Transverse Magnetic Field Gradient Research Articles

Analysis of effects of magnetic field gradient on atomic spin polarization and relaxation in optically pumped atomic magnetometers.

The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.

Nuclear Spin Readout in a Cavity-Coupled Hybrid Quantum Dot-Donor System

Nuclear spins show long coherence times and are well isolated from the environment, which are properties making them promising for quantum information applications. Here, we present a method for nuclear spin readout by probing the transmission of a microwave resonator. We consider a single electron in a silicon quantum dot-donor device interacting with a microwave resonator via the electric dipole coupling and subjected to a homogeneous magnetic field and a transverse magnetic field gradient. In our scenario, the electron spin interacts with a $^{31}\mathrm{P}$ defect nuclear spin via the hyperfine interaction. We theoretically investigate the influence of the P nuclear spin state on the microwave transmission through the cavity and show that nuclear spin readout is feasible with current state-of-the-art devices. Moreover, we identify optimal readout points with strong signal contrast to facilitate the experimental implementation of nuclear spin readout. Furthermore, we investigate the potential for achieving coherent excitation exchange between a nuclear spin qubit and cavity photons.

Controlling Synthetic Spin-Orbit Coupling in a Silicon Quantum Dot with Magnetic Field

Tunable synthetic spin-orbit coupling (s-SOC) is one of the key challenges in various quantum systems, such as ultracold atomic gases, topological superconductors, and semiconductor quantum dots. Here we experimentally demonstrate controlling the s-SOC by investigating the anisotropy of spin-valley resonance in a silicon quantum dot. As we rotate the applied magnetic field in-plane, we find a striking nonsinusoidal behavior of resonance amplitude that distinguishes s-SOC from the intrinsic spin-orbit coupling (i-SOC), and associate this behavior with the previously overlooked in-plane transverse magnetic field gradient. Moreover, by theoretically analyzing the experimentally measured s-SOC field, we predict the quality factor of the spin qubit could be optimized if the orientation of the in-plane magnetic field is rotated away from the traditional working point.

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope.

Electron paramagnetic resonance (EPR) spectroscopy is widely used to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. We present an extensive experimental study and modeling of EPR-STM of Fe and hydrogenated Ti atoms on a MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe. Also, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.

Spin and motion dynamics with zigzag ion crystals in transverse magnetic gradients

We investigate the dynamics of ion crystals in zigzag configuration in transverse magnetic field gradients. A surface-electrode Paul trap is employed to trap 40Ca+ ions and features submerged current wires to generate magnetic field gradients of up to 16.3(9) T m−1 at the ions position. With the gradient aligned in the direction perpendicular to the axis of weakest confinement, along which linear ion crystals are formed, we demonstrate magnetic field gradient induced coupling between the spin and ion motion. We perform sideband spectroscopy upon directly driving the spins with a radiofrequency field for crystals of three ions on their linear-to-zigzag structural transition. Furthermore, we observe the rich excitation spectrum of vibrational modes in a planar crystal comprised of four ions.

Structure of current and plasma in current sheets depending on the conditions of sheet formation

The structure of current sheets created under laboratory conditions is characterized by a large variety, which depends substantially on the conditions under which the sheet if formed. In this work, we present the results of an experimental study of the structure and evolution of current sheets that were formed in magnetic configurations with a singular line of the X type. It has been shown that the change in the transverse magnetic field gradient, the strength of the longitudinal magnetic field, and the mass of the ions in plasma makes it possible to significantly vary the main parameters of the current sheets. This offers the challenges of using laboratory experimental results for analyzing and simulating the cosmophysical processes.

Very-low-field MRI of laser polarized xenon-129

We describe a homebuilt MRI system for imaging laser-polarized xenon-129 at a very low holding field of 2.2mT. A unique feature of this system was the use of Maxwell coils oriented at so-called “magic angles” to generate the transverse magnetic field gradients, which provided a simple alternative to Golay coils. We used this system to image a laser-polarized xenon-129 phantom with both a conventional gradient-echo and a fully phase-encoded pulse sequence. In other contexts, a fully phase-encoded acquisition, also known as single-point or constant-time imaging, has been used to enable distortion-free imaging of short-T2∗ species. Here we used this technique to overcome imperfections associated with our homebuilt MRI system while also taking full advantage of the long T2∗ available at very low field. Our results demonstrate that xenon-129 image quality can be dramatically improved at low field by combining a fully phase-encoded k-space acquisition with auxiliary measurements of system imperfections including B0 field drift and gradient infidelity.

Electrical control of a long-lived spin qubit in a Si/SiGe quantum dot.

Nanofabricated quantum bits permit large-scale integration but usually suffer from short coherence times due to interactions with their solid-state environment. The outstanding challenge is to engineer the environment so that it minimally affects the qubit, but still allows qubit control and scalability. Here, we demonstrate a long-lived single-electron spin qubit in a Si/SiGe quantum dot with all-electrical two-axis control. The spin is driven by resonant microwave electric fields in a transverse magnetic field gradient from a local micromagnet, and the spin state is read out in the single-shot mode. Electron spin resonance occurs at two closely spaced frequencies, which we attribute to two valley states. Thanks to the weak hyperfine coupling in silicon, a Ramsey decay timescale of 1 μs is observed, almost two orders of magnitude longer than the intrinsic timescales in GaAs quantum dots, whereas gate operation times are comparable to those reported in GaAs. The spin echo decay time is ~40 μs, both with one and four echo pulses, possibly limited by intervalley scattering. These advances strongly improve the prospects for quantum information processing based on quantum dots.

Magnetophoretic mixing for in situ immunochemical binding on magnetic beads in a microfluidic channel

Functionalized magnetic beads offer promising solutions to a host of micro-total analysis systems ranging from immunomagnetic biosensors to cell separators. Immunochemical binding of functional biochemical agents or target biomolecules serves as a key step in such applications. Here we show how magnetophoretic motion of magnetic microspheres in a microchannel is harnessed to promote in situ immunochemical binding of short DNA strands (probe oligonucleotide) on the bead surface via streptavidin–biotin bonds. Using a transverse magnetic field gradient, the particles are transported across a co-flowing analyte stream containing biotinylated probe oligonucleotides that are labeled with a Cy3-fluorophore. Quantification of the resulting biotin–streptavidin promoted binding has been achieved through fluorescence imaging of the magnetophoretically separated magnetic particles in a third stream of phosphate buffered saline. Both the experimental and numerical data indicate that for a given flow rate, the analyte binding per bead depends on the flow fraction of the co-flowing analyte stream through the microchannel, but not on the fluid viscosity. Parametric studies of the effects of fluid viscosity, analyte flow fraction, and total flow rate on the extent of binding and the overall analyte separation rate are also conducted numerically to identify favorable operating regimes of a flow-through immunomagnetic separator for biosensing, cell separation, or high-throughput applications.

Characterization of Nonspecific Crossover in Split-Flow Thin Channel Fractionation

Split-flow thin channel (SPLITT) fractionation is a technique for continuous separation of particles or macromolecules in a fluid stream into fractions according to the lateral migration induced by application of a field perpendicular to the direction of flow. Typical applications have involved isolation of different fractions from a polydisperse sample. Some specialized applications involve the separation of the fraction influenced by the transverse field from the fraction that is not. For example, immunomagnetically labeled biological cells may be separated from nonlabeled cells with the application of a transverse magnetic field gradient. In such cases, it may be critically important to minimize contamination of the labeled cells with nonlabeled cells while at the same time maximizing the throughput. Such contamination is known as nonspecific crossover (NSC) and refers to the real or apparent migration of nonmobile particles or cells across stream lines with the mobile material. The possible mechanisms for NSC are discussed, and experimental results interpreted in terms of shear-induced diffusion (SID) caused by viscous interactions between particles in a sheared flow. It is concluded that SID may contribute to NSC, but that further experiments and mathematical modeling are necessary to more fully explore the phenomenon.

Nonperturbative broadening of paramagnetic resonance lines by transverse magnetic field gradients

Nonperturbative broadening of paramagnetic resonance lines by transverse magnetic field gradients

A novel simplified two-dimensional magneto-optical trap as an intense source of slow cesium atoms

We describe the design and performance of a slow atom source based on a 2D magneto-optical trap (MOT) that uses an innovative simple optical configuration. Metal-coated retro-reflecting prisms replace mirrors and quarter-wave plates so the optical power of the cooling laser beam is recycled. This source has been characterised for three different configurations: with and without transverse magnetic field gradient, and with a pusher beam to obtain a 2D + -MOT. The longitudinal velocity is of the order of 25 m·s −1 , with a transverse velocity spread ≤1 m·s −1 , while the typical atomic flux density obtained is up to 1.3×10 14 at ·s −1 ·m −2 for a cesium vapour pressure of ~4×10 −8 mbar in the source. We use this slow atom beam, instead of cesium vapour, to load a 3D moving optical molasses that feeds a continuous cold atom fountain. We obtain a gain of a factor ~20 in the atomic flux launched by the fountain.

Experimental study of current sheet evolution in magnetic configurations with null-points and X-lines

A review is presented on experimental research of current sheet formation in 3D magnetic configurations containing null-points and/or singular lines of the X-type. Formation of current sheets is revealed to occur in various 3D configurations, both with and without isolated magnetic null-points, specifically in configurations with X-lines. Local characteristics of 3D magnetic configuration define the parameters of the current sheet, which forms usually a twisted surface with an angular orientation determined by a local value of the transverse magnetic field gradient. A degree of plasma compression into the sheet decreases with a rise of longitudinal magnetic field component, displaying a transition to a behavior of uncompressible plasma. It is established that current sheet formation and magnetic reconnection processes can take place within a limited range of initial conditions, while a gradient of transverse magnetic field is the most important parameter.

Magnetically stimulated inhomogeneity of conductivity and nonlocal transport phenomena in metals

Charge transport in a conducting medium with a magnetically stimulated inhomogeneity of kinetic coefficients along the direction of transport is investigated both experimentally and analytically. Measurements were made on samples in the form of high-purity polycrystalline aluminum plates whose conductivity inhomogeneity was simulated by the technique of curving of current lines so that the local normal component of the applied magnetic field varies according to an exponential or quadratic law. The relations describing the tensor connection between the electric field and charge flux density are used to calculate spatial dependence of the potential. The sign reversal of the electric field is described as the result of competition between the potential contributions from the current along the transverse magnetic field gradient and the Hall current at right angles to it.

Penetration of a transverse magnetic field by an accelerated field-reversed configuration

The field-reversed configuration (FRC) is a compact toroid with near unity β that is an ideal candidate to provide central fueling for large tokamaks. The study of the acceleration and penetration physics of the FRC into a transverse magnetic field gradient is necessary in order to evaluate the fueling efficiency of the FRC. To this end, experiments were conducted on the LSX/mod facility [J. T. Slough and A. L. Hoffman, 16th IAEA Fusion Energy Conference 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. II, p. 237], where large mass (0.8 mg) FRCs were accelerated to high velocity (2×105 m/s), and then guided into a transverse magnetic field. Various diagnostics were employed to characterize the penetration process. These included thermocouple probes, magnetic probes, and emission arrays. A simple analytical model is developed that explains the basic features of the penetration process. Further modeling with two-dimensional numerical calculations provided for scaling laws to reach the conditions necessary to penetrate a large fusion tokamak.

Transverse gradient coil with circle current paths.

The paper describes a new type of transverse magnetic field gradient coil for applications in MRI and MRS. It uses full circle current paths rather than current arcs and has a particularly simple unit construction of symmetric form. It features a large volume of uniform transverse field gradient, high efficiency and low inductance for rapid switching. A prototype coil has been constructed and evaluated to check computer simulations. Two smaller sets have subsequently been made for NMR microscopy. This coil design may be used for all sizes.

Optimization of Transverse Gradient Coils with Coaxial Return Paths by Simulated Annealing

Coils with coaxial return paths are used to generate transverse magnetic field gradients. This paper describes optimization of such coils by the method of simulated annealing, a method known to be able to find the global minimum of a function. The adaptive simulated annealing (ASA) program has been analyzed and applied to optimization of a family of coils with 8–16 building blocks, each carrying equal current. Positions of the blocks along the longitudinal axis of the coils were optimized. A new subclass of coils is proposed; the diameter of return paths of this subclass of coils is not fixed but may be varied. The new coils provide greater gradient uniformity than those for which only positions of the blocks are optimized. All optimized coils should find applications in high-precision and high-resolution imaging and spectroscopic experiments.

Optical collimation and compression of a thermal atomic beam

We have performed an experiment to optically cool and compress a thermal beam of sodium atoms by employing a two-dimensional version of a magneto-optical trap. The beam compressor employs a tapered transverse magnetic field gradient formed by four pairs of permanent magnets. We measure the efficiency of collimation and compression as function of longitudinal velocity group.