Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Devoret, Michel

Abstract

Quantum bits built from microwave superconducting circuit components such as capacitors, inductors, and tunnel junctions are one of the leading platforms for realizing a fault-tolerant quantum computer. A roadblock towards this goal is the insufficient fidelity of operations -- single-qubit gates, two-qubit gates, and readout measurements -- performed on the quantum bits via microwave pulses. The main limitation of operation fidelity comes from decoherence. If the operation is performed too slowly compared to the decoherence time, a qubit error will happen. Usually, it is possible to speed up the operations by increasing the power of the microwave pulse. However, reducing the pulse duration and increasing its power was found to induce unwanted state transitions of the quantum bit, often to non-computational excited states. What is the origin of unwanted state transitions of a superconducting qubit caused by microwave pulses? Through a set of systematic experiments with transmon superconducting qubits as well as theoretical calculations, we classify all such transitions into three categories, according to their origin. First, the transitions may stem from activation of spurious resonances in the spectrum of the quantum bit. Second, the transitions may be caused by inelastic scattering of photons constituting the microwave pulse off the qubit and into the qubit environment. Finally, microwaves may accelerate the qubit decay through the AC Stark shift. The latter happens when the coupling of the qubit to its environment is stronger at the Stark-shifted frequency. The dissertation provides a comprehensive roadmap for suppressing microwave-induced state transitions, thus contributing to the improvement of the fidelity of superconducting qubit operations.

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