Quantentechnologie

High-bandwidth, fiber-based optical cavities are a promising building block for future quantum networks. They are used to resonantly couple stationary qubits such as single or multiple atoms with photons routing quantum information into a fiber network at high rates. In high-bandwidth cavities, standard fluorescence imaging on the atom-cavity resonance line for controlling atom positions is impaired since the Purcell effect strongly suppresses all-directional fluorescence. Here, we restore imaging of ⁸⁷Rb atoms strongly coupled to such a fiber Fabry-Pérot cavity by detecting the repumper fluorescence which is generated by continuous and three-dimensional Raman sideband cooling. We have carried out a detailed spectroscopic investigation of the repumper-induced differential light shifts affecting the Raman resonance, dependent on intensity and detuning. Our analysis identifies a compromise regime between imaging signal-to-noise ratio and survival rate, where physical insight into the role of dipole-force fluctuations in the heating dynamics of trapped atoms is gained.

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.

We demonstrate the storage of 5 ns light pulses in a single rubidium atom coupled to a fiber-based optical resonator. Our storage protocol addresses a regime beyond the conventional adiabatic limit and approaches the theoretical bandwidth limit. We extract the optimal control laser pulse properties from a numerical simulation of our system and measure storage efficiencies of (8.1±1.1)%, in close agreement with the maximum expected efficiency. Such well-controlled and high-bandwidth atom-photon interfaces are key components for future hybrid quantum networks.