Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Applied Physics
First Advisor
Devoret, Michel
Abstract
Nonequilibrium quasiparticle excitations (QPs) are a significant loss mechanism inherentto superconducting devices. There are two distinct mechanisms by which transmon qubits couple to QPs. First, when QPs tunnel across the Josephson junction (JJ) of a transmon, they couple to the phase difference across the junction and may cause decoherence of the quantum state. Second, when QPs are generated by a high‑energy photon absorbed at the JJ, this process likewise may cause a qubit transition. Both of these mechanisms result in a transfer of a single charge across the JJ, causing a switch in the charge parity of the qubit. While these mechanisms share an experimental signature of a charge‑parity switch, the effectiveness of strategies to suppress charge‑parity‑switching decoherence depend on which mechanism is responsible. How these charge‑parity‑switching mechanisms may be experimentally distinguished, and subsequently suppressed, is the question answered by this thesis. In this dissertation, we present these distinct charge‑parity‑switching mechanisms, re‑ ferred to as NUmber‑conserving Parity Switching (NUPS, tunneling of pre‑existing QPs) and Photon‑Assisted Parity Switching (PAPS, generation of QPs with photon absorption at the JJ), and demonstrate their impact on transmon qubits. In doing so, we highlight the influence of a difference in the superconducting energy gaps of the aluminum films of the transmon on QP tunneling. By tuning the qubit energy relative to this gap difference, we elucidate the contributions of PAPS to charge‑parity switching and QP generation. Having determined that both charge‑parity‑switching mechanisms are occurring in the qubit, we then demonstrate how both can be suppressed. PAPS is suppressed by improved shielding and filtering, making it possible to measure a charge‑parity‑switching rate dom‑ inated by NUPS. A novel experimental protocol for extracting the qubit‑state dependence of the charge‑parity‑switching rate is then used to demonstrate that QP relax to a cold, thermalized distribution despite their highly non‑equilibrium density. As a result, NUPS is suppressed by the gap difference of the aluminum films. We demonstrate control over this suppression by engineering the thickness of the aluminum films forming our qubits.
Recommended Citation
Diamond, Spencer, "Quasiparticles and Charge-Parity Switching in Transmon Qubits" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1426.
https://elischolar.library.yale.edu/gsas_dissertations/1426
