Spin-exchange Cross Section Research Articles

Magnetic resonance frequency shifts in quantum magnetometers based on the phenomenon of the optical orientation of atoms

Subject of study. Magnetic resonance frequency shifts caused by spin exchange collisions involving optically oriented alkali atoms in the ground state are studied. Aim of study. Spin-exchange collisions involving optically oriented pairs of alkali atoms of different types are theoretically studied to determine the shifts in the magnetic resonance line frequencies as a function of temperature for various pairs of alkali atoms under various optical orientation conditions in order to determine optimum constraints for the construction of quantum magnetometers with optical pumping using mixtures of alkali atoms. Method. Collisions between optically oriented alkali atoms are analyzed within the framework of quantum scattering theory, and data on the interaction potentials of alkali-atom dimers are used to calculate the scattering phases for collisions involving these potentials and the imaginary parts of the complex spin-exchange cross-section. The resulting cross sections as a function of energy were used to determine the magnetic resonance frequency shifts as a function of temperature. Main results. The magnetic resonance line frequency shifts for the following pairs of alkali atoms were obtained as a function of temperature: 39K−133Cs, 39K−85Rb, and 133Cs−85Rb. The shift in the magnetic resonance line involving the F=1 hyperfine state for a 39K−85Rb alkali-atom pair was found to pass through zero near temperature 480 K. Practical significance. The results obtained in this paper can be used to develop zero-spin-exchange-shift quantum electronic devices based on the optical orientation of atoms. In particular, it is possible to develop co-magnetometers based on the optical orientation of alkali atoms.

Light-narrowed parametric resonance magnetometer with the fundamental sensitivity beyond the spin-exchange limit.

In the field of biomagnetic measurements, one of the most important recent challenges is to perform measurements in a magnetically unshielded environment. This first requires that atomic magnetometers can operate in a finite magnetic field, and have enough high sensitivity. To meet these requirements, we develop a light-narrowed parametric resonance (LPR) magnetometer. By adding a modulation magnetic field to the large longitudinal magnetic field, our LPR magnetometer can measure small transverse magnetic fields with an intrinsic sensitivity of 3.5 fT/Hz1/2 in a longitudinal magnetic field of μT range. Moreover, we have also demonstrated that in contrast to the previous light-narrowed scalar magnetometers, our LPR magnetometer has the potential to achieve higher sensitivity. Because in our case spin-exchange relaxation suppression by using light narrowing can lead to an improvement of fundamental sensitivity limit regardless of which quantum noise is dominant, and hence the fundamental sensitivity is no longer limited by spin-exchange, and approaches the fundamental limit set by the spin-exchange and spin-destruction cross sections.

Cold collisions between alkali metals and alkaline-earth metals in the heteronuclear atom-ion system Li+Ba+

In a recent experiment [P. Weckesser et al., Nature (London) 600, 429 (2021)], the quantum $s$-wave regime was attained for an alkali-metal and alkaline-earth atom-ion combination $({\mathrm{LiBa}}^{+})$. We investigate possible outcomes from the interaction of this ion-atom pair at quantum regimes from a theoretical point of view. For this purpose, Born-Oppenheimer potential energy surfaces are constructed for the three lowest dissociation channels of the ${(\mathrm{Ba}\text{\ensuremath{-}}\mathrm{Li})}^{+}$ molecular system using a multireference configuration interaction electronic-structure calculation. We present elastic, spin-exchange (SE), and diffusion cross sections in different energy regimes. The collisional properties of this system are calculated in terms of the scattering phase shifts and scattering cross sections, and the semiclassical behavior at a relatively high energy limit is also examined. For SE collisions, phase locking is obtained towards lower partial waves.

Investigating Rubidium Density and Temperature Distributions in a High-Throughput 129Xe-Rb Spin-Exchange Optical Pumping Polarizer.

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of 129Xe-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput 129Xe-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S1/2→52P1/2 (D1)/52P3/2 (D2)) and violet (52S1/2→62P1/2/62P3/2) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D2 and 62P3/2 absorption by comparison with measurements of the D1 and 62P1/2 absorption lines, respectively. D2 absorption deviates from the Beer-Lambert law at [Rb]D2>4×1017 m−3 whilst 52S1/2→62P3/2 absorption deviates from the Beer-Lambert law at [Rb]6P3/2>(4.16±0.01)×1019 m−3. The measured [Rb] was used to estimate a 129Xe-Rb spin exchange cross section of γ′=(1.2±0.1)×10−21 m3 s−1, consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.

Enhanced Coupling of Electron and Nuclear Spins by Quantum Tunneling Resonances.

Noble-gas spins feature hours-long coherence times, owing to their great isolation from the environment, and find practical usage in various applications. However, this isolation leads to extremely slow preparation times, relying on weak spin transfer from an electron-spin ensemble. Here we propose a controllable mechanism to enhance this transfer rate. We analyze the spin dynamics of helium-3 atoms with hot, optically excited potassium atoms and reveal the formation of quasibound states in resonant binary collisions. We find a resonant enhancement of the spin-exchange cross section by up to 6 orders of magnitude and 2 orders of magnitude enhancement for the thermally averaged, polarization rate coefficient. We further examine the effect for various other noble gases and find that the enhancement is universal. We outline feasible conditions under which the enhancement may be experimentally observed and practically utilized.

Collisions of Lithium Atoms in Ground State. Complex Spin-Exchange Cross Sections

Collisions of Lithium Atoms in Ground State. Complex Spin-Exchange Cross Sections

Theoretical models of spin-exchange optical pumping: Revisited and reconciled

Theoretical models for continuous-flow and stopped-flow spin-exchange optical pumping of 129Xe have long predicted much higher 129Xe polarization values than are measured experimentally, leading to a search for additional depolarization mechanisms. In this work, we show that a misapplication of the general theory of spin-exchange optical pumping along with the incorrect use of previously measured spin-exchange constants has been perpetuated in the past 20 years and is the main cause of the long-held discrepancy between theoretical and experimental 129Xe polarization values. Following the standard theory of spin-exchange optical pumping developed almost 40 years ago by Happer et al., we outline the common mistake made in the application of this theory in modern theoretical models and derive a simplified expression of the spin-exchange cross section that can be used to correctly predict 129Xe polarization values under any set of experimental conditions. We show that the complete expression of the spin-exchange cross section derived using the work of Happer et al. predicts spin-exchange rates tenfold higher than those previously assumed in theoretical models of continuous-flow and stopped-flow spin-exchange optical pumping and can fully rectify the long-standing discrepancy between theoretical and experimental polarization values.

Complete Quantum Coherent Control of Ultracold Molecular Collisions.

We show that quantum interference-based coherent control is a highly efficient tool for tuning ultracold molecular collision dynamics that is free from the limitations of commonly used methods that rely on external electromagnetic fields. By varying the relative populations and phases of initial coherent superpositions of degenerate molecular states, we demonstrate complete coherent control over integral scattering cross sections in the ultracold s-wave regime of both the initial and final collision channels. The proposed control methodology is applied to ultracold O_{2}+O_{2} collisions, showing extensive control over s-wave spin-exchange cross sections and product branching ratios over many orders of magnitude.

Measuring the enhancement factor of the hyperpolarized Xe in nuclear magnetic resonance gyroscopes

In atomic gyroscopes, measuring the effective noble-gas magnetic field enables the determination of the noble-gas polarization. The enhancement factor is introduced to relate the effective magnetic field and the polarization in a certain magnetic-field environment. Earlier experiments concentrate on the strong magnetic-field environment for measuring the spin-exchange cross section precisely, so the results on the enhancement factor in the weak magnetic field are still insufficient. In our nuclear magnetic resonance gyroscope based on Cs-$^{129}\mathrm{Xe}$ and -$^{131}\mathrm{Xe}$, the magnetic field is usually set to 1--10 $\ensuremath{\mu}\mathrm{T}$. In this paper, we measure the effective magnetic field and the polarization of Xe atoms to estimate the Cs-Xe enhancement factor in the weak magnetic field of 2.5 $\ensuremath{\mu}\mathrm{T}$ and give the estimation of the Cs-$^{129}\mathrm{Xe}$ enhancement factor of $(1.60\ifmmode\pm\else\textpm\fi{}0.06)\ifmmode\times\else\texttimes\fi{}{10}^{4}$.

Rapid 129Xe-Rb spin-exchange rate measurement by using an atomic magnetometer.

We propose a fast and accurate method for $^ {129} {\rm Xe}{-}{\rm Rb}$129Xe-Rb spin-exchange rate determination based on an atomic magnetometer at low magnetic field and low pressure. The $^ {129} {\rm Xe}{-}{\rm Rb}$129Xe-Rb spin-exchange rate can be estimated via the product of the maximum transverse spin polarization of $^ {129} {\rm Xe}$129Xe nuclei and the corresponding Rabi frequency. The velocity-averaged binary spin-exchange cross section and the rate constant for spin-exchange in $^ {129} {\rm Xe}{-}{\rm Rb}$129Xe-Rb van der Waals complexes are measured to be ${\langle \sigma \nu \rangle _{{\rm SE}}} = ( {1.26 \pm 0.13} ) \times {10^{ - 16}}\;{{\rm cm}^ 3 }{\rm /s}$⟨σν⟩SE=(1.26±0.13)×10-16cm3/s and ${\kappa _{^4{\rm He}}} = 1326 \pm (47)\,{{\rm s}^{ - 1 }}$κ 4He=1326±(47)s-1 in $^4{\rm He}$4He buffer gas, respectively. With respect to the method based on the longitudinal relaxation rate, the method presented in this work can greatly facilitate the measurement process, determining the spin-exchange rate in several minutes. Moreover, this method is more suitable for vapor cells with long longitudinal relaxation time.

Collisions with Participation of Polarized Cesium and Lithium Alkali Atoms

The spin-exchange cross sections and the magnetic resonance frequency shift in collision of lithium and cesium atoms in the ground state are calculated. The calculations are performed based on the data on the singlet ($$X{}^{1}{{\Sigma }^{ + }}$$) and triplet ($$a{}^{3}{{\Sigma }^{ + }}$$) interaction potentials of 7Li 133Cs dimer. The passage from the energy dependences of the real and imaginary parts of the complex spin-exchange cross section to the temperature dependences makes it possible to obtain information on both the broadening and the frequency shift of the magnetic resonance line in collisions of the atoms under study.

Spin-Exchange Collisions in the K–Li System

Spin-exchange cross sections and the magnetic-resonance frequency shift in collision of lithium and potassium atoms in the ground state have been calculated for the first time. The cross sections are calculated based on the data on the singlet ($$X{}^{1}{{\Sigma }^{ + }}$$) and triplet ($$a{}^{3}{{\Sigma }^{ + }}$$) interaction potentials of 39K7Li dimer. A passage from the energy dependences to the temperature ones of the real and imaginary parts of the complex spin-exchange cross section yields information about both the broadening of the magnetic resonance line of the atoms under study and the magnetic-resonance frequency shift in their collision.

Спин-обменные столкновения в системе K-Li

AbstractSpin-exchange cross sections and the magnetic-resonance frequency shift in collision of lithium and potassium atoms in the ground state have been calculated for the first time. The cross sections are calculated based on the data on the singlet ( $$X{}^{1}{{\Sigma }^{ + }}$$ ) and triplet ( $$a{}^{3}{{\Sigma }^{ + }}$$ ) interaction potentials of ^39K^7Li dimer. A passage from the energy dependences to the temperature ones of the real and imaginary parts of the complex spin-exchange cross section yields information about both the broadening of the magnetic resonance line of the atoms under study and the magnetic-resonance frequency shift in their collision.

Interaction between spin polarized alkali atoms

Collisions of alkali-metal atoms in the ground state with the electron spin S = 1/2 are accompanied by exchange of electron coordinates between colliding particles, which leads to polarization transfer between them (i.e., to the well-known phenomenon of spin exchange). In addition, along with the polarization transfer from one partner to another, the magnetic resonance lines of colliding atoms are broadened and shifted in spin-exchange collisions. In particular the last two processes depend on the complex spin-exchange cross section. The real part of the cross section determines the so-called “spin-exchange cross section,” which is responsible for the broadening of magnetic resonance lines, while the imaginary part determines the shift cross section, which governs the frequency shift of magnetic resonance.In order to describe the spin-exchange process one have to know complex spin-exchange cross sections. Complex spin-exchange cross sections are calculated on the basis of the data on the singlet (X1Σ+) and triplet (a3Σ+) potentials describing the interaction alkali-metal atoms in the ground state.Collisions of alkali-metal atoms Cs, Rb, K, Na in the ground state are considered in the energy interval of 10−4−10−2 au.

Phase Locking between Different Partial Waves in Atom-Ion Spin-Exchange Collisions

We present a joint experimental and theoretical study of spin dynamics of a single ^{88}Sr^{+} ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin relaxation of the Sr^{+} ion Zeeman qubit occurs after 48(7) Langevin collisions, which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate is observed as the magnetic field is increased. Interestingly, we find that while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross section, remains locked to a single value over a wide range of partial waves, which leads to quantum interference effects.

Magnetic Resonance Frequency Shifts of Spin-Polarized Cesium Atoms in a Cs–Rb Mixture

The interaction of spin-polarized cesium atoms with cesium and rubidium atoms under conditions of optical orientation of atoms is considered. On the basis of data on spin-exchange complex cross sections in the Cs–Cs and Cs–Rb systems, a calculation of a magnetic resonance frequency shift of cesium atoms in a Cs–Rb mixture, as well as comparison of the calculated values of the frequency shift with experimental data, is performed.

Scattering Properties of Ground-State 23Na Vapor Using Generalized Scattering Theory

The scattering properties of ground-state 23Na vapor are investigated within the framework of the Galitskii–Migdal–Feynman formalism. Viewed as a generalized scattering theory, this formalism is used to calculate the medium phase shifts. The scattering properties of the system—the total, viscosity, spin-exchange, and average cross sections—are then computed using these phase shifts according to standard recipes. The total cross section is found to exhibit the Ramsauer–Townsend effect as well as resonance peaks. These peaks are caused by the large difference between the potentials for electronic spin-singlet and spin-triplet states. They represent quasi-bound states in the system. The results obtained for the complex spin-exchange cross sections are particularly highlighted because of their importance in the spectroscopy of the Na2 dimer. So are the results for the scattering lengths pertaining to both singlet and triplet states. Wherever possible, comparison is made with other published results.

Quantitative study of optical pumping in the presence of spin-exchange relaxation

We have performed quantitative measurements of the variation of the on-resonance absorption coefficients ${\ensuremath{\kappa}}_{0}$ of the four hyperfine components of the Cs ${D}_{1}$ transition as a function of laser power $P$, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate ${\ensuremath{\gamma}}_{1}$) yield algebraic ${\ensuremath{\kappa}}_{0}(P)$ dependences that do not reproduce the experimental findings from Cs vapor in a paraffin-coated cell. However, numerical results that consider spin-exchange relaxation (at a rate ${\ensuremath{\gamma}}_{\mathrm{se}}$) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of ${\ensuremath{\kappa}}_{0}$, the optical pumping saturation power ${P}_{\mathrm{sat}}$, and the ratio ${\ensuremath{\gamma}}_{\mathrm{se}}/{\ensuremath{\gamma}}_{1}$, are well described by the experimental conditions and yield absolute values for ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{\mathrm{se}}$. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section.

Magnetic-resonance-frequency shifts in spin-exchange collisions of Rb and Cs alkali-metal atoms

On the basis of data on spin-exchange complex cross sections in the Rb–Rb and Cs–Rb systems, the magnetic-resonance-frequency shifts of rubidium atoms upon collisions in a mixture of Cs–Rb alkalimetal atoms are calculated. The results obtained are compared with the experimental data and the previous theoretical data.

Collisions of alkali-metal atoms Cs and Rb in the ground state. Spin exchange cross sections

Collisions of alkali-metal atoms 133Cs and 85Rb in the ground state are considered in the energy interval of 10–4–10–2 au. Complex cross sections of the spin exchange, which allow one to calculate the processes of polarization transfer and the relaxation times, as well as the magnetic resonance frequency shifts caused by spin exchange Cs–Rb collisions, are obtained.