Atomic Physics Seminars

Counter-intuitive predictions of quantum mechanics are most readily explored in the field of optics, where table-top experiments suffice to make sensitive measurements. A single atom, radiating in free space, is the simplest and most fundamental quantum optical system. It is particularly attractive for study of multiple measurements on a quantum system because, quite unlike the situation for a classical radiator, the detection of a radiated photon directly affects the probability of a subsequent emission. Indeed, measurement of fluctuations in the radiated intensity provided the first experimental evidence for such counter-intuitive quantum effects as photon antibunching and sub-Poissonian light. However, quantum fluctuations in the optical phase of the radiated light remain relatively unexplored. Especially remarkable is the fact that "squeezing" in single-atom fluorescence, a phase-sensitive quantum effect first predicted in 1981, has long eluded direct observation despite receiving considerable attention. The reason is that measurement of phase-sensitive nonclassical effects in atomic fluorescence presents severe experimental challenges. In this talk I will describe how we recently overcame these challenges to make the first measurements of single-atom squeezing spectra in the phase-dependent fluorescence of atoms radiating in free space. Our experimental scheme permits a valid comparison of the observations with our predictions, thus yielding a new and simple physical picture of phase-dependent quantum noise in atomic fluorescence. Results of a direct measurement of the two-time field correlations will also be presented. Our measurements help elucidate the basic atomic processes underlying "squeezing". Our observations are especially important because the measurement accuracy in current state-of-the-art cold atom interferometers and frequency standards is limited by quantum noise. Controlling the phase-dependent quantum noise may enable measurement beyond quantum limits.