PhD Defense - Spencer Diamond

Event time: 
Thursday, November 2, 2023 - 9:30am to 10:30am
YQI Researchers
Yale Community
General Public
YQI Seminar Room See map
Event description: 
Quasiparticles and charge parity switching in transmon qubits
Nonequilibrium quasiparticle excitations (QPs) are a significant loss mechanism inherent
to 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.

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