Direct observation and manipulation of non-equilibrium quasiparticle tunneling: Part 1/2

Tunneling of excess non-equilibrium quasiparticles is a source of decoherence in superconducting qubits. In transmons, identifying quasiparticle tunneling events is difficult since their experimental signature -- charge-parity switching -- can be mimicked by high-energy photons that break Cooper pairs. Here, we strongly suppress photon-induced pair breaking with radiation filtering and shielding, which allows us to directly probe the tunneling of the remaining excess quasiparticles. By measuring the temperature dependence of the quasiparticle tunneling rates in the ground and excited states of the transmon, we find evidence for the effective thermalization of the quasiparticles. A resulting asymmetry between the quasiparticle tunneling rates in the two qubit states allows us to transfer quasiparticles on-demand, thus actively manipulating the charge-parity of the transmon.

In the first part of this two-part talk, we will introduce the concept and setup of our experiment.