Spin-exchange Relaxation Research Articles

Sensitivity Improvement of Spin-Exchange Relaxation Free Atomic Magnetometers by Hybrid Optical Pumping of Potassium and Rubidium

An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fTrms/Hz1/2.

Stroboscopic Backaction Evasion in a Dense Alkali-Metal Vapor

We explore experimentally quantum nondemolition measurements of atomic spin in a hot potassium vapor in the presence of spin-exchange relaxation. We demonstrate a new technique for backaction evasion by stroboscopic modulation of the probe light. With this technique we study spin noise as a function of polarization for atoms with spin greater than 1/2 and obtain good agreement with a simple theoretical model. We point that, in a system with fast spin exchange, where the spin-relaxation rate is changing with time, it is possible to improve the long-term sensitivity of atomic magnetometry by using quantum nondemolition measurements.

Femtotesla atomic magnetometry in a microfabricated vapor cell

We describe an optically pumped 87Rb magnetometer with 5 fT/Hz(1/2) sensitivity when operated in the spin-exchange relaxation free (SERF) regime. The magnetometer uses a microfabricated vapor cell consisting of a cavity etched in a 1 mm thick silicon wafer with anodically bonded Pyrex windows. The measurement volume of the magnetometer is 1 mm3, defined by the overlap region of a circularly polarized pump laser and a linearly polarized probe laser, both operated near 795 nm. Sensitivity limitations unique to the use of microfabricated cells are discussed.

Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

A potentially biocompatible class of spin-labeled macromolecules, spin-labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser-type dynamic nuclear polarization (DNP). All presented SL-heparins show high (1)H DNP enhancement factors up to E=-110, which validates that effectively more than one hyperfine line can be saturated even for spin-labeled polarizing agents. The parameters for the Overhauser-type DNP are determined and discussed. A striking result is that for spin-labeled heparins, the off-resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non-negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin-nuclear spin relaxation (T(1ne)). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.

Ultrahigh sensitivity magnetic field and magnetization measurements with an atomic magnetometer

We describe an ultrasensitive atomic magnetometer based on optically pumped potassium atoms operating in a spin-exchange relaxation free regime. We demonstrate magnetic field sensitivity of 160 aT/Hz1/2 in a gradiometer arrangement with a measurement volume of 0.45 cm3 and energy resolution per unit bandwidth of 44ℏ. As an example of an application enabled by such a magnetometer, we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10−10 emu/cm3/Hz1/2 and temperatures up to 420°C.

Collision-induced spin exchange of alkali-metal atoms with 3He

Collisions of atoms and molecules in external electromagnetic fields can depolarize electronic and nuclear spins, leading to decoherence of quantum superposition states. The fundamental microscopic processes giving rise to spin depolarization are spin exchange and spin relaxation. In this work, we present an analysis of spin exchange transitions in collision of alkali metal atoms with 3He. We use accurate ab initio calculations to evaluate the electron spin density of Li-He and the interaction energy of Rb-He. The Fermi contact interaction constants for the complexes of Na, K, and Rb atoms with 3He are extracted from the measured frequency shift enhancement factors using the ab initio interaction potentials. The spin density calculations also yield the hyperfine pressure shift of Li-He.

Spin-exchange relaxation-free magnetometry using elliptically polarized light

Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of $7\text{ }\text{fT}/\sqrt{\text{Hz}}$ with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.

Spin relaxation in silicon coupled quantum dots

We present a detailed investigation for spin relaxation processes in silicon coupled quantum dots. Low-field magnetoconductance measurements have been employed to deduce phase dephasing and spin relaxation rates. On the basis of the dephasing theory containing triplet channel interaction, we have demonstrated that small energy transfer scattering process is the dominant dephasing mechanism, and strong electron-electron interaction results in an interdot spin-exchange relaxation process. Triplet-singlet relaxation is found to be another important spin relaxation process in the inner quantum dots, taking into account the triplet-singlet splitting induced by spin-orbit coupling.

A large sample study of spin relaxation and magnetometric sensitivity of paraffin-coated Cs vapor cells

We have manufactured more than 250 nominally identical paraffin-coated Cs vapor cells (28 mm inner diameter bulbs) for multi-channel atomic magnetometer applications. We describe our dedicated cell characterization apparatus. For each cell we have determined the intrinsic longitudinal, Γ 01, and transverse, Γ 02, relaxation rates. Our best cell shows Γ 01/2π≈0.5 Hz, and Γ 02/2π≈2 Hz. We find a strong correlation of both relaxation rates which we explain in terms of reservoir and spin exchange relaxation. For each cell we have determined the optimal combination of rf and laser powers which yield the highest sensitivity to magnetic field changes. Out of all produced cells, 90% are found to have magnetometric sensitivities in the range of 9 to 30 fT $\sqrt{\mathrm{Hz}}$ . Noise analysis shows that the magnetometers operated with such cells have a sensitivity close to the fundamental photon shot noise limit.

Collision-induced spin depolarization of alkali-metal atoms in coldHe3gas

We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with $^{3}\mathrm{He}$ in a magnetic field. A rigorous quantum theory for spin-changing transitions is developed and applied to calculate the spin exchange and spin relaxation rates of Li and K atoms in cryogenic $^{3}\mathrm{He}$ gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for $\mathrm{Li}\text{\ensuremath{-}}^{3}\mathrm{He}$ and $\mathrm{K}\text{\ensuremath{-}}^{3}\mathrm{He}$, which are in agreement with the present theory. Our calculations demonstrate that the alkali-metal atoms have extremely slow spin depolarization rates, suggesting a number of potential applications in precision spectroscopy and quantum optics.

Quantum Zeno effect in atomic spin-exchange collisions

The suppression of spin-exchange relaxation in dense alkali-metal vapors discovered in 1973 and governing modern atomic magnetometers is here reformulated in terms of quantum measurement theory and the quantum Zeno effect. This provides a new perspective of understanding decoherence in spin-polarized atomic vapors.

Electron spin exchange relaxation of radicals in low magnetic field

Spin relaxation of radicals caused by electron spin exchange (ESE) in solution at low and zero magnetic fields have been studied theoretically and experimentally using stable nitroxide radicals. It is shown that the probabilities of relaxation transitions in low and zero magnetic fields differ from the probabilities in high magnetic fields. The use of high-field expressions in low and zero magnetic fields is not correct. It is shown that ESE-induced relaxation leads to transfer of electron polarization to nuclear polarization. The theoretical predictions have been compared to experimental EPR spectra of nitroxide radicals in low magnetic fields.

Nuclear Spin Gyroscope Based on an Atomic Comagnetometer

We describe a nuclear spin gyroscope based on an alkali-metal-noble-gas comagnetometer. Optically pumped alkali-metal vapor is used to polarize the noble-gas atoms and detect their gyroscopic precession. Spin precession due to magnetic fields as well as their gradients and transients can be cancelled in this arrangement. The sensitivity is enhanced by using a high-density alkali-metal vapor in a spin-exchange relaxation free regime. With a K-3He comagnetometer we demonstrate rotation sensitivity of 5 x 10(-7) rad s(-1) Hz(-1/2), equivalent to a magnetic field sensitivity of 2.5 fT/Hz(1/2). The rotation signal can be increased by a factor of 10 using 21Ne with a smaller magnetic moment. The comagnetometer is also a promising tool in searches for anomalous spin couplings beyond the standard model.

Unshielded three-axis vector operation of a spin-exchange-relaxation-free atomic magnetometer

We describe a vector alkali–metal magnetometer that simultaneously and independently measures all three components of the magnetic field. Using a feedback system, the total field at the location of the magnetometer is kept near zero, suppressing the broadening due to spin-exchange collisions. The resonance linewidth and signal strength of the magnetometer compare favorably with two different scalar operation modes in which spin-exchange relaxation is only partially suppressed. Magnetic field sensitivity on the order of 1pT∕Hz is demonstrated in a laboratory environment without magnetic shields.

High-sensitivity atomic magnetometer unaffected by spin-exchange relaxation.

Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of 10 fT Hz(-1/2), limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be 2x10(-18) T Hz(-1/2).

Manifestation of spin exchange relaxation in microwave field induced magnetic field effects on recombination of confined radical pairs

Within the simple exponential kinetic model, possible manifestations of the strong electron spin exchange interaction (ESEI) and relaxation (ESER) in microwave field induced magnetic field effects on recombination of confined radical pairs are analysed theoretically. It is shown that not only ESEI but also ESER strongly affects the spectra of reaction yield detected magnetic resonance, stimulated nuclear polarisation and EPR leading to broadening and change of line shape.

The second order recombination and spin-exchange relaxation of atomic deuterium at low temperatures in a low magnetic field

We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a4He coated sample cell, using the hyperfine resonance of β(F = 1/2, mF= −1/2) − δ(F = 3/2, mF= −1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1DD= (5.5 ± 1.3) × 10−9cm and the adsorption energy of D on4He surface was ea= 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1DDat low field was orders of magnitude smaller than what was expected when the scaling of 1/B2dependence of the direct recombination mechanism was used, and in addition, eawas significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, GDD= (1.4 ± 0.6) × 10−10cm3sec−1. It was smaller than theoretical calculations.

Spin-correlated radical pairs in micellar systems: mechanism of CIDEP and the micelle size dependence

Abstract A new model of time-resolved EPR in micellized radical pairs is introduced. The model is based on numerical integration of the master Liouville equation for spin-correlated (micellized) pairs and free (escaped) radicals. The diffusion of radicals is considered in terms of a supercage model. This approach is used to analyze data on laser flash photolysis of 13 C-carbonyl labelled ketone α-deoxybenzoin in aqueous sodium alkyl (10–12) sulphate solutions. EPR lines of 13 C-benzoyl radicals exhibit antiphase structure (APS) typical of spin-correlated pairs. Due to very large APS splitting, 0.8–1.5 mT, the ST 0 polarized lines from free benzoyl radicals can be isolated spectrally. The observed line shape of the APS cannot be accounted for in the standard model of effective exchange potential. The shape of the APS is shown to be controlled by spin exchange relaxation in micellized pairs.

Theory of magnetic field effects on radical pair recombination in micelles

Magnetic field effects (MPE) of radical pair (RP) recombination in micelles are investigated both analytically and numerically. The confinement effect of a micelle is modeled by a spherically symmetric potential well. The results show that the contribution from ST_ transitions to all kind of MFEs is significantly increased due to the confinement effect. The spin exchange relaxation is also increased causing the STo-contribution to CIDEP to be essentially reduced. Surprisingly a pronounced effect of ST+ transitions on CIDEP is found when the confinement is strong. Simple analytical expressions for the recombination probability and for CIDNP and CIDEP are rigorously derived and a high accuracy is found by comparison with the numerical results. The results are interpreted in terms of a simple supercage model which is based on the assumption that the process can be separated into two stages: (1) a fast initial geminate stage in which the radicals are thermalised within the micelle and (2) a slower decay of a quasi-equilibrium state by escape from the micelle.

An analytical and numerical investigation of multiplet CIDEP

A detailed analysis of the accuracy of the analytical expressions for the chemically induced dynamical electron polarization (CIDEP) generated in radical pair (RP) recombination is performed by comparison with very accurate numerical calculations. The numerical results are obtained by a direct numerical solution of the stochastic Liouville equation. A large variety of limiting cases corresponding to different relations between the parameters of the problems are analyzed. For example, strong and weak exchange interactions, low and high reactivity of RPs, etc. The dependences of CIDEP on spin-lattice relaxation in the radicals and other dephasing processes are also considered. The analyses of the accuracy is accompanied by a short discussion of the basic concepts of the theory and, in particular, the important concept of effective radii of the spin exchange relaxation and the reactivity. New expressions were derived to correctly describe the slow diffusion region in the weak exchange interaction limit. It is shown that the analytical formulae reproduce the numerical results with a very high accuracy for all realistic values of the parameters.