}_1$ with periodic polarization reversals from the polarization modulators. Spin-exchange collisions between Xe atoms and the oscillating Rb spins drive the NMR resonance. The field produced by the precessing Xe nuclei rotates the Rb spins, causing a Faraday rotation of the polarization of the probe light.

Figure 2

Xe precession signals detected by the embedded Rb magnetometer. The magnetometer gain reversals square-wave modulate the sinusoidal Xe precession. When the Rb spin-exchange field is compensated, signals detuned by one half-linewidth (blue) are smaller in amplitude and phase shifted with respect to the on-resonance case (red). Uncompensated, the spin-exchange fields greatly suppress phase shifts.

Figure 3

NMR line shapes for synchronously pumped ${}^{131}\mathrm{Xe}$, $n_S=7.1\times {10}^{12}{\mathrm{cm}}^{-3}$. (a) When the spin-exchange field is compensated, the rotating-frame polarizations show the expected Lorentzian forms (red: in phase or ${\tilde{K}}_x$, blue: ${\tilde{K}}_y$, black: $K_z$), and little longitudinal polarization is generated. (b) An uncompensated spin-exchange field not only broadens the NMR resonance, but produces large $z$ polarizations and dramatically suppresses the rotating frame quadrature component.

Figure 4

Comparison of observed spin-exchange broadening with Eqs. (8) and (10). Bottom: The broadening of the NMR linewidth due to the spin-exchange field can be eliminated by applying a compensation field that oscillates 180° out of phase with the spin-exchange field. Solid lines and dots show ${}^{131}\mathrm{Xe}$ linewidths, dashes and unfilled dots show a sample for ${}^{129}\mathrm{Xe}$. Densities are listed in units of ${10}^{12}{\mathrm{cm}}^{-3}$. Top: Representative measurement of the slope of the quadrature signal as a function of compensation field.