Date of Award

5-11-2025

Date Published

June 2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor(s)

Britton Plourde

Keywords

correlated errors;phonons;quantum computation;quasiparticles;superconducting qubits

Subject Categories

Physical Sciences and Mathematics | Physics

Abstract

Quantum computers promise to solve currently intractable problems of great interest for a diverse range of applications. For a quantum computer to solve these problems, it must handle occasional errors with quantum error correction, which distributes the quantum information across many physical qubits, increasing resilience. A promising technology for implementing such a system uses superconducting circuits as physical qubits. Standard quantum error correction schemes can accommodate random, local errors, but errors that are correlated across multiple qubits are fatal. Correlated errors can arise from ionizing radiation impacting the device substrate, which creates free charge that can be sensed by the qubits and energetic phonons that travel throughout the chip. When theses phonons impinge on the device layer, they create dissipative excitations, called quasiparticles, in the superconducting qubit electrodes, which can poison many of the qubits simultaneously, leading to correlated errors. Correlated errors can also arise from another source that has no corresponding charge signature, so-called "phonon-only" events, which have been observed in other communities but never investigated in superconducting qubits previously. In this thesis, we explore both sources of correlated errors and use various mitigation strategies to inhibit correlated errors. We first describe the experiment observing phonon-only events in our superconducting qubit devices and characterize a decrease in the poisoning rates from these events during the cooldown. We believe the source of these events is the relaxation of thermal stresses between the substrate and metal films contracting differently upon cooling to cryogenic temperatures. In another experiment, we use a $\gamma$-ray source outside of the cryostat to controllably poison our qubits. We develop a new technique to characterize the poisoning footprint of devices with and without explicit mitigation strategies, which is vital knowledge for quantum error correction schemes based on identifying and managing poisoning events. The work presented in this thesis serves to inform future quantum processor designs that minimize correlated errors, forming a pathway towards fault-tolerant operation.

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Open Access

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