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.

Introduction

The rapid development in recent decades of techniques for producing, trapping, and manipulating cold atoms has, as a side-benefit, made possible new methods of atomic magnetometry. The properties of cold atoms, including long coherence times and excellent spatial localization, are often desirable for high-precision magnetic sensing and allow the techniques of atomic magnetometry to be extended to previously inaccessible regions of parameter space.

Specifically, the appeal of cold atoms for magnetometry lies in the demonstrated potential for high sensitivity at high spatial resolution. Magnetic-field measurements with atoms at finite temperature are generally characterized by motional averaging, in which atoms statistically sample a volume of space determined by the measurement time, the velocity distribution, and, if present, the confinement. For high-spatial-resolution magnetometry, atomic motion must be limited. The average displacement of atoms can be reduced by decreasing the measurement time, but a shorter measurement time is unappealing because it degrades the sensitivity of the measurement. Tighter confinement is an alternative means of reducing atomic motion, and is indeed attractive provided the confinement does not adversely affect the atomic spin coherence. The use of buffer gases in vapor-cell magnetometers is effectively a form of confinement, and indeed allows magnetometry with millimeter-scale spatial resolution. However, buffer gases are not entirely benign for atomic spins, having small but finite spin-destruction collisional cross-sections [1], whose effects would be increasingly deleterious at the high pressures necessary to achieve micrometer-scale resolution. A final alternative is to reduce the velocity spread by cooling the atomic ensemble; indeed, the use of cold atoms permits a significant reduction in motional averaging, and can be achieved with no loss (and potentially an increase) in spin-coherence time.

Introduction

In this chapter, we discuss miniaturized atomic magnetometers, and the technology and applications relevant to this somewhat unusual direction in magnetometer research and development [1]. By “miniaturized,” we mean, in addition to their small size, magnetometers that have associated desirable qualities such as low power consumption, low cost, high reliability, and the potential for mass fabrication. Together with the high sensitivity usually obtained from the use of atoms, these properties result in magnetic sensors that fill a unique application space and may in fact enable new applications for which atomic magnetometers have not before been used.

It is perhaps surprising that atomic magnetometers in general are not more widely used in the world today. The main application areas at present are geophysical surveying and magnetic anomaly detection. Geophysical surveying is important in oil and mineral exploration, archeology, and unexploded ordnance detection and is typically carried out by moving one or more atomic magnetometers over the area to be surveyed. The magnetic “map” generated from this data can show the locations and in some cases the size and shape of magnetic objects or structures buried beneath the surface of the earth. Magnetic anomalies include vehicles, ships, and submarines and are typically detected via magnetic gradiometry. There are, however, only three major companies in North America, employing perhaps a few hundred people, that manufacture and sell atomic magnetometers. This effort represents a rather small fraction of the worldwide yearly market for magnetic sensors, which was estimated in 2005 to be about §1 billion [2]. Commercial atomic magnetometers are described in Chapter 20.

Introduction and history

Paraffin films and other surface coatings have played a decisive role in the emergence and development of optical magnetometry. When alkali atoms in the vapor phase collide with the bare surface of a glass container, they disappear inside the glass and are replaced in the vapor phase by another atom with random spin orientation. With a mean free path of the dimensions of the cell (typically on the order of 1 to several cm), the collision frequency is much too high, 104 s−1, to maintain the substantial spin polarization required for practical applications. In order to prevent this detrimental effect, vapor cells include either an inert buffer gas [1–3] or an antirelaxation surface coating [4]. In the presence of a noble gas at a pressure from 10−2 to a few atmospheres, the alkali atoms diffuse very slowly from the center of the cell to the glass walls, and their orientation is only very slightly affected by gas collisions. However, there are several advantages to the use of a surface coating instead of buffer gas. If the static magnetic field is not homogeneous, then resonance lines suffer from inhomogeneous broadening in the presence of the gas [5–7]. In addition, the optical pumping process is perturbed by the buffer gas [8, 9]: (i) it is more efficient at the center of the cell than near the uncoated walls, so that the atomic orientation is inhomogeneous inside the cell; (ii) the pump beam absorption line is broadened, and its profile varies with the distance from the entrance window. These effects are unfavorable for the production of alignment in the ground state.

Introduction

Magnetometry has been an invaluable tool in all stages of space exploration, from the first ionospheric sounding rockets to the most modern interplanetary probes. Our solar system is fundamentally a magnetically active environment – indeed, one might define the extent of the solar system by its heliopause, as it is the magnetic influence of the Sun which separates us from the interstellar medium. The interactions between the solar wind and the bodies of the solar system are varied and complex, and they have strong implications for the past and future of these bodies. Most importantly, a planet's magnetic field is one of the few characteristics which can be measured from space to yield information about the nature and dynamics of its interior. Recognizing these scientific imperatives, mission designers have included precise magnetometers on nearly all the spacecraft used to explore our solar system; this in turn has driven advances in magnetometer technology over the past fifty years.

Achievements of space magnetometry

Discoveries made by space magnetometers have been among the most profound achievements of space exploration. Rocket-borne magnetometers gave the first definitive evidence of electrical currents in the Earth's ionosphere and their effect on diurnal variations of the geomagnetic field [1]. These data not only shed light on the interaction between the solar wind and the Earth; they also complemented radiation studies which mapped out the Van Allen belts and thus paved the way for manned space flight. Later spacecraft magnetometers advanced dynamo theory by confirming the lack of a planet-scale dipolar field on Venus [2,3] and discovering, to much surprise, a still-active dynamo within Mercury [4].

Sources of biomagnetism

Magnetic fields arise from many parts of the body and are produced by two distinct types of sources–ionic currents and magnetic tissues. Although most of the body is weakly diamagnetic, the magnetic tissues of greatest interest are paramagnetic or ferromagnetic. The major source of paramagnetism in the body is the liver, which contains iron compounds [1]. The strongest sources of ferromagnetism in the body are ingested or inhaled ferromagnetic substances, which can be detected even in trace amounts [2].

Like other electrical currents, ionic currents generated by nerve and muscle tissue produce magnetic fields and potential differences, which can be detected at the body surface or even outside the body. The ionic current has two components – a “primary” current and a “volume” current [3] (Fig. 16.1). Primary currents result directly from physiological activity and are confined largely within the electrically excited cells. Volume currents are passive return currents that extend far into the surrounding medium in response to the electromotive forces that drive the primary currents. Bioelectric signals are conducted to the body surface by volume currents and are the main source of electroencephalographic (EEG) and electrocardiographic (ECG) signals. Bioelectric signals are strongly influenced by the highly inhomogeneous electrical conductivity of the body. In contrast, biomagnetic signals arise predominantly from primary currents and are affected to a much lesser extent. This accounts for a key advantage of biomagnetic versus bioelectric fields – their simpler signal-transmission properties – which enables a more accurate determination of source location.