Cavity QED experiments with a whispering-gallery-mode bottle resonator

Abstract

The interaction of a two-level atom with a single mode of the quantized electromagnetic field constitutes one of the most fundamental systems investigated in quantum optics. We have pursued such an investigation where rubidium atoms are strongly coupled to the modes of a whispering-gallery-mode (WGM) resonator that is itself interfaced with an optical fiber. In order to facilitate studies of this atom-light interaction, an experimental apparatus was constructed around a novel type of WGM resonator developed in our group. The spectral and spatial mode structure of this resonator yield an intriguing atom-light response arising principally from the existence of two frequency-degenerate modes.<br />This thesis reports on high resolution experiments studying the transmission and reflection spectra of modes with a high quality factor (Q=10^7-10^8) in our WGM resonator. Light is coupled into and out of WGMs by frustrated total internal reflection using an optical nanofiber.<br />The atom-light interaction is facilitated by an atomic fountain that delivers a cloud of atoms to the location of the resonator. At random moments, single-atoms are clearly observed transiting the evanescent field of the resonator modes with a transit time of a few microseconds. A high-speed experimental control system was developed to firstly detect the coupling of individual atoms to the resonator and secondly to perform time-resolved spectroscopy on the strongly coupled atom-resonator system.<br />Spectral measurements clearly resolve an atom-induced change in the resonant transmission of the coupled system (65% absolute change) that is much larger than predicted in the standard Jaynes-Cummings model (25% absolute change) and that has thus far not been observed. To gain further insight, we experimentally explored the properties of the interaction and performed supporting simulations.<br />Spectroscopy was performed on the atom-resonator system using two nanofibers to in- and out-couple light for probing/observing the system.<br />Using this setup, we find an asymmetric response in the fraction of reflected light from the empty resonator mode. The coupling of atoms to a mode similarly produces asymmetric transmission and reflection spectra that critically depend on the direction of light propagation in the mode. Possible explanations for the spectral properties are identified and possible routes to verifying the exact nature of the atom-light interaction are suggested.<br />The observation of directional asymmetries and large atom-induced changes in the transmission and reflection spectra provide important new perspectives on the fundamental dynamics of atom-light coupling with WGM resonators. Moreover, our novel resonator design features four-port functionality using two optical fibers as well as very low intrinsic losses, which altogether makes the system a versatile platform for fundamental studies of open quantum systems. Within the architecture of an optical fiber-based quantum network, for example, our resonator can in principle serve as a platform for the deterministic generation of light-matter entanglement while simultaneously operating as a four-port optical switch, i.e., a quantum-mechanical switch for light.