BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:1 UID:UW-Physics-Event-4563 DTSTART:20170928T150000Z DURATION:PT1H0M0S DTSTAMP:20260419T021557Z LAST-MODIFIED:20170918T132358Z LOCATION:5310 Chamberlin SUMMARY:Dynamical control techniques with superconducting qubits\, R. G. Herb Condensed Matter Seminar\, Dr. Simon Gustavsson \, MIT DESCRIPTION:Dynamical error suppression techniques are commonly used t o improve coherence in quantum systems. They reduce dephasing errors b y applying control pulses designed to reverse erroneous coherent evolu tion driven by environmental noise. However\, such methods cannot corr ect for irreversible processes such as energy relaxation (T1). In this work\, we investigate a complementary\, stochastic approach to reduci ng errors: instead of deterministically reversing the unwanted qubit e volution\, we use control pulses to shape the noise environment dynami cally. In the context of superconducting qubits\, we implement a pumpi ng sequence to reduce the number of unpaired electrons - quasiparticle s - in close proximity to the device. We report a 70% reduction in the quasiparticle density\, resulting in a threefold enhancement in qubit relaxation times\, and a comparable reduction in coherence variabilit y [1].
\nIn a separate experiment\, we investigate qubit dephasin g (T2) due to photon shot noise in a flux qubit transversally coupled to a coplanar microwave resonator. Due to the AC Stark effect\, photon fluctuations in the resonator cause frequency shifts of the qubit\, w hich in turn lead to dephasing. While this is universally understood\, we have made the first quantitative spectroscopy of this noise for bo th thermal (i.e.\, residual photons from higher temperature stages) an d coherent photons (residual photons from the readout and control puls es). By mapping out the noise power spectral density seen by the qubit \, we uniquely identify thermal shot noise as the dominant source of d ephasing. When implementing the CPMG dynamical-decoupling protocol\, w e are able mitigate to the adverse influence of the photon shot noise\ , and improve T2 Echo ~ 40 us to reach T2 CPMG ~ 80 us ~ 2*T1. Further more\, by improving the filtering for thermal noise in a subsequent co oldown\, we are able to reduce the residual photon population to 0.000 4\, resulting in T2 echo times approaching 100 us [2].
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\n[ 1] Science 354\, 1573 (2016)
\n[2] Nature Communications 7\, 12964 (2016)
\n URL:https://www.physics.wisc.edu/events/?id=4563 END:VEVENT END:VCALENDAR