Engineering quantum electronic chips: realization and characterization of circuit quantum electrodynamics systems

Circuit quantum electrodynamics (circuit QED) studies the strong coupling of microwave photons stored in a transmission line cavity to superconducting qubits on the level of single quanta and realizes coupling strengths which can hardly be achieved in atomic cavity QED systems. Within the scope of this thesis, microand nano-fabrication processes for the realization of circuit QED systems have been successfully realized and optimized in the ETH clean room facility FIRST. These achievements allowed to perform a number of new circuit QED experiments. The circuit QED systems discussed here consist of aluminum/aluminum-oxide Josephson junction qubits integrated into superconducting coplanar waveguide resonators. The physical properties of high quality resonators, made by using optical lithography and reactive ion etching of niobium or evaporation of aluminum thin films were analyzed in detail. In particular, resonators with fundamental frequencies from 2 to 9 GHz and quality factors ranging from a few hundreds to several hundred thousands controlled by appropriately designed input and output coupling capacitors were realized, their microwave transmission spectra were measured at temperatures of 20 mK and well described with theoretical lumped element and distributed element transmission matrix models. The ability to realize superconducting qubits with precisely controlled properties like Josephson energy and charging energy is essential for defining a certain qubit energy level structure. In particular, the maximum qubit transition frequency, the level of anharmonicity, the charge dispersion and the coupling strength between qubit and cavity must be controlled in order to perform a specific circuit QED experiment. This in turn requires a high level of control over the applied fabrication procedures. Josephson junctions were fabricated by using electron beam lithography and shadow evaporation. Fabrication processes were set up and process parameters like resist exposure dose and junction oxidation time were calibrated. Electrical and material properties of aluminum/aluminum-oxide Josephson junctions like the room temperature tunnel resistance, the current-voltage characteristic at milli-Kelvin temperatures or the resistance alteration with time due to post-oxidation processes (junction aging) were studied. The development of tunable coplanar waveguide resonators with integrated direct current superconducting quantum interference devices (DC-SQUIDs) with which the resonator frequency can be varied by an external magnetic flux threading the SQUID loop has been started and transmon type qubits have been designed and fabricated for performing circuit QED experiments. The ability to realize circuit QED systems with controlled parameters has been demonstrated by fabricating and characterizing circuit QED devices with one or several transmon type qubits integrated into a transmission line cavity. To realize qubit-qubit coupling in quantum computer architectures, the virtual photon exchange between two qubits in a transmission line cavity has been studied in dependence on the detuning of both qubits from the cavity resonance. The qubit-qubit interaction strength is explained by taking into account higher order cavity modes. Furthermore, experiments were enabled where the Jaynes-Cummings energy ladder, the Lamb shift, the Mollow triplet and the Autler-Townes doublet were spectroscopically observed in a circuit QED system and where two-qubit entangled states were prepared and characterized using quantum state tomography.