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Double-resonance magnetometry in arbitrarily oriented fields

Stuart J. Ingleby, Erling Riis, Aidan S. Arnold, Paul F. Griffin, Carolyn O’Dwyer, Iain C. Chalmers

Published Aug 21, 2017• 0 citations
OpenAlex

Abstract

Our magnetometry development is focussed on compact, portable sensors for geophysical field measurements. In order to develop practical sensors with minimised size and power requirements, single-beam double-resonance magnetometry is used, avoiding any requirement for extensive optical hardware or full-field compensation. Magnetic resonance in atomic polarisation is detected using a polarimeter to measure optical rotation in transmitted pump light, allowing rejection of common-mode optical noise. The application of this technique to unshielded geophysical field measurements requires detailed understanding of systematic effects arising in arbitrary orientations of the static field B0. We report the development and calibration of a test system for double resonance measurements in generated static fields of well-controlled magnitude and orientation, including systems for automated B0 control and software generated modulation/demodulation [1]. The sensitivity of these devices depends on signal amplitude, phase and RF broadening, all of which vary with B0 orientation. By working in a low-field, weak-pumping regime, we obtain experimental measurements of anisotropy in these parameters, in agreement with theoretically derived distributions [2]. We comment on the importance of measurement mode and signal demodulation in optimising double-resonance sensitivity and bandwidth, and the suitability of these techniques for compact, portable magnetometers. This work is supported by the UK Quantum Technology Hub in Sensors and Metrology [3].

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Double-resonance optically pumped magnetometry can be used to measure static magnetic fields with high sensitivity by detecting a resonant atomic spin response to a small oscillating field perturbation. Determination of the resonant frequency yields a scalar measurement of static field $({B}_{0})$ magnitude. We present calculations and experimental data showing that the on-resonance polarimeter signal of light transmitted through an atomic vapor in arbitrarily oriented ${B}_{0}$ may be modeled by considering the evolution of alignment terms in atomic polarization. We observe that the amplitude and phase of the magnetometer signal are highly dependent upon ${B}_{0}$ orientation and present precise measurements of the distribution of these parameters over the full $4\ensuremath{\pi}$ solid angle.

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Our magnetometry development is focussed on compact, portable sensors for geophysical field measurements. In order to develop practical sensors with minimised size and power requirements, single-beam double-resonance magnetometry is used, avoiding any requirement for extensive optical hardware or full-field compensation. Magnetic resonance in atomic polarisation is detected using a polarimeter to measure optical rotation in transmitted pump light, allowing rejection of common-mode optical noise. The application of this technique to unshielded geophysical field measurements requires detailed understanding of systematic effects arising in arbitrary orientations of the static field B0. We report the development and calibration of a test system for double resonance measurements in generated static fields of well-controlled magnitude and orientation, including systems for automated B0 control and software generated modulation/demodulation [1]. The sensitivity of these devices depends on signal amplitude, phase and RF broadening, all of which vary with B0 orientation. By working in a low-field, weak-pumping regime, we obtain experimental measurements of anisotropy in these parameters, in agreement with theoretically derived distributions [2]. We comment on the importance of measurement mode and signal demodulation in optimising double-resonance sensitivity and bandwidth, and the suitability of these techniques for compact, portable magnetometers. This work is supported by the UK Quantum Technology Hub in Sensors and Metrology [3].
Double-resonance optically pumped magnetometry can be used to measure static magnetic fields with high sensitivity by detecting a resonant atomic spin response to a small oscillating field perturbation. Determination of the resonant frequency yields a scalar measurement of static field (B0)({B}_{0})(B0​) magnitude. We present calculations and experimental data showing that the on-resonance polarimeter signal of light transmitted through an atomic vapor in arbitrarily oriented B0{B}_{0}B0​ may be modeled by considering the evolution of alignment terms in atomic polarization. We observe that the amplitude and phase of the magnetometer signal are highly dependent upon B0{B}_{0}B0​ orientation and present precise measurements of the distribution of these parameters over the full $4\ensuremath{\pi}$ solid angle.
The field of magnetometry has played a pivotal role in developing our understanding of the world around us, by providing a wealth of unique information via non-invasive magnetic measurements. In many applications the characteristics of magnetic fields — their strength and temporal dependence, for example — constitute a unique fingerprint of their source, and provide a window into both their origin and any underlying physical processes that produced them. The invention of the laser, and recent advancements in anti-relaxation coatings for vapour cells, has revolutionised the field of magnetometry and given rise to a myriad of optical magnetometers which demonstrate exquisite sensitivity. This has seen a movement away from traditional cryogenic magnetometry techniques — which utilise superconducting quantum interference devices — and has ushered in a slew of optical techniques which offer a whole host of benefits such as room-temperature operability, intrinsic accuracy, and portability. Applications which were previously inaccessible due to demanding requirements such as high sensitivity at ambient temperatures, are now permissible using these newly developed optical techniques. This thesis details the development of an ultrastable optical magnetometer based on nonlinear magneto-optical rotation in rubidium vapour, that has been optimised for high sensitivity over long timescales. Additionally, a novel measurement technique — which relies on tracking the instantaneous phase of atomic spin precession — has been developed and demonstrated, resulting in significant improvements to both the amplitude and frequency response of the device when compared to conventional measurement techniques described in the literature. Significant attention is paid to technical and fundamental noise sources, which must be minimised in order to achieve high performance — especially over long timescales. The latter chapters discuss the development of the magnetometer into a sensitive and reliable system, capable of measuring extremely small magnetic-field fluctuations, and transient fields with arbitrarily complex temporal dependence. The capability of the device to measure minute field fluctuations, as well as rapid transient disturbances, may find use in real-world applications such as geophysical exploration, medical diagnostics and imaging, and magnetic anomaly detection. Furthermore, any fundamental physics applications which demand accurate and precise measurements of magnetic fields may find application of this device — and the novel measurement techniques developed within this thesis—extremely beneficial.
Introduction Shortly after the inception of atomic magnetometry, alkali-vapor magnetometers were being used to measure the Earth's magnetic field to unprecedented precision. During the same era, Bell and Bloom first demonstrated all-optical atomic magnetometry through synchronous optical pumping [1] (see Chapters 1 and 6). In this approach, optical-pumping light is frequency- or amplitude-modulated at harmonics of the Larmor frequency ω L to generate a precessing spin polarization within an alkali vapor at finite magnetic field [2, 3]. Although this technique received considerable attention from the atomic physics community for its applicability to optical pumping experiments, Earth's-field alkali-vapor atomic magnetometers continued to rely on radiofrequency (RF) field excitation for several decades (see Chapter 4). Upon the advent of diode lasers addressing alkali and metastable helium transitions, synchronously pumped magnetometers experienced a revival beginning in the late 1980s. In recent years, such magnetometers have found applications in nuclear magnetic resonance detection [4] (see also Chapter 14), quantum control experiments [5], and chip-scale devices intended for spacecraft use [6] (see also Chapters 7 and 15). All-optical magnetometers possess several advantages over devices that employ RF coils. RF-driven magnetometers can suffer from cross-talk if two sensors are placed in close proximity, since the AC magnetic field driving resonance in one vapor cell can adversely affect the other. All-optical magnetometers are free from such interference. When operated in self-oscillating mode [7], RF-driven magnetometers require an added ±90° electronic phase shift in the feedback loop to counter the intrinsic phase shift between the RF field and the probe-beam modulation.
Optically pumped atomic magnetometers (OPAMs) offer high sensitivity at room temperature and are increasingly considered for portable magnetic sensing in geomagnetic-field environments. Here, we report a handheld-scale, single-beam scalar 87Rb OPAM with a sensor-head volume of approximately 110 ml. The device operates in an all-optical Bell–Bloom configuration and uses digital lock-in, dispersive tracking of the 87Rb Larmor resonance, implemented with a hybrid electronics stack that combines in-house control hardware with commercial modules. A single frequency-modulated laser beam performs both pumping and probing without RF coils. All signal processing is realized in Python on a single-board computer paired with a commercial off-the-shelf data-acquisition module, enabling immediate deployment without dedicated signal-processing hardware. The magnetometer has an intrinsic in-band field sensitivity of approximately 21 pT/Hz, estimated from the lock-in dispersion slope, over a 0.1–30 Hz closed-loop in-band region with a digital-output rate of 200 samples/s. In an unshielded Earth-field deployment, we detect repeatable transient magnetic signatures from a controlled elevator motion sequence and quantify standoff observability over sensor-elevator distances from 1.25 m to 10 m. These results show that compact scalar OPAMs can provide bandwidth and range-resolved event sensitivity suitable for field-deployable magnetic anomaly detection and infrastructure monitoring in realistic geomagnetic environments.
Optically pumped magnetometers are important instruments for imaging biological magnetic signals without the need for cryogenic cooling. These magnetometers are presently available in the commercial market and utilize the principles of atomic alignment or orientation, enabling remarkable sensitivity and precision in the measurement of magnetic fields. This research focuses on utilizing a spin-aligned atomic ensemble for magnetometry at zero field. An approach is introduced which involves evaluating how the linear polarization of light rotates as it passes through the atomic vapor to null the magnetic field. Analytical expressions are derived for the resulting spin alignment and photodetection signals. Experimental results are provided, demonstrating good agreement with the theoretical predictions. The sensitivity and bandwidth of the magnetometer are characterized based on the detected polarization rotation signal. Lastly, the practical utility of the magnetometer for medical applications is demonstrated by successfully detecting a synthetic cardiac signal.
The ability to measure the magnetic fields around celestial bodies enables various insights into processes and phenomena deep within such bodies.The use of magnetometry in space missions dates back to the 1950s, and a number of different technologies have been used to measure these fields, ranging from fluxgates to Anisotropic Magneto Resistance (AMR) sensors.Optically-pumped magnetometers (OPMs) represent the state-of-the-art in magnetic field sensitivity. 1 Such sensors rely upon the interaction of laser light to spin polarise a warm vapour of neutral atoms.Detecting the precession of these spins, again with laser light, is a direct way to measure a magnetic field and is relatable to fundamental constants.Improvements in quantum technology within the last decade have led to the miniaturisation of these sensors.We have now reached the point where compact, lightweight, and low power devices offer greater sensitivity to magnetic fields than any other technology.In this work, we identify the following four key space application areas in which OPMs may offer significant advantages over existing technologies: Environments in which high sensitivity is needed at low magnetic fields (10s of nT), full vector read-out with high directional sensitivity and reduced SWaP compared to helium magnetometers, miniaturised and lightweight scalar magnetometers to complement vector fluxgate measurements, vector or scalar sensors that need high (40 dB) gradiometer performance.This work identifies OPM configurations for each area and makes a first estimate of size, weight and power requirements for each.By enabling improvements in magnetic field sensitivity, this represents an exciting prospect for precision magnetic field measurements while also saving mass and reducing complexity.