Advances in Superconducting Circuit Quantum Electrodynamics

Abstract

The topics of this thesis are based on circuit quantum electrodynamics (cQED), a theoretical and experimental platform allowing the study of light--matter interaction. This platform is rich both in observable physical phenomena and future practical applications. A "circuit" in cQED may comprise various elements, with the two main types being electromagnetic quantum harmonic oscillators, or resonators, and superconducting Josephson quantum bits, qubits. Because of the relative ease to fabricate and control quantum circuits—especially when compared to the more traditional cavity quantum electrodynamics—cQED has quickly grown in popularity in research labs across the world and is regarded as one of the major contenders for quantum computing.

The advances referred to in the title of this thesis address three significant challenges to practical applications of cQED; they are relevant not only to quantum computing, but also to other applications, such as simulations of physical systems. The first advance is related to control scalability. Practical applications require large circuits, and the current approaches used to send control signals to those circuits will not scale indefinitely. A solution to this challenge, the quantum socket, is presented and evaluated in depth. The second advance concerns calibration. Any application of cQED requires knowing the precise parameters defining the interactions between the various components of a circuit. Two cutting edge methods for the calibration of interaction parameters are explained and benchmarked; they show a remarkable improvement over existing, inefficient, methods. The third advance involves the physics of dielectric defects in the samples on which circuits are fabricated. These unwanted defects are modeled as two-level systems (TLS) that interact with circuit elements such as qubits. Experimental measurements and novel simulations conclusively demonstrate that interactions between TLS are responsible for the stochastic relaxation-time fluctuations observed in superconducting qubits.