Optical magnetometry, in which a magnetic field is measured by observing changes in the properties of light interacting with matter immersed in the field, is not a new field. It has its origins in Michael Faraday's discovery in 1845 of the rotation of the plane of linearly polarized light as it propagated through a dense glass in the presence of a magnetic field. Faraday's historic discovery marked the first experimental evidence relating light and electromagnetism.

A century later, atomic magnetometers based on optical pumping were introduced and gradually perfected by such giants as Alfred Kastler, Hans Dehmelt, Jean Brossel, William Bell, Arnold Bloom, and Claude Cohen-Tannoudji, to name but a few of the pioneers. Recent years have seen a revolution in the field related to the development of tunable diode lasers, efficient antirelaxation wall coatings, techniques for elimination of spin-exchange relaxation, and, most recently, the advent of optical magnetometers based on color centers in diamond. Today, optical magnetometers are pushing the boundaries of sensitivity and spatial resolution, and, in contrast to their able competition from super-conducting quantum interference device (SQUID) magnetometers, they do not require cryogenic temperatures. Numerous novel applications of optical magnetometers have flourished, from detecting signals in microfluidic nuclear-magnetic resonance chips to measuring magnetic fields of the human brain to observing single nuclear spins in a solid matrix.

Introduction

Nuclear magnetic resonance (NMR) is a powerful analytical tool for elucidation of molecular form and function, finding application in disciplines including medicine (magnetic resonance imaging), materials science, chemistry, biology, and tests of fundamental symmetries [1–6]. Conventional NMR relies on a Faraday pickup coil to detect nuclear spin precession. The voltage induced in a pickup coil is proportional to the rate of change of the magnetic flux through the coil. Hence, for a given nuclear spin polarization, the signal increases linearly with the Larmor precession frequency of the nuclear spins. Since the thermal nuclear spin polarization is also linear in the field strength, the overall signal is roughly proportional to B2, motivating the development of stronger and stronger magnetic fields. Additionally, an important piece of information in NMR is the so-called chemical shift, which effectively modifies the gyromagnetic ratios of the nuclear spins depending on their chemical environment. This produces different precession frequencies for identical nuclei on different sites of a molecule, and the separation in precession frequencies is linear in the magnetic field. For these reasons, tremendous expense has been spent on the development of stronger magnets. Typical spectrometers feature 9.4 T superconducting magnets, corresponding to 400 MHz proton precession frequencies, and state-of-the-art NMR facilities may feature 24 T magnets, corresponding to 1 GHz proton precession frequency. While the performance of such machines is impressive, there are a number of drawbacks: superconducting magnets are immobile and expensive (roughly §500 000 for a 9.4 T magnet and console) and require a constant supply of liquid helium.

Overview and introduction

At present, we know of four fundamental forces, three of which (electromagnetism, the strong force, and the weak force) are well described by what has come to be known as the Standard Model, a theory developed in the 1960s by Glashow, Weinberg, Salam, and others [1–3]. The fourth, gravity, is well understood at macroscopic scales in terms of Einstein's theory of general relativity [4, 5]. In spite of the spectacular agreement between these theoretical descriptions and numerous experimental measurements, it has been exceedingly challenging to develop a consistent theory of gravitation at the quantum scale, primarily because of the extreme difference between the mass and distance scales at which experimental tests of the two theories are performed. Furthermore, there are a variety of observations that have defied satisfactory explanation within this framework, prominent among them the matter–antimatter asymmetry of the universe [6], evidence for dark matter [7], and the accelerating expansion of the universe, attributed to a mysterious “dark energy” permeating spacetime [8]. It is always of great interest to carry out experiments testing the agreement between theory and experiment beyond the frontier of present precision, and the abundant mysteries confronting our modern understanding of fundamental particles and interactions make the present era an especially auspicious time for the discovery of new physics.

The techniques of optical magnetometry are ideally suited for experimental tests of fundamental physical laws involving atomic spins. For example, a variety of optical magnetometry techniques are being used to search for heretofore undiscovered spin-dependent forces that would indicate the existence of new fundamental interactions.

Airborne magnetometers and gradiometers

Along with electromagnetic (EM), gravity, and radiation detection methods, magnetometry is a basic method for geophysical exploration for minerals, including diamonds and oil. Fixed-wing and helicopter-borne magnetometers and gradiometers are generally used for assessment explorations, with ground and marine methods providing for follow-up mapping of interesting areas.

Magnetometers have been towed by or mounted on airborne platforms for resource exploration since the 1940s [1]. Mapping of the Earth's magnetic field can illuminate structural geology relating to rock contacts, intrusive bodies, basins, and bedrock. Susceptibility contrasts associated with differing amounts of magnetite in the subsurface can identify areas that are good candidates for base and precious metal mineral deposits or diamond pipes. Existing magnetic anomalies associated with known mineralization are often extrapolated to extend drilling patterns and mining activities into new areas.

After World War II fluxgate sensors, originally employed for submarine detection, replaced dipping needle and induction coil magnetic field sensing systems as the air-borne magnetometer of choice. While the fluxgate and induction magnetometers could measure the components of the Earth's field rapidly (100 Hz or faster), their sensitivity to orientation made them a poor choice for installation on moving platforms. Experiments by Packard and Varian in 1953 on nuclear magnetic resonance resulted in the invention of the orientation-independent total-field proton precession magnetometer and total-field magnetometers replaced vector magnetometer systems in mobile platforms.