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) 2π 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×, and Rb magnetometer gain measurements project photon shot-noise limited NMR frequency uncertainties below 10 nHz/sqrt[Hz].
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) 2π 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×, and Rb magnetometer gain measurements project photon shot-noise limited NMR frequency uncertainties below 10 nHz/sqrt[Hz].
We demonstrate a transversely polarized spin-exchange pumped noble gas comagnetometer which suppresses systematic errors from longitudinal polarization. Rb atoms as well as 131Xe and 129Xe 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 nonmagnetic spin-dependent interactions the sensing bandwidth is 1 Hz, the white-noise level is $7\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{Hz}/\sqrt{\text{Hz}}$, and the bias instability is $\ensuremath{\sim}1$ $\ensuremath{\mu}\mathrm{Hz}$.
We demonstrate a novel quantum sensor for measuring non-magnetic spin-dependent interactions. This sensor utilizes 131Xe, 129Xe, and 85Rb which are continuously polarized transverse to a pulsed bias field. The transverse geometry of this spin-exchange pumped comagnetometer suppresses longitudinal polarization, which is an important source of systematic error. Simultaneous excitation of both Xe isotopes is accomplished by frequency modulating the repetition rate of the bias field pulses at subharmonics of the Xe Larmor resonance frequencies. The area of each bias pulse causes 2π Larmor precession of the Rb. We present continuous dual-species Xe excitation and discuss a temperature-dependent wall interaction that limits the 129Xe polarization. The Rb atoms serve as an embedded magnetometer for detection of the Xe precession. We discuss Rb magnetometer phase shifts, and show that even first-order treatments of these phase shifts can result in order-of-magnitude improvements in the achieved field suppression when performing comagnetometry. The sensing bandwidth of the presented device is 1 Hz, and we demonstrate a white-noise level of 7 μHz/Hz and a bias instability of ∼1μHz.
Recently the demand for hyperpolarized noble gases is arising to improve NMR spectroscopy resolution for imaging. Hyperpolarization of noble gases is achieved by spin- exchange with alkali metal. atoms that undergo optical pumping by circularly polarized radiation. Optical pumping is usually achieved by high power semiconductor lasers. These sources have a bandwidth very large when compared with the transition bandwidth of the optically active species. This has two serious consequences: (i) the request of high power lasers, (ii) the losses that the unemployed energy delivered to the Rb vapor can provide in the efficiency of the energy transfer. Object of our investigation will be the comparison of two experimental set ups for optical pumping of Rb vapor where pumping is given in one case by a dye laser pumped by a UV nitrogen laser built in our laboratory according to the Blumlein scheme and in the other case by a semiconductor laser. Major elements that will be discussed are (1) the role of stimulated emission of Rb vapor to increase the pumping efficiency, (2) the fate of resonant photons that are emitted during Rb decays, (3) the role of applied magnetic fields in the efficiency of pumping processes.
We present a constructed setup for polarizing 129 Xe noble gas. Hyperpolarized 129 Xe has been obtained via spin exchange with an optically pumped rubidium vapor. Optical pumping is based on polarizing the valence electron of rubidium by the resonant absorption of a circularly polarized laser light. The magnetic field of 30 G was used for obtaining 129 Xe polarization. The apparatus for detecting polarization is a nuclear magnetic resonance spectrometer. The highest 129 Xe polarization of 54% has been obtained using 60 W circularly polarized laser light with wavelength of 794.7 nm. The measured longitudinal relaxation time of the hyperpolarized 129 Xe was 72.3 minutes.
We present the principles, applications, and performance of new techniques developed to measure the frequency spectrum of freely precessing polarized noble-gas atoms. The noble-gas polarization technique of spin exchange with laser optically pumped Rb enables us to simultaneously polarize several species (3He, 21Ne, and 129Xe). With signal-to-noise ratios of 104, the free precession frequencies are determined with a precision of less than ${10}^{\mathrm{\ensuremath{-}}6}$ Hz out of 103 Hz in 1024 s. The technique will be applied to determine frequency shifts due to dipole couplings to applied fields and quadrupole splittings due to local Lorentz-invariance-violating spatial anisotropy. In all cases, the frequencies of two or more species are measured simultaneously in order to provide magnetometry for increased precision and to provide measurement and cancellation of systematic effects.
Analytical model for polarization transfer during gas-phase collision events in spin-exchange optical pumping: Spin-21129Xe versus spin-23131Xe
Spin-exchange optical pumping (SEOP) is a method for producing spin-hyperpolarized noble gas nuclei, such as 129Xe and 131Xe, which are used in various magnetic resonance applications from fundamental physics to quantum sensing and medical imaging. In SEOP, optically polarized alkali-metal atoms transfer their spin polarization to the noble gas nuclei in gas-phase collision events via the hyperfine coupling (HFC) between the alkali valence electron and the noble gas nucleus. While the polarization transfer physics of spin I=1/2 nuclei, such as 129Xe, is relatively well understood, that of spin I>1/2 nuclei, such as 131Xe (I=3/2), has been far less studied, and no rigorous theoretical model has been presented to date. To this end, we derive a simple analytical model for the upper limit, neglecting relaxation, of the SEOP polarization transfer, applicable to noble gases with arbitrary nuclear spin. Analytical evaluation of the Baker-Campbell-Hausdorff expansion for the time evolution of the spin density operator ρ^(t) reveals that only even-order terms in the HFC contribute to the polarization transfer, with the leading-order quadratic term being the most significant. We obtain a result similar to that derived for the spin-exchange cross section by Herman [Phys. Rev. 137, A 1062 (1965)], but in a more general framework for the time evolution of ρ^(t) that is also more familiar to magnetic resonance researchers. The model is applied to understand the difference in the polarization transfer efficiency between 129Xe and 131Xe, yielding results in agreement with previous experiments. We also validate the model by comparison to detailed numerical multiscale simulations of the SEOP process, where full quantum-chemically computed spin Hamiltonians sampled from molecular dynamics simulations of the gas-phase collision events are used to propagate the spin dynamics.