2 research outputs found
Coexistence of nonequilibrium density and equilibrium energy distribution of quasiparticles in a superconducting qubit
The density of quasiparticles typically observed in superconducting qubits
exceeds the value expected in equilibrium by many orders of magnitude. Can this
out-of-equilibrium quasiparticle density still possess an energy distribution
in equilibrium with the phonon bath? Here, we answer this question
affirmatively by measuring the thermal activation of charge-parity switching in
a transmon qubit with a difference in superconducting gap on the two sides of
the Josephson junction. We then demonstrate how the gap asymmetry of the device
can be exploited to manipulate its parity.Comment: Updated acknowledgements, corrected typo
Distinguishing parity-switching mechanisms in a superconducting qubit
Single-charge tunneling is a decoherence mechanism affecting superconducting
qubits, yet the origin of excess quasiparticle excitations (QPs) responsible
for this tunneling in superconducting devices is not fully understood. We
measure the flux dependence of charge-parity (or simply, ``parity'') switching
in an offset-charge-sensitive transmon qubit to identify the contributions of
photon-assisted parity switching and QP generation to the overall
parity-switching rate. The parity-switching rate exhibits a
qubit-state-dependent peak in the flux dependence, indicating a cold
distribution of excess QPs which are predominantly trapped in the low-gap film
of the device. Moreover, we find that the photon-assisted process contributes
significantly to both parity switching and the generation of excess QPs by
fitting to a model that self-consistently incorporates photon-assisted parity
switching as well as inter-film QP dynamics