arXiv subjects

Svenja Knappe

Publications and source records attributed to Svenja Knappe.

12 recordsLinked to original sources

arXiv2026

An Accurate Vector Magnetometer via Zeeman Rabi Oscillations

Accurate magnetic field direction sensing in compact platforms is critical in applications spanning magnetic navigation, space science, and biomedical imaging. We demonstrate a single-optical-axis vector optically pumped magnetometer based on Rabi oscillations between Zeeman sublevels driven by a series of resonant radiofrequency (RF) polarization ellipses (PEs). A calibration protocol based on controlled rotations of the DC magnetic field determines the spatial orientation of each PE. We develop a detailed theoretical model describing the angular dependence of the Rabi frequencies, incorporating key systematics including RF Stark shifts and Bloch-Siegert shifts. We also account for an RF-based heading-error systematic affecting Rabi-frequency measurements arising from the nonlinear Zeeman effect. Simultaneous Larmor measurements yield the magnitude of the magnetic field, enabling integrated vector-scalar measurements. The magnetometer achieves deadzone-free vector operation with 80 $\mu$rad mean angular accuracy and angular noise densities as low as 8 $\mu$rad$/\sqrt{\mathrm{Hz}}$, offering a pathway towards miniaturized sensors without requiring 3D optical access or sensor rotations.

physics.atom-ph↗

arXiv2024

Accurate vector optically pumped magnetometer with microwave-driven Rabi frequency measurements

Robust calibration of vector optically pumped magnetometers (OPMs) is a nontrivial task, but increasingly important for applications requiring high-accuracy such as magnetic navigation, geophysics research, and space exploration. Here, we showcase a vector OPM that utilizes Rabi oscillations driven between the hyperfine manifolds of $^{87}$Rb to measure the direction of a DC magnetic field against the polarization ellipse structure of a microwave field. By relying solely on atomic measurements -- free-induction decay (FID) signals and Rabi measurements across multiple atomic transitions -- this sensor can detect drift in the microwave vector reference and compensate for systematic shifts caused by off-resonant driving, nonlinear Zeeman (NLZ) effects, and buffer gas collisions. To facilitate dead-zone-free operation, we also introduce a novel Rabi measurement that utilizes dressed-state resonances that appear during simultaneous Larmor precession and Rabi driving (SPaR). These measurements, performed within a microfabricated vapor cell platform, achieve an average vector accuracy of 0.46 mrad and vector sensitivities down to 11 $\mu$rad$/\sqrt{\text{Hz}}$ for geomagnetic field strengths near 50 $\mu$T. This performance surpasses the challenging 1-degree (17 mrad) accuracy threshold of several contemporary OPM methods utilizing atomic vapors with an electromagnetic vector reference.

physics.atom-ph↗

arXiv2023

Correcting heading errors in optically pumped magnetometers through microwave interrogation

We demonstrate how to measure in situ for heading errors of optically pumped magnetometers (OPMs) in the challenging parameter regime of compact vapor cells with imperfect optical pumping and high buffer gas pressure. For this, we utilize microwave-driven Ramsey and Rabi frequency spectroscopy (FS) to independently characterize scalar heading errors in free induction decay (FID) signals. Both of these approaches suppress 5-nT inaccuracies in geomagnetic fields caused by nonlinear Zeeman (NLZ) shifts in FID measurements to below 0.6 nT. For Ramsey FS, we implement short periods of microwave interrogation within a $\pi/2-t_R-3\pi/2$ Ramsey interferometry sequence, effectively circumventing systematic errors from off-resonant driving. Conversely, Rabi FS leverages an atom-microwave Hamiltonian for accurate modeling of Rabi oscillation frequencies, achieving a measurement precision down to 80 pT$/ \sqrt{\text{Hz}}$ that is limited primarily by technical microwave noise. We show that the fundamental sensitivity of Rabi FS is 30 pT/$\sqrt{\text{Hz}}$ with our vapor cell parameters through a Cram\'er-Rao lower bound (CRLB) analysis. This work paves the way for future investigations into the accuracy of hyperfine structure (HFS) magnetometry and contributes to the broader applicability of OPMs in fields ranging from navigation and geophysics to space exploration and unexploded ordinance detection, where heading error mitigation is essential.

physics.atom-ph↗

arXiv2022

Coherence of Rabi oscillations with spin exchange

Rabi measurements in atomic vapor cells are of current interest in a range of microwave imaging and sensing experiments, but are increasingly in a parameter space outside of theoretical studies of coherence defined by spin-exchange collisions. Here, we study the coherence of Rabi oscillations in vapor cells by employing continuous non-destructive readout of the hyperfine manifold of $^{87}$Rb using Faraday rotation. We develop a full model for spin-exchange (SE) coherence for hyperfine transitions that takes into account a non-static population distribution. In this regime, Rabi oscillations exhibit nontrivial time-domain signals that allow verification of vapor-cell parameters. We find excellent agreement between theory and experiment, which will aid in benchmarking sensitivities of Rabi measurement applications.

physics.atom-ph↗

arXiv2021

A study of scalar optically-pumped magnetometers for use in magnetoencephalography without shielding

Scalar optically-pumped magnetometers (OPMs) are being developed in small packages with high sensitivities. The high common-mode rejection ratio of these sensors allows for detection of very small signals in the presence of large background fields making them ideally suited for brain imaging applications in unshielded environments. Despite a flurry of activity around the topic, questions remain concerning how well a dipolar source can be localized under such conditions, especially when using few sensors. In this paper, we investigate the source localization capabilities using an array of scalar OPMs in the presence of a large background field while varying dipole strength, sensor count, and forward model accuracy. We also consider localization performance as the orientation angle of the background field changes. Our results are validated experimentally through accurate localization using a phantom virtual array mimicking a current dipole in a conducting sphere in a large background field. Our results are intended to give researchers a general sense of the capabilities and limitations of scalar OPMs for magnetoencephalography systems.

physics.ins-det↗

arXiv2017

A microfabricated optically-pumped magnetic gradiometer

We report on the development of a microfabricated atomic magnetic gradiometer based on optical spectroscopy of alkali atoms in the vapor phase. The gradiometer, which operates in the spin-exchange relaxation free regime, has a length of 60 mm and cross sectional diameter of 12 mm, and consists of two chip-scale atomic magnetometers which are interrogated by a common laser light. The sensor can measure differences in magnetic fields, over a 20 mm baseline, of 10 fT/Hz$^{1/2}$ at frequencies above 20 Hz. The maximum rejection of magnetic field noise is 1000 at 10 Hz. By use of a set of compensation coils wrapped around the sensor, we also measure the sensor sensitivity at several external bias field strengths up to 150 mG. This device is useful for applications that require both sensitive gradient field information and high common-mode noise cancellation.

physics.ins-det↗

arXiv2014

An optically modulated zero-field atomic magnetometer with suppressed spin-exchange broadening

We demonstrate an optically pumped $^{87}$Rb magnetometer in a microfabricated vapor cell based on a zero-field dispersive resonance generated by optical modulation of the $^{87}$Rb ground state energy levels. The magnetometer is operated in the spin-exchange relaxation-free regime where high magnetic field sensitivities can be achieved. This device can be useful in applications requiring array-based magnetometers where radio frequency magnetic fields can induce cross-talk among adjacent sensors or affect the source of the magnetic field being measured.

physics.ins-det↗

arXiv2014

The hyperfine Paschen-Back Faraday effect

We investigate experimentally and theoretically the Faraday effect in an atomic medium in the hyperfine Paschen-Back regime, where the Zeeman interaction is larger than the hyperfine splitting. We use a small permanent magnet and a micro-fabricated vapour cell, giving magnetic fields of the order of a Tesla. We show that for low absorption and small rotation angles, the refractive index is well approximated by the Faraday rotation signal, giving a simple way to measure the atomic refractive index. Fitting to the atomic spectra, we achieve magnetic field sensitivity at the $10^{-4}$ level. Finally we note that the Faraday signal shows zero crossings which can be used as temperature insensitive error signals for laser frequency stabilisation at large detuning. The theoretical sensitivity for $^{87}$Rb is found to be $\sim 40$ kHz/$^\circ$C.

physics.atom-ph↗

arXiv2013

Optical hyperpolarization and NMR detection of $^{129}$Xe on a microfluidic chip

Optically hyperpolarized $^{129}$Xe gas has become a powerful contrast agent in nuclear magnetic resonance (NMR) spectroscopy and imaging, with applications ranging from studies of the human lung to the targeted detection of biomolecules. Equally attractive is its potential use to enhance the sensitivity of microfluidic NMR experiments, in which small sample volumes yield poor sensitivity. Unfortunately, most $^{129}$Xe polarization systems are large and non-portable. Here we present a microfabricated chip that optically polarizes $^{129}$Xe gas. We have achieved $^{129}$Xe polarizations greater than 0.5$\%$ at flow rates of several microliters per second, compatible with typical microfluidic applications. We employ in situ optical magnetometry to sensitively detect and characterize the $^{129}$Xe polarization at magnetic fields of 1 $μ$T. We construct the device using standard microfabrication techniques, which will facilitate its integration with existing microfluidic platforms. This device may enable the implementation of highly sensitive $^{129}$Xe NMR in compact, low-cost, portable devices.

physics.ins-det↗

arXiv2012

Absolute absorption and dispersion of a rubidium vapour in the hyperfine Paschen-Back regime

Here we report on measurements of the absolute absorption and dispersion properties of an isotopically pure 87Rb vapour for magnetic fields up to and including 0.6 T. We discuss the various regimes that arise when the hyperfine and Zeeman interactions have different magnitudes, and show that we enter the hyperfine Paschen- Back regime for fields greater than 0.33 T on the Rb D2 line. The experiment uses a compact 1 mm3 microfabricated vapour cell that makes it easy to maintain a uniform and large magnetic field with a small and inexpensive magnet. We find excellent agreement between the experimental results and numerical calculations of the weak probe susceptibility where the line positions and strengths are calculated by matrix diagonalization.

physics.atom-ph↗

arXiv2011

Detection of a single cobalt microparticle with a microfabricated atomic magnetometer

We present magnetic detection of a single, 2 μm diameter cobalt microparticle using an atomic magnetometer based on a microfabricated vapor cell. These results represent an improvement by a factor of 105 in terms of the detected magnetic moment over previous work using atomic magnetometers to detect magnetic microparticles. The improved sensitivity is due largely to the use of small vapor cells. In an optimized setup, we predict detection limits of 0.17 μm^3.

physics.atom-ph↗

arXiv2011

Parahydrogen enhanced zero-field nuclear magnetic resonance

Nuclear magnetic resonance (NMR), conventionally detected in multi-tesla magnetic fields, is a powerful analytical tool for the determination of molecular identity, structure, and function. With the advent of prepolarization methods and alternative detection schemes using atomic magnetometers or superconducting quantum interference devices (SQUIDs), NMR in very low- (~earth's field), and even zero-field, has recently attracted considerable attention. Despite the use of SQUIDs or atomic magnetometers, low-field NMR typically suffers from low sensitivity compared to conventional high-field NMR. Here we demonstrate direct detection of zero-field NMR signals generated via parahydrogen induced polarization (PHIP), enabling high-resolution NMR without the use of any magnets. The sensitivity is sufficient to observe spectra exhibiting 13C-1H J-couplings in compounds with 13C in natural abundance in a single transient. The resulting spectra display distinct features that have straightforward interpretation and can be used for chemical fingerprinting.

physics.chem-ph↗