arXiv subjects

M. E. Limes

Publications and source records attributed to M. E. Limes.

12 recordsLinked to original sources

arXiv2026

Probe-assisted Depopulation Pumping in Low-pressure Alkali-metal Vapor Cells for Magnetometry

For precision atomic magnetometry, inert buffer gas is included in alkali-metal vapor cells to significantly broaden hyperfine transitions, which facilitates optical pumping and reduces diffusive relaxation, while also providing non-radiative excited state quenching. We show low-buffer gas pressure (below 50 Torr) alkali vapor cells with resolved hyperfine manifolds can also yield high-performance magnetometers. For high polarization in $^{87}$Rb, we optically pump $F=2$ states with narrow linewidth $σ_+$ light, while tuning a probe beam to depopulate $F=1$ states ($Δν= 6.8$ GHz from $F=2$). The probe tuning then also provides $F=2$ detection with high optical rotation and low probe broadening; we demonstrate top-bottom gradiometry, within a single 25 Torr, 0.5 cc cell, that yields an Earth's field free-precession magnetometer sensitivity of 18 fT/$\sqrt{\text{Hz}}$ with a 1 kHz bandwidth, as well as RF magnetometer sensitivity of 12 fT/$\sqrt{\text{Hz}}$ in a small band about 110 kHz.

physics.atom-ph↗

arXiv2024

Frequency-dependent amplitude correction to free-precession scalar magnetometers

Pump and probe scalar atomic magnetometers show incredible potential for real-world, traditionally difficult measurement environments due to their high dynamic range and linearity. Previously, it has been assumed these scalar magnetometer have a flat response across their bandwidth, and flat noise floor. Here we show that standard fitting routines, used to extract the magnetic field, result in a non-linear frequency dependent response across the sensor bandwidth, due to the time-averaged nature of such free precession measurements. We present an analytic correction dependent on dead-time, and show how this equation can also correct the sensor spectral density. The maximum in-band amplitude loss approaches 29\% as the frequency of interest becomes the Nyquist frequency, making a significant correction for applications such as source localization in magnetoencephalography. These pump and probe atomic magnetometers also are known to have large aliasing of out-of-band signals, and we propose a scheme where the frequency of out-of-band signals can be identified by performing fits with varying dead-time on the raw free-precession sensor data.

physics.atom-ph↗

arXiv2024

Long spin-1/2 noble gas coherence times in mm-sized anodically bonded batch-fabricated $^{3}$He-$^{129}$Xe-$^{87}$Rb cells

As the only stable spin-1/2 noble gas isotopes, $^{3}$He and $^{129}$Xe are promising systems for inertial rotation sensing and searches for exotic spin couplings. Spin-1/2 noble gases have intrinsic coherence times on the order of hours to days, which allows for incredibly low frequency error of free-precession measurements. However relaxation in miniature cells is dominated by interactions with the cell wall, which limits the performance of a chip-scale sensor that uses noble gases. While $^{129}$Xe wall relaxation times have previously been limited to 10's of seconds in mm-sized cells, we demonstrate the first anodically bonded batch-fabricated cells with dual $^{3}$He-$^{129}$Xe isotopes and $^{87}$Rb in a 6~mm$^3$ volume with $^{3}$He and $^{129}$Xe $T_2$ coherence times of, respectively, 4 h and 300 s. We use these microfabricated cells in a dual noble-gas comagnetometer and discuss its limits.

physics.atom-ph↗

arXiv2021

Femtotesla nearly quantum-noise-limited pulsed gradiometer at Earth-scale fields

We describe a finite fields magnetic gradiometer using an intense pulsed laser to polarize a $^{87}$Rb atomic ensemble and a compact VCSEL probe laser to detect paramagnetic Faraday rotation in a single multipass cell. We report differential magnetic sensitivity of $14$ fT/Hz$^{1/2}$ over a broad dynamic range including Earth's field magnitude and common-mode rejection ratio higher than $10^4$. We also observe a nearly quantum-noise-limited behaviour of the gradiometer, by comparing the experimental standard deviation of the estimated frequency difference against the Cramér-Rao lower bound in the presence of white photon shot-noise, atomic spin noise and atomic diffusion.

physics.atom-ph↗

arXiv2021

Heading errors in all-optical alkali-vapor magnetometers in geomagnetic fields

Alkali-metal atomic magnetometers suffer from heading errors in geomagnetic fields as the measured magnetic field depends on the orientation of the sensor with respect to the field. In addition to the nonlinear Zeeman splitting, the difference between Zeeman resonances in the two hyperfine ground states can also generate heading errors depending on initial spin polarization. We examine heading errors in an all-optical scalar magnetometer that uses free precession of polarized $^{87}\text{Rb}$ atoms by varying the direction and magnitude of the magnetic field at different spin polarization regimes. In the high polarization limit where the lower hyperfine ground state $F = 1$ is almost depopulated, we show that heading errors can be corrected with an analytical expression, reducing the errors by two orders of magnitude in Earth's field. We also verify the linearity of the measured Zeeman precession frequency with the magnetic field. With lower spin polarization, we find that the splitting of the Zeeman resonances for the two hyperfine states causes beating in the precession signals and nonlinearity of the measured precession frequency with the magnetic field. We correct for the frequency shifts by using the unique probe geometry where two orthogonal probe beams measure opposite relative phases between the two hyperfine states during the spin precession.

physics.atom-ph↗

arXiv2020

Portable magnetometry for detection of biomagnetism in ambient environments

We present a method of optical magnetometry with parts-per-billion resolution that is able to detect biomagnetic signals generated from the human brain and heart in Earth's ambient environment. Our magnetically silent sensors measure the total magnetic field by detecting the free-precession frequency of highly spin-polarized alkali metal vapor. A first-order gradiometer is formed from two magnetometers that are separated by a 3 cm baseline. Our gradiometer operates from a laptop consuming 5 W over a USB port, enabled by state-of-the-art micro-fabricated alkali vapor cells, advanced thermal insulation, custom electronics, and laser packages within the sensor head. The gradiometer obtains a sensitivity of 16 fT/cm/Hz$^{1/2}$ outdoors, which we use to detect neuronal electrical currents and magnetic cardiography signals. Recording of neuronal magnetic fields is one of a few available methods for non-invasive functional brain imaging that usually requires extensive magnetic shielding and other infractructure. This work demonstrates the possibility of a dense array of portable biomagnetic sensors that are deployable in a variety of natural environments.

physics.med-ph↗

arXiv2018

Dipolar and scalar $^3$He and $^{129}$Xe frequency shifts in mm-sized cells

We describe a $^{3}$He-$^{129}$Xe comagnetometer operating in stemless anodically bonded cells with a 6 mm$^3$ volume and a $^{129}$Xe spin coherence time of 300 sec. We use a $^{87}$Rb pulse-train magnetometer with co-linear pump and probe beams to study the nuclear spin frequency shifts caused by spin polarization of $^{3}$He. By systematically varying the cell geometry in a batch cell fabrication process we can separately measure the cell shape dependent and independent frequency shifts. We find that a certain aspect ratio of the cylindrical cell can cancel the effects of $^3$He magnetization that limit the stability of vapor-cell comagnetometers. Using this control we also observe for the first time a scalar $^{3}$He-$^{129}$Xe collisional frequency shift characterized by an enhancement factor $κ_{\text{HeXe}} = -0.011\pm0.001$.

physics.atom-ph↗

arXiv2017

$^{3}$He-$^{129}$Xe Comagnetometery using $^{87}$Rb Detection and Decoupling

We describe a $^{3}$He-$^{129}$Xe comagnetometer using $^{87}$Rb atoms for noble-gas spin polarization and detection. We use a train of $^{87}$Rb $π$ pulses and $σ^+/σ^-$ optical pumping to realize a finite-field Rb magnetometer with suppression of spin-exchange relaxation. We suppress frequency shifts from polarized Rb by measuring the $^{3}$He and $^{129}$Xe spin precession frequencies in the dark, while applying $π$ pulses along two directions to depolarize Rb atoms. The plane of the $π$ pulses is rotated to suppress the Bloch-Siegert shifts for the nuclear spins. We measure the ratio of $^{3}$He to $^{129}$Xe spin precession frequencies with sufficient absolute accuracy to resolve the Earth's rotation without changing the orientation of the comagnetometer. A frequency resolution of 7 nHz is achieved after integration for 8 hours without evidence of significant drift.

physics.atom-ph↗

arXiv2016

Robust solid $^{129}$Xe longitudinal relaxation times

We find that if solid xenon is formed from liquid xenon, denoted "ice", there is a 10% increase of $^{129}$Xe longitudinal relaxation $T_1$ time (taken at 77 K and 2 Tesla) over a trickle-freeze formation, denoted "snow". Forming xenon ice also gives unprecedented reproducibility of $^{129}$Xe $T_1$ measurements across a range of 77-150 K. This temperature dependence roughly follows the theory of spin-rotation mediated by Raman scattering of harmonic phonons (SRRS), though it results in a smaller-than-predicted spin-rotation coupling strength $c_{K0}/h$. Enriched ice $^{129}$Xe $T_1$ experiments show no isotopic dependence in bulk relaxation mechanisms at 77 K and at kilogauss fields.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic resonance in slowly modulated longitudinal field: Modified shape of the Rabi oscillations

The sensitivity of the Rabi oscillations of a resonantly driven spin-1/2 system to a weak and slow modulation of the static longitudinal magnetic field, B_0, is studied theoretically. We establish the mapping of a weakly driven two-level system with modulation onto a strongly driven system without modulation. The mapping suggests that different regimes of spin dynamics, known for a strongly driven system, can be realized under common experimental conditions of weak driving (driving field B_ 1 << B_ 0) upon proper choice of the domains of modulation frequency, ω_m, and amplitude, B_2. Fast modulation ω_m >> Ω_R, where Ω_R is the Rabi frequency, emulates the regime of driving frequency much bigger than the resonant frequency. Strong modulation, B_2 >> B_1, emulates the regime B_1 >> B_0. Resonant modulation, ω_m \approx Ω_R, gives rise to an envelope of the Rabi oscillations. The shape of this envelope is highly sensitive to the detuning of the driving frequency from the resonance. Theoretical predictions for different domains of B_2 and ω_m were tested experimentally using NMR of protons in water, where, without modulation, the pattern of Rabi oscillations could be observed over many periods. We present experimental results which reproduce the three predicted modulation regimes, and agree with theory quantitatively.

cond-mat.mes-hall↗

arXiv2012

Numerical study of spin-dependent transition rates within pairs of dipolar and strongly exchange coupled spins with (s=1/2) during magnetic resonant excitation

The effect of dipolar and exchange interactions within pairs of paramagnetic electronic states on Pauli-blockade-controlled spin-dependent transport and recombination rates during magnetic resonant spin excitation is studied numerically using the superoperator Liouville-space formalism. The simulations reveal that spin-Rabi nutation induced by magnetic resonance can control transition rates which can be observed experimentally by pulsed electrically (pEDMR) and pulsed optically (pODMR) detected magnetic resonance spectroscopies. When the dipolar coupling exceeds the difference of the pair partners' Zeeman energies, several nutation frequency components can be observed, the most pronounced at sqrt{2} gamma B_1 (gamma is the gyromagnetic ratio, B_1 is the excitation field). Exchange coupling does not significantly affect this nutation component; however, it does strongly influence a low-frequency component < gamma B_1. Thus, pEDMR/pODMR allow the simultaneous identification of exchange and dipolar interaction strengths.

cond-mat.mtrl-sci↗

arXiv2012

Analytical study of spin-dependent transition rates within pairs of dipolar and strongly exchange coupled spins with (S = 1/2) during magnetic resonant excitation

We study theoretically the spectrum, F(s), of spin-dependent transition rates within dipolar D and exchange J coupled pairs of two spins with S=1/2 undergoing Rabi oscillations due to a coherent magnetic resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange is stronger than the Rabi frequency (J>>Ω_R), the frequency components of the Rabi oscillation do not depend on J, rather they are determined by the relation between Ω_R and D. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of Ω_R/D and δ/D, where δis detuning of the driving ac field from the Larmor frequency. When Ω_R>>D, the two lower frequencies approach s=Ω_R, while the upper line approaches s=2Ω_R. Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different Ω_R/D and δ/D. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with Ω_R: its frequency decreases with Ω_R at Ω_R<D. Upon further increase of Ω_R this frequency then passes through a minimum and, eventually, approaches s=Ω_R. Nonmonotonic behavior of the frequencies is accompanied by nonmonotonic behavior of the respective oscillation intensity.

cond-mat.mtrl-sci↗