PhD Defense - Spencer Diamond | Yale Quantum Institute

Event time:

Thursday, November 2, 2023 - 9:30am to 10:30am

Audience:

YQI Researchers

Yale Community

General Public

Location:

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.