Introduction

Soon after the development of optical magnetometers based on the radio-optical double resonance method (see Chapter 4), it was realized by Bell and Bloom [1] that an alternative method for optical magnetometry was to modulate the light used for optical pumping at a frequency resonant with the Larmor precession of atomic spins. In a Bell-Bloom optical magnetometer, circularly polarized light resonant with an atomic transition propagates through an atomic vapor along a direction transverse to a magnetic field B. Atomic spins immersed in B precess at the Larmor frequency ΩL, and when the light intensity is modulated at Ωm = ΩL, a resonance in the transmitted light intensity is observed. The essential ideas of the Bell–Bloom optical magnetometer are reviewed in Chapter 1 (Section 1.2), and can be summarized in terms of what Bell and Bloom termed optically driven spin precession: in analogy with a driven harmonic oscillator, in a magnetic field B atomic spins precess at a natural frequency equal to ΩL and the light acts as a driving force oscillating at the modulation frequency Ωm. From another point of view, the Bell-Bloom optical magnetometer can be described in terms of synchronous optical pumping: when Ωm = ΩL, there is a “stroboscopic” resonance in which atoms are optically pumped into a spin state stationary in the frame rotating with ΩL. Depending on the details of the atomic structure, the spin state stationary in the rotating frame can be either a dark state that does not interact with the modulated light or a bright state for which the strength of the light–atom interaction is increased.