The development of optically pumped magnetometers (OPMs) capable of operating in fields up to and beyond Earth's field has been accelerated in recent years by advances in the micro-fabrication of components, such as alkali-vapour cells and chip-scale lasers, and the application of spin-exchange relaxation suppression techniques, such as light-narrowing. Using these techniques, OPMs can be built for highly-accurate scalar geo-magnetometry. Sub-picotesla sensitivity can be achieved in compact packages, and commercialisation of these technologies is progressing globally. We review the underlying principles for OPM operation, including optimisation of microfabricated alkali vapour cells, and summarise the current state-of-the-art in their application.
The development of optically pumped magnetometers (OPMs) capable of operating in fields up to and beyond Earth's field has been accelerated in recent years by advances in the micro-fabrication of components, such as alkali-vapour cells and chip-scale lasers, and the application of spin-exchange relaxation suppression techniques, such as light-narrowing. Using these techniques, OPMs can be built for highly-accurate scalar geo-magnetometry. Sub-picotesla sensitivity can be achieved in compact packages, and commercialisation of these technologies is progressing globally. We review the underlying principles for OPM operation, including optimisation of microfabricated alkali vapour cells, and summarise the current state-of-the-art in their application.
The optically pumped magnetometer (OPM) is a class of atomic devices that rely on the measurement of Larmor precession of an atomic spin ensemble in a magnetic field. They need the specific frequency light to complete the pumping and detection process. OPMs typically work under the Earth's magnetic field or a limited field. When operating in near-zero field, they can realize the spin-exchange relaxation-free (SERF) regime, thereby promoting a substantial increase in sensitivity. At present, OPMs have been widely used in magnetoencephalography, magnetocardiography, geomagnetic detection, and inertia measurement (comagnetometer). There are a large number of researchers engaged in the study of relevant mechanisms, devices, technologies, and applications. More importantly, supported by MEMS technology and micro/nano optics, OPMs have the potential to move toward chip-scale sensors. This special issue contains some latest developments in OPMs and related areas. We hope the reprint can provide valuable references for researchers in the related field.
Optically pumped magnetometry in unshielded environments is potentially of great advantage in a wide range of surveying and security applications. Optimisation of OPM modulation schemes and feedback in the Mx scheme offers enhanced sensitivity through noise cancellation and decoherence suppression. The work presented demonstrates capability for softwarecontrolled optimisation of OPM performance in ambient fields in the 0.5G range. Effects on magnetometer bandwidth and sensitivity are discussed. Supported by UK National Quantum Technologies Programme.
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.
We demonstrate a way of operating an optically pumped magnetometer with miniaturized cesium cell using the light-narrowing effect. The magnetometer setup shows improvement of shot-noise-limited sensitivity (42fT/Hz in a cell of only 9.3mm3 volume) due to the suppression of spin-exchange relaxation to a large extent and the use of a strikingly increased fraction of alkali-metal atoms for signal generation, working even in $\ensuremath{\mu}\mathrm{T}$ magnetic fields, by using only a single high-intensity laser beam both for pumping and probing of atomic spins.
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.