When atoms interact with a phase-modulated field, the probability of finding the atom in the excited-state oscillates at the second harmonic of the modulation frequency, $2{\omega }_m.$ The amplitude of this oscillating probability is a resonant function of the Rabi frequency Ω, and this is termed a β Rabi resonance. In this work, we examine the line shape of the β Rabi resonance both theoretically and experimentally. We find that a small-signal theory of the β-Rabi-resonance condition captures much of the line shape’s character, and, in particular, that the resonance’s “line Q” (i.e., $2\delta {\Omega }_{1/2}/\Omega )$ is proportional to the modulation frequency. This result can be applied to the atomic candle, where β Rabi resonances are employed to stabilize field strength. Considering our results in the context of developing an optical atomic candle, we find that a free-running diode laser’s intensity noise could be improved by orders of magnitude using the atomic candle concept.

• Received 11 March 2002

DOI:https://doi.org/10.1103/PhysRevA.66.023806