In this thesis, I present fluorescence imaging of 87Rb atoms inside an optical cavity using the Raman imaging technique. The first part describes precision measurements of differential light shifts that occur during continuous Raman sideband cooling. The light shifts are caused by the optical repumper beam and modify the two-photon resonance of the Raman coupling. The cooling process is modeled by means of a three-level system that takes into account the Raman coupling and the repumping process. We find qualitative agreement between the estimated and the measured light shifts. In addition, we identified the optimum cooling parameters and confirmed that near ground-state cooling is achieved. We also observe that it is beneficial to detune the optical repumping beam away from resonance in order to suppress detrimental heating due to dipole-force fluctuations.
The characterization of differential light shifts is subsequently used to optimize the fluorescence imaging of atoms inside the resonator. We implement Raman imaging which is based on detecting the repumper fluorescence during continuous Raman sideband cooling. The complexity of the parameter space is reduced via a two-photon feed-forward which maintains the resonance with the Raman cooling sideband when varying the repumper parameters. A new imaging system was installed in order to suppress spurious background light that was blinding the camera. The optimization of the fluorescence is discussed and a mean signal-to-noise ratio of about six is obtained for an exposure time of 1 s. Using the cavity-based atom detection we independently measure the probability that a single atom survives the exposure time. We obtain survival probabilities exceeding 80 % and lifetimes up to 55 s which are believed to be vacuum-limited. In conclusion, Raman imaging is successfully applied to image small atomic ensembles inside the resonator. This constitutes the first step towards photon storage experiments with multiple atoms.
In addition, I present theoretical calculations regarding photon generation in an atom-cavity system. The shaping of the single-photon temporal wave function is discussed which is based on tailoring the corresponding control laser pulse. For sufficiently smooth temporal envelopes any pulse shape can be generated as long as the dynamics are adiabatic. We study the breakdown of the adiabatic approximation for our cavity parameters by means of numerical simulations and find that the photon generation fidelity drops when the characteristic pulse time approaches the inverse cavity linewidth. The control laser pulse with time-dependent Rabi frequency induces light shifts on the atomic levels. Thereby, a phase chirp is imprinted onto the generated photon which reduces the photon generation fidelity if no chirp-compensation is applied. As an alternative to chirp compensation via active phase modulation of the control laser beam, I present a mechanism for passive chirp compensation based on a bichromatic driving field. This scheme makes use of two optical frequency components that mutually cancel each others light shifts. Implementing the latter requires only a single amplitude modulator and no additional phase modulator.
Theoretical work mostly describes photon generation by means of an atomic Λ-system with one electronic excited state. In real atoms there are, however, additional excited states present. The off-resonant coupling to several excited states can cause a destructive interference which reduces the photon emission efficiency for certain parameter choices. I presented a model of photon generation that takes into account a second degenerate cavity mode with orthogonal polarization and includes off-resonant couplings to multiple excited states. In this case, the off-resonant couplings can be exploited to tune the branching ratio of photon emission into the two degenerate orthogonal polarization modes of the cavity. Thereby, the photon emission can be guided into a balanced polarization superposition state. This provides means of maximizing the Bell state projection probability of entanglement distribution schemes.
}, Author = {Ammenwerth, M.}, Journal = {}, Pages = {}, Title = {{Raman Imaging of Small Atomic Ensembles Inside an Optical Cavity}}, Volume = {}, Year = {2020} }