Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, so too is it possible to stabilize the amplitude (or 'brightness') of a field to atomic parametric resonances, the so-called Rabi-resonances. For ease of reference, and by analogy to the atomic clock, we have coined the term 'atomic candle' for this quantum-mechanical, amplitude-stabilization system. Though the atomic candle was originally developed to stabilize microwave power in gas-cell atomic clocks, thereby eliminating a source of timekeeping instability in these devices, the atomic candle's applications extend well beyond the area of precise timekeeping. Basically, the atomic-candle provides a means for detecting and controlling subtle amplitude changes in electromagnetic fields at very low Fourier frequencies (i.e., f < 0.1 Hz). In the present work, we discuss a number of atomic candle applications: laser stabilization, absorption/refractive-index measurements, observations of cavity mode stability over very long time scales (i.e., 50 days), and measurements of low-frequency absorption/scattering fluctuations along a propagation path.
In the weak-field limit, resonant absorption is viewed as a passive process: an optical field impinges on an atom, and within some cross-sectional area the atom has a high probability for absorbing the radiant energy. Absorption, however, is a dynamic process. Consequently, though a singlemode laser is highly monochromatic, the field's phase noise (i.e., quantum noise) generates fluctuations in the atom's absorption cross section. Laser phase noise (PM) thereby gives rise to absorption cross-section noise, and hence fluctuations in the medium's transmitted light intensity (AM). Following a brief overview of the PM-to-AM conversion process, we consider the role of collisions on PM-to-AM conversion efficiency in the weak-field regime. Specifically, the relative-intensity-noise of a diode laser, tuned to the Rb D1 transition, was measured after it passed through a rubidium/nitrogen vapor. Varying the nitrogen pressure, we found that rapid collisional dephasing decreased the efficiency of PM-to-AM conversion. Examining the rubidium hyperfine transition lineshape as a function of nitrogen pressure, we then found that pressure-broadening increased the transition's signal-to-noise ratio when limited by the PM-to-AM conversion process.