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DTSTAMP:20261022T111935Z
UID:6ad87b1d35c24118206910@ist.ac.at
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DESCRIPTION:Speaker: Adria Rospars\nhosted by Johannes Fink\nAbstract: To p
 reserve quantum information\, a qubit must be isolated from its environmen
 t. To be useful\, it must remain accessible to control and measurement. In
  this talk\, I will present three projects from my PhD at ENS de Lyon. Eac
 h addresses this tension for a different circuit.The transmon is a circuit
  whose lowest levels are charge-insensitive\, but whose readout is not: th
 e photon number at which it stops being quantum non-demolition depends on 
 offset charge\, and reported offset charges drift over minutes to hours. O
 n two devices\, we observed transmons whose offset charges were pinned at 
 zero for more than ten days and up to nearly three months. This stability 
 is fragile and disappeared in later cooldowns. I will show how materials c
 haracterization points to a thin superconducting layer that shunts the Jos
 ephson junction and pins the offset charge.The fluxonium shunts the juncti
 on deliberately\, with a superinductance\, which makes all its levels inse
 nsitive to offset charge. However\, fluxonium readout remains slower than 
 transmon readout\, even at lower target fidelity. Here\, I will present a 
 cross-resonance readout\, in which the fluxonium\, driven at the resonator
  frequency\, transmits the drive to the resonator with an amplitude that s
 cales with the dispersive shift of each state. With appropriate circuit pa
 rameters\, the resonator can then either stay close to vacuum for one stat
 e while being populated for the other\, even when the dispersive shift is 
 smaller than the resonator linewidth\, or be displaced in opposite directi
 ons for two states. I will show both regimes experimentally\, using the th
 ree lowest levels of the fluxonium.Finally\, I will present the xiqubit\, 
 a protected qubit in which only pairs of Cooper pairs can tunnel. It addre
 sses two obstacles faced by existing protected designs: sensitivity to jun
 ction asymmetry\, and the conflict between protection against relaxation a
 nd protection against dephasing. The first devices are fabricated and will
  be measured in the coming months.
LOCATION:Office Bldg West / Ground floor / Heinzel Seminar Room (I21.EG.101
 )\, ISTA
ORGANIZER:swiddman@ist.ac.at
SUMMARY:Adria Rospars: Improving superconducting quantum circuits: transmon
  charge stability\, fluxonium readout and a new protected qubit
URL:https://talks-calendar.ista.ac.at/events/6713
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