| Energy dissipation in phase qubits leads to short relaxation times and is a primary obstacle to their further development. Identifying the causes of this energy loss, then, is imperative in order to design a more coherent qubit. In order to quantify energy loss in the Josephson junction itself, we have developed a Tunable Josephson Resonator (TJR).;The TJR consists of a single Josephson junction in a superconducting loop with a shunt capacitor. The Josephson inductance, and hence the resonant frequency of the device can be tuned by applying a DC magnetic flux bias to the superconducting loop. The device typically has a maximum resonant frequency of several gigahertz. Because of the nonlinear element of the Josephson inductance, the resonance is non-linear for high drive amplitudes. For low drive amplitudes, however, the resonance is linear and the circuit can be analyzed using simple techniques. In the linear regime, the Quality factor of the device can be easily measured. By measuring devices with and without junctions, we show that the Josephson junction is the dominant source of energy dissipation in the circuit.;Because the dominant source of energy dissipation is the Josephson junction, the Quality factor of the device can be improved by increasing the shunt capacitance. We use this improved TJR as a tunable cavity for circuit-QED experiments. The tunable cavity is capacitively coupled to two phase qubits. The cavity can act as a tunable coupler mediating the interaction between two qubits which are on resonance with each other. It can also act as a bus, shuttling a state between two qubits tuned to different frequencies.;This thesis describes all aspects of the experiments, including theory, device fabrication, experimental setup, measurement techniques, and results. |
