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.

Introduction

Optically pumped helium (He) magnetometers have provided magnetic field data for military, aeromagnetic survey, space exploration and geophysical laboratory applications for over five decades. The characteristics of He magnetometers that have made them instruments of choice for these varied applications include high sensitivity, high accuracy, simplicity of the resonance line, small heading errors due to light shifts, temperature independence of resonance cells, linear relationship between the magnetic field and the resonance frequency, excellent stability for gradiometer operation and robustness for field and space use. Scalar He magnetometers can easily be configured for omnidirectional operation with no moving parts to provide full sensitivity on all headings relative to the magnetic field direction.

Helium magnetometers have two types of optical pumping radiation sources. All He magnetometers manufactured from 1960 to 1990 utilized an RF electrodeless discharge He-4 lamp as an optical pumping source of 1083 nm resonance radiation which is composed of three closely spaced He-4 resonance lines D0, D1, and D2. In the 1980s, the development efforts for a single-line pump source for both He magnetometers and basic research on He isotopes resulted in both high-efficiency semiconductor lasers and optical fiber lasers at 1083 nm. Laser-pumped He magnetometers are characterized by sensitivities up to two orders of magnitude better than lamp-pumped He magnetometers and are more accurate, smaller, and very stable for use in magnetic gradiometers. L. D. Schearer provided a comprehensive review of the beginning science of He-4 magnetometers [1] and a review of the first 25 years of progress in optically pumped He magnetometers [2].

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 ωL 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.

Introduction

Applications such as geophysical exploration for minerals and oil, anti-submarine warfare, volcanology, earthquake studies, magnetic observatories, and the detection of buried objects at sites of archaeological significance require instruments that can measure small variations of the Earth's magnetic field in time and space. Other applications such as laboratory metrology, space exploration, and biomedicine may require measurements at lower or higher fields. The demand for precise measurements of magnetic fields is met in part by commercial atomic magnetometers supplied by a number of manufacturers over the last fifty years. Commercial optical magnetometers based on cesium, potassium, and helium are now firmly established. Commercial magnetometers designed for measuring anomalies in the Earth's field are typically operated as self-oscillating devices or VCO (voltage-controlled oscillator) lock-in devices with the oscillation or lock-in frequency proportional to the magnetic field. This chapter examines the specifications for atomic magnetometers, compares the most widely used approaches, describes some of the features demanded by different applications, and surveys the history of atomic magnetometers. (The online supplemental material contains a table listing many of the United States patents related to the field of atomic magnetometry.)

Earth's field at the surface ranges from 20 to 80 μT. Typically the user is seeking to generate a map of the local magnetic field upon which magnetic anomalies can be discerned. The map may be made by means of a walking survey, for instance in the case of archaeological sites. For mineral exploration much wider areas must be surveyed, so that typically airborne and ground surveys are required. The magnetic anomalies can be in the pT range or even smaller. For such applications, the most significant requirement for the magnetometer is its sensitivity.