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.
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 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.
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].
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.
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.
Cardiomagnetometry is a growing field of noninvasive medical diagnostics that has triggered a need for affordable high-sensitivity magnetometers. Optical pumping magnetometers (OPMs) are promising candidates for satisfying that need since it has been demonstrated that they can be used to map the magnetic field of the beating human heart. We discuss the principle of a phase-detecting OPM and describe the procedures used to optimize its performance. The optimized OPM has an intrinsic magnetometric sensitivity of 63 fT/Hz^1/2 and a measurement bandwidth of 140 Hz with a spatial resolution of 28 mm, measured in a weakly shielded environment. We further discuss the fundamental limitations of frequency- and phase-detecting magnetometers on the basis of information theory.
Rb optically pumped magnetometer (OPM) based on electron paramagnetic resonance (EPR) show advantages to measure the nuclear magnetization and have succeeded in fundamental physics and rotation sensing, etc . The magnetometry sensitivity is a key performance of these Rb OPMs which should be improved. In this study, a high sensitivity Rb OPM is demonstrated theoretically and experimentally. To improve the sensitivity, acousto-optic modulation based on balanced detection is applied to suppress the probe noises. Compared with the conventional optical rotation detection for this OPM configuration, the probe noise shows a significant suppression especially in low frequencies. Eventually, a simultaneous dual-axis transverse measurement with 30 fT/Hz 1/2 sensitivity is achieved in a 200 Hz bandwidth and a 250nT linear working range. In addition, we utilize a closed-loop feedback to improve the stability and enlarge the transverse measurement range to 10µT order of magnitude while maintain the open-loop performances. A quasi-static magnetic field measurement can also be achieved in the longitudinal direction in the closed-loop mode. This OPM can serve for the nuclear magnetization measurement with a high sensitivity especially in environments with a large magnitude of the external magnetic field.