Interfacing cold atoms and superconductors

One of the major challenges of the last decade in physics has been the practical implementation of quantum computing. One very promising candidate for this task are processors using superconducting qubits. While superconducting circuits working in the quantum regime can process quantum information at high rates, they lack the ability to store quantum information on timescales which are longer than a few microseconds. Unfortunately, there is no single quantum system which fulfills all the criteria for a quantum computer. It is therefore very interesting to combine two different quantum systems in order to exploit their respective advantages. This thesis describes a path towards constructing a hybrid quantum system of ultracold atoms and superconducting microstructures. The ultimate goal of this is a hybrid system in which quantum information is processed by superconducting qubits, transferred using a superconducting coplanar microwave resonator and stored in an ensemble of cold rubidium-87 atoms. The experimental system used to pursue this goal combines a cold atom setup at room temperature with a helium flow cryostat, which is used to cool superconducting structures to a temperature of 4.2 K. Atoms are prepared and trapped in magnetic potentials created by currents in a superconducting microtrap. We study the influence of the Meissner effect on the magnetic field, which greatly perturbs the magnetic trapping potential when atoms are brought close to superconducting structures. It is demonstrated that lifetimes of atomic ensembles in the vicinity of superconductors are not limited by Johnson noise induced spin flips, as it would be the case for normal conductors. We trap atomic ensembles on a superconducting atom chip and subsequently transport them into the gap of a coplanar microwave resonator. The transport is greatly facilitated by screening currents in the resonator ground planes, which keep the magnetic flux inside the superconducting resonator constant. Using these screening currents, a magnetic trap based on persistent currents is created. We prepare atomic ensembles on the atom chip in a quantum superposition state using a two-photon radio-frequency and microwave transition. The coherence of this superposition is shown to be on the order of T2∝10 s, five orders of magnitude longer than the coherence time of superconducting circuits. It is demonstrated that long atomic coherence times can be achieved even in the presence of the nearby superconducting cavity, making atomic ensembles attractive as quantum memories in a hybrid quantum architecture. In a further experiment, we study the temporal evolution of electric fields close to a metallic surface, when atoms are repeatedly deposited on it. Atoms which are adsorbed at the surface give rise to an electrostatic field which impacts the energy of Rydberg atoms by the Stark effect. The energy of the Rydberg levels is observed using electromagnetically induced transparency. It is shown that these adsorbate fields can be detrimental in experiments which aim at coupling Rydberg atoms to coplanar resonators, as the Rydberg levels are shifted out of the cavity resonance.