arXiv subjects

Thad G. Walker

Publications and source records attributed to Thad G. Walker.

16 recordsLinked to original sources

arXiv2026

Production of Intense Spin-Polarized Beams of Hydrogen Isotopes by Charge Transfer with High Density Raman-Pumped Alkali-Metal Vapors

It should be possible to generate multi-ampere spin-polarized beams of hydrogen isotopes by repeated charge-transfer collisions in highly spin-polarized Cs vapor. Estimates suggest that off-resonant Raman pumping with kW scale narrowband tunable light at 895 nm should be able to produce a 1 m long, 10 cm diameter volume of 80\% polarized Cs vapor. The charge transfer collisions between the Cs and hydrogen result in a high nuclear spin-polarized negative ion beam that can be subsequently accelerated to high energy, neutralized, and be used to heat fusion plasmas with resulting increases in the fusion conversion efficiency.

physics.atom-ph↗

arXiv2025

Exact steady state of perturbed open quantum systems

We present a general non-perturbative method to determine the exact steady state of open quantum systems under perturbation. The method works for systems with a unique steady state and the perturbation may be time-independent or periodic, and of arbitrarily large amplitude. Using the Drazin inverse and a single diagonalization, we construct an operator that generates the entire dependence of the steady state on the perturbation parameter. The approach also enables exact analytic operations-such as differentiation, integration, and ensemble averaging-with respect to the parameter, even when the steady state is computed numerically. We apply the method to three non-trivial open quantum systems, showing that it achieves exact results, with a computational speedup of one to several orders of magnitude for calculations requiring large sampling, compared to previous approaches.

quant-ph↗

arXiv2025

Perspective: Practical Atom-Based Quantum Sensors

Atomic vapors, manipulated and probed by light and other electromagnetic fields, constitute versatile and powerful quantum systems for sensing applications. Atoms are identical, isolatable, interfaceable, and intelligible. These features, coupled with the relative simplicity with which quantum properties can be exploited in state preparation and detection using modern laser and electro-optic tools, make atoms very attractive for sensing applications. This Perspective discusses the potential and process for realizing practical quantum sensors using atoms.

quant-ph↗

arXiv2025

Fast and Accurate Method for Doppler Averaging of Rydberg EIT Signals

Modeling the effect of Doppler broadening due to the thermal atomic motion of Rydberg sensors typically relies on sampling a large velocity class, solving the equations of motion for each velocity, and then averaging the atomic density matrix over that velocity class. This process is inexact, slow, and presents a bottleneck in simulating Rydberg sensors. We present an approach that allows for fast, exact velocity averaging just by solving the equations of motion twice. We find the "propagator" that acts on the zero-velocity solution to generate the velocity-dependent atomic state for all velocities. By averaging this propagator over the Maxwell-Boltzmann velocity distribution of the atoms, we obtain an explicit, analytic formula that generates the averaged atomic state. This method is expected to save memory and time computational resources by one to several orders of magnitude, compared to traditional sampling approaches.

quant-ph↗

arXiv2022

Rydberg state engineering: A comparison of tuning schemes for continuous frequency sensing

On-resonance Rydberg atom-based radio-frequency (RF) electric field sensing methods remain limited by the narrow frequency signal detection bands available by resonant transitions. The use of an additional RF tuner field to dress or shift a target Rydberg state can be used to return a detuned signal field to resonance and thus dramatically extend the frequency range available for resonant sensing. Here we investigate three distinct tuning level schemes based on adjacent Rydberg transitions, which are shown to have distinct characteristics and can be controlled with mechanisms based on the tuning field frequency or field strength. We further show that a two-photon Raman feature can be used as an effective tuning mechanism separate from conventional Autler-Townes splitting. We compare our tuning schemes to AC Stark effect-based broadband RF field sensing and show that although the sensitivity is diminished as we tune away from a resonant state, it nevertheless can be used in configurations where there is a low density of Rydberg states, which would result in a weak AC Stark effect.

physics.atom-ph↗

arXiv2020

A Synchronous Spin-Exchange Optically Pumped NMR-Gyroscope

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 16 $μ\text{Hz}/\sqrt{\text{Hz}}$ and bias instability of $\sim$800 nHz.

physics.atom-ph↗

arXiv2019

Dual-Species Synchronous Spin-Exchange Optical Pumping

We demonstrate a novel quantum sensor for measuring non-magnetic spin-dependent interactions. This sensor utilizes $^{131}$Xe, $^{129}$Xe, and $^{85}$Rb 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 $^{129}$Xe 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 $μ$Hz/$\sqrt{\text{Hz}}$ and a bias instability of $\sim1$ $μ$Hz.

physics.atom-ph↗

arXiv2017

Parallel low-loss measurement of multiple atomic qubits

We demonstrate low-loss measurement of the hyperfine ground state of Rubidium atoms in a dipole trap array of five sites by state dependent fluorescence detection. The presence of atoms and their internal states are minimally altered by utilizing circularly polarized probe light and a strictly controlled quantization axis. We achieve mean state detection fidelity of $97\%$ without correcting for imperfect state preparation or background losses, and $98.7\%$ when corrected. After state detection and correction for background losses, the probability of atom loss due to the state measurement is $<2\%$ and the initial hyperfine state is preserved with $>98\%$ probability.

quant-ph↗

arXiv2016

Spin-Exchange Pumped NMR Gyros

We present the basic theory governing spin-exchange pumped NMR gyros. We review the basic physics of spin-exchange collisions and relaxation as they pertain to precision NMR. We present a simple model of operation as an NMR oscillator and use it to analyze the dynamic response and noise properties of the oscillator. We discuss the primary systematic errors (differential alkali fields, quadrupole shifts, and offset drifts) that limit the bias stability, and discuss methods to minimize them. We give with a brief overview of a practical implementation and performance of an NMR gyro built by Northrop-Grumman Corporation, and conclude with some comments about future prospects.

physics.atom-ph↗

arXiv2013

Atomic Fock State Preparation Using Rydberg Blockade

We use coherent excitation of 3-16 atom ensembles to demonstrate collective Rabi flopping mediated by Rydberg blockade. Using calibrated atom number measurements, we quantitatively confirm the expected $\sqrt{N}$ Rabi frequency enhancement to within 4%. The resulting atom number distributions are consistent with essentially perfect blockade. We then use collective Rabi $π$ pulses to produce ${\cal N}=1,2$ atom number Fock states with fidelities of 62% and 48% respectively. The ${\cal N}=2$ Fock state shows the collective Rabi frequency enhancement without corruption from atom number fluctuations.

quant-ph↗

arXiv2012

Entanglement of Two Atoms using Rydberg Blockade

Over the past few years we have built an apparatus to demonstrate the entanglement of neutral Rb atoms at optically resolvable distances using the strong interactions between Rydberg atoms. Here we review the basic physics involved in this process: loading of single atoms into individual traps, state initialization, state readout, single atom rotations, blockade-mediated manipulation of Rydberg atoms, and demonstration of entanglement.

quant-ph↗

arXiv2012

Optical Magnetometer Array for Fetal Magnetocardiography

We describe an array of spin-exchange relaxation free optical magnetometers designed for detection of fetal magnetocardiography (fMCG) signals. The individual magnetometers are configured with a small volume with intense optical pumping, surrounded by a large pump-free region. Spin-polarized atoms that diffuse out of the optical pumping region precess in the ambient magnetic field and are detected by a probe laser. Four such magnetometers, at the corners of a 7 cm square, are configured for gradiometry by feeding back the output of one magnetometer to a field coil to null uniform magnetic field noise at frequencies up to 200 Hz. Using this array, we present the first measurements of fMCG signals using an atomic magnetometer.

physics.atom-ph↗

arXiv2011

Formation and dynamics of van der Waals molecules in buffer-gas traps

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxations are too slow to affect trap lifetimes. However, helium-3-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag3He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps.

physics.atm-clus↗

arXiv2011

Polarization Limits in K-Rb Spin-Exchange Mixtures

We present measurements of the optical absorption of K vapor at 795 nm due to the presence of high pressure He gas. The results set a limit on the polarization attainable in hybrid spin-exchange optical pumping of He-3.

physics.atom-ph↗

arXiv2007

Consequences of Zeeman Degeneracy for van der Waals Blockade between Rydberg Atoms

We analyze the effects of Zeeman degeneracies on the long-range interactions between like Rydberg atoms, with particular emphasis on applications to quantum information processing using van der Waals blockade. We present a general analysis of how degeneracies affect the primary error sources in blockade experiments, emphasizing that blockade errors are sensitive primarily to the weakest possible atom-atom interactions between the degenerate states, not the mean interaction strength. We present explicit calculations of the van der Waals potentials in the limit where the fine-structure interaction is large compared to the atom-atom interactions. The results are presented for all potential angular momentum channels invoving s, p, and d states. For most channels there are one or more combinations of Zeeman levels that have extremely small dipole-dipole interactions and are therefore poor candidates for effective blockade experiments. Channels with promising properties are identified and discussed. We also present numerical calculations of Rb and Cs dipole matrix elements and relevant energy levels using quantum defect theory, allowing for convenient quantitative estimates of the van der Waals interactions to be made for principal quantum numbers up to 100. Finally, we combine the blockade and van der Waals results to quantitatively analyze the angular distribution of the blockade shift and its consequence for angular momentum channels and geometries of particular interest for blockade experiments with Rb.

quant-ph↗

arXiv2004

Zeros of Rydberg-Rydberg Foster Interactions

Rydberg states of atoms are of great current interest for quantum manipulation of mesoscopic samples of atoms. Long-range Rydberg-Rydberg interactions can inhibit multiple excitations of atoms under the appropriate conditions. These interactions are strongest when resonant collisional processes give rise to long-range C_3/R^3 interactions. We show in this paper that even under resonant conditions C_3 often vanishes so that care is required to realize full dipole blockade in micron-sized atom samples.

physics.atom-ph↗