We describe a new approach to precision NMR with hyperpolarized gases designed to mitigate NMR frequency shifts due to the alkali spin exchange field. The electronic spin polarization of optically pumped alkali atoms is square-wave modulated at the noble-gas NMR frequency and oriented transverse to the DC Fourier component of the NMR bias field. Noble gas NMR is driven by spin-exchange collisions with the oscillating electron spins. On resonance, the time-average torque from the oscillating spin-exchange field produced by the alkali spins is zero. Implementing the NMR bias field as a sequence of alkali 2π-pulses enables synchronization of the alkali and noble gas spins despite a 1000-fold discrepancy in gyromagnetic ratio. We demonstrate this method with Rb and Xe, and observe novel NMR broadening effects due to the transverse oscillating spin exchange field. When uncompensated, the spin-exchange field at high density broadens the NMR linewidth by an order of magnitude, with an even more dramatic suppression (up to 70x) of the phase shift between the precessing alkali and Xe polarizations. When we introduce a transverse compensation field, we are able to eliminate the spin-exchange broadening and restore the usual NMR phase sensitivity. The projected quantum-limited sensitivity is better than 1 nHz// √ Hz.The ability to produce highly magnetized noble gases via spin-exchange collisions with spin-polarized alkali atoms [1] has greatly impacted scientific studies of magnetic resonance imaging [2], high-energy nuclear physics with spin-polarized targets [3], and chemical physics [4]. Applications in precision measurements began with NMR gyros [5] and have continued with fundamental symmetry tests using multiple cell free induction decay [6], dual-species masers [7,8], selfcompensating co-magnetometers [9], NMR oscillators [10], and free spin-precession co-magnetometers [11][12][13][14].Some of these approaches [5, 9-11, 14] take advantage of enhanced NMR detection by the embedded alkali magnetometer. The alkali and noble-gas spin ensembles experience enhanced polarization sensitivity due to the Fermi-contact interaction during collisions between the two species. The effective Fermi-contact fields experienced by the two species arewhere S, K are the electron and nuclear spin operators, n S , n K the atomic densities, µ K the nuclear magnetic moment of the noble gas, µ B the Bohr magneton, and the atomic g-factor g ≈ 2. The frequency-shift enhancement factor κ [15,16] was recently measured to be 493 ± 31 [17] for RbXe. Thus the detected NMR field B SK is ∼500× larger for the embedded magnetometer than for any external sensor. This seemingly decisive advantage comes with the cost of similarly enhancing the B KS field due to the spin-polarized alkali atoms, 190 µG at 2n S S = 10 13 cm −3 . In typical longitudinally polarized NMR this field produces large frequency shifts of order 0.1 Hz. One approach for mitigating this effect is to compare two Xe isotopes [5,11], for which the enhancement factors are equal to about 0....

We demonstrate a transversely polarized spin-exchange pumped noble gas comagnetometer which suppresses systematic errors from longitudinal polarization. Rb atoms as well as 131 Xe and 129 Xe nuclei are simultaneously polarized perpendicular to a pulsed bias field. Both Xe isotopes' nuclear magnetic resonance conditions are simultaneously satisfied by frequency modulation of the pulse repetition rate. The Rb atoms detect the Xe precession. We highlight the importance of magnetometer phase shifts when performing comagnetometry. For detection of non-magnetic spin-dependent interactions the sensing bandwidth is 1 Hz, the white-noise level is 7 µHz / √ Hz, and the bias instability is ≈ 1 µHz.Spin-exchange (SE) pumped comagnetometers [1, 2] utilize co-located ensembles of spin-polarized alkali-metal atoms and noble gas nuclei [3] to suppress magnetic field noise. Such devices have been used to place upper bounds on spin-mass couplings [4,5], Lorentz violations [6-10], and atomic electric dipole moments [11][12][13], and for the measurement of inertial rotation [2,[14][15][16]. The fundamental uncertainty of a SE pumped comagnetometer's measure of inertial rotation scales favorably with sensor size compared to alternative technologies [17].Longitudinal SE fields are important sources of systematic uncertainty in devices which utilize the embedded alkali-metal atoms for high SNR detection. Consider an ensemble of two noble gas species (a and b) which are spin-exchange optically pumped (SEOP) in a common magnetic field B z and are each subject to some spin-dependent interaction X. The Larmor resonance frequency of isotope a can be written as [2,[18][19][20]]where γ is the gyromagnetic ratio, S and K are the respective alkali-metal and noble gas polarizations, z subscripts refer to the longitudinal components (i.e., parallel to the bias field direction), and b i j is the SE coefficient characterizing the influence of j's polarization on i. With knowledge of ρ = γ a /γ b , simultaneous measurement of Ω a and Ω b allows B z to be suppressed while sensitivity to X a z and X b z is retained [21]. In this paper we demonstrate a SE pumped 131 Xe-129 Xe comagnetometer which suppresses time-averaged S z and K z such thatwhere the superscripts a and b refer to 129 Xe and 131 Xe, respectively, and ρ = 3.373417(38) [22]. The comagnetometer relies on a dual-species version of synchronous SEOP [23], wherein noble gas nuclei are continuously polarized transverse to a frequency modulated pulsed bias field. We demonstrate that the correlation between the frequency of precession of 131 Xe and 129 Xe is sufficient FIG. 1: Schematic of apparatus. Field shim coils are not shown. The green trace depicts the frequency modulated bias field pulses.to resolve 7 µHz / √ Hz of white frequency noise with a low frequency field noise suppression of > 10 3 . We also demonstrate the influence of alkali-metal magnetometer phase shifts on the field noise suppression of the comagnetometer.A schematic of the experimental setup is shown in Fig. 1. 85 Rb atoms...

Inertial navigation systems generally consist of timing, acceleration, and orientation measurement units. Although much progress has been made towards developing primary timing sources such as atomic clocks, acceleration and orientation measurement units often require calibration. Nuclear Magnetic Resonance (NMR) gyroscopes, which rely on continuous measurement of the simultaneous Larmor precession of two co-located polarized noble gases, can be configured to have scale factors that depend to first order only on fundamental constants. The noble gases are polarized by spin-exchange collisions with co-located optically pumped alkali-metal atoms. The alkali-metal atoms are also used to detect the phase of precession of the polarized noble gas nuclei. Here we present a version of an NMR gyroscope designed to suppress systematic errors from the alkali-metal atoms. We demonstrate rotation rate angle random walk (ARW) sensitivity of 16μHz/Hz and bias instability of ∼800 nHz.

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