Particles that have both angular momentum and a magnetic moment precess in a magnetic field at their Larmor frequency
ωL = γ B, (1)
where γ is the gyromagnetic constant (the ratio of the magnetic dipole moment to the angular momentum) of the atom and B is the local magnetic field. Atomic (or optical) magnetometers are based on a measurement of the Larmor precession by use of an optical field tuned to an appropriate optical transition in the atoms. In the most common optical configuration, often referred to as the Mx magnetometer, atoms are first polarized along some direction by use of a circularly polarized “pump” laser field. The precessing transverse polarization is then driven with a resonant transverse excitation magnetic field. Finally, the magnitude of the precessing polarization is measured with a transverse probe optical field. A schematic of this method is shown in the Figure (a) below. The precessing polarization can also be driven by a modulated light field, which resonantly optically pumps the atoms. In this configuration, known as the Bell-Bloom arrangement and shown in (b), the pump and drive fields (and even the probe) can be the same laser beam.
If the magnetic field is sufficiently weak, the longitudinal component of the field ceases to cause a resonant precession of the atomic polarization, but instead the transverse component of the field causes a small static reorientation of the polarization, as shown in (c) below. This reorientation can be monitored with a transverse probe field in a manner similar to the precession. In this case the magnitude of the transverse atomic polarization is proportional not only to the magnetic field but also to the transverse relaxation rate (T2) of the atomic polarization.
Atomic magnetometers range from large, highly precise laboratory apparatus to smaller, but less sensitive, instruments that can be used in the field. Magnetometers are typically characterized by their sensitivity, but also by a range of other features such as vector or scalar operation, bandwidth, heading error, size, weight, power, cost and reliability. These characteristics determine the range of applications for which the magnetometer is suitable. Over the last few years, we have adapted the processes and designs developed at NIST for chip-scale atomic clocks to compact atomic magnetometers. The basic idea behind many chip-scale atomic devices is to combine the use of advanced, low-power diode lasers (usually vertical-cavity surface emitting lasers or VCSELs) with the existing techniques of atomic spectroscopy and processes common in MEMS. This allows us to develop millimeter-scale structures that operate at low power, but retain much of the precision of their larger counterparts.
Figure 1. (a) Magnetically driven atomic precession, in which the atoms are polarized along the direction of the strong magnetic field B0 and then resonantly driven into a precessing state. The amplitude of the precession is monitored by a probe beam. (b) Optically driven precession in which the precessing atomic polarization is driven by a modulated optical field. (c) Static reorientation of the atomic polarization due to a weak magnetic field. All three methods are used in the magnetometers under development at NIST.