Adam Sears

Circuit quantum electrodynamics (Circuit QED) is the extremely successful framework for studying quantum devices developed along with the transmon, a superconducting charge qubit with an insensitivity to several types of dephasing. It involves the description of superconducting qubits and harmonic oscillators as quantized circuits. This thesis describes the implementation of two experiments that reduce circuit QED to its simplest components. Both experiments utilize elements that are known to have low dissipation: excited electron spin defects in crystals may take seconds to decay at cryogenic temperatures and the Josephson junction in superconducting qubits is nearly lossless; we begin by discussing the proper perspective for the remaining lossy elements. In the first experiments, a collection of magnetic dipoles is coupled as an ensemble to a superconducting resonator to investigate their suitability as a quantum memory; in the second a transmon “artificial atom” is placed inside a three-dimensional superconducting box. We further extend the study of the “3D transmon” and the harmonic oscillator modes of its rectangular waveguide cavity in terms of a new description of their hybridization (Black-box Quantization). Finally, we identify and resolve issues of photon induced dephasing in the first new devices. This thesis follows the evolution of superconducting qubits from coherence times of several microseconds to nearly a millisecond.