BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:1 UID:UW-Physics-Event-8182 DTSTART:20230221T180000Z DTEND:20230221T193000Z DTSTAMP:20260414T121419Z LAST-MODIFIED:20230220T025156Z LOCATION:https://uwmadison.zoom.us/j/5205327844 SUMMARY:Photon Assisted Quasiparticle Poisoning and Single Flux Quantu m-Based Digital Control of Superconducting Qubits\, Thesis Defense\, C huanhong Liu\, Physics Graduate Student DESCRIPTION:The ideal superconductor provides a pristine environment f or the delicate states of a quantum computer: because there is an ener gy gap to excitations\, there are no spurious modes with which the qub its can interact\, causing irreversible decay of the quantum state. As a practical matter\, however\, there exists a high density of excitat ions out of the superconducting ground state even at ultralow temperat ure\; these are known as quasiparticles. Observed quasiparticle densit ies are of order 1~$\\mu$m$^{-3}$\, tens of orders of magnitude greate r than the equilibrium density expected from theory. Nonequilibrium qu asiparticles extract energy from the qubit mode and can induce dephasi ng. Here we show that a dominant mechanism for quasiparticle poisoning is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qub it circuits with millimeter-wave radiation\, and we use an interferome tric quantum gate sequence to reconstruct the charge parity of the qub it. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation\, providing an efficient path fo r photons to generate quasiparticles. A deep understanding of this phy sics will pave the way to realization of next-generation superconducti ng qubits that are robust against quasiparticle poisoning. The single flux quantum (SFQ) digital superconducting logic family has been propo sed for the scalable control of next-generation superconducting qubit arrays. In the initial implementation\, SFQ-based gate fidelity was li mited by quasiparticle (QP) poisoning induced by the dissipative on-ch ip SFQ driver circuit. In this work\, we introduce a multi-chip module architecture to suppress phonon-mediated QP poisoning. Here\, the SFQ elements and qubits are fabricated on separate chips that are joined with In bump bonds. We use interleaved randomized benchmarking to char acterize the fidelity of SFQ-based gates\, and we demonstrate an error per Clifford gate of 1.2(1)%\, an order-of-magnitude reduction over t he gate error achieved in the initial realization of SFQ-based qubit c ontrol. We use purity benchmarking to quantify the contribution of inc oherent error at 0.96(2)%\; we attribute this error to photon-mediated QP poisoning mediated by the resonant mm-wave antenna modes of the qu bit and SFQ-qubit coupler. We anticipate that a straightforward redesi gn of the SFQ driver circuit to limit the bandwidth of the SFQ pulses will eliminate this source of infidelity\, allowing SFQ-based gates wi th fidelity approaching theoretical limits\, namely 99.9% for resonant sequences and 99.99% for more complex pulse sequences involving varia ble pulse-to-pulse separation. URL:https://www.physics.wisc.edu/events/?id=8182 END:VEVENT END:VCALENDAR