Exploring fluxon tunneling dynamics in the IST qubit*

In a heavy fluxonium, the wave functions of fluxons become increasingly disjoint as the barrier height between the two potential wells is raised by increasing the Josephson energy. In the IST qubit limit [1], this suppression of fluxon transitions can lead to remarkable relaxation times measured to be on the order of several hours. In this work, we delve into the dynamics of fluxon tunneling and initialization studied in such an IST qubit. We induce inter-well transitions in the qubit by populating the cavity with a large number of photons. Our semi-classical model accurately replicates the tunneling dynamics, encompassing both photon-assisted and non-resonant tunneling between plasmon levels in the two fluxon wells. This model suggests that, at specific values of external flux through the rf-SQUID loop, direct transitions between the wells might become feasible. Furthermore, photon-assisted tunneling enables precise fluxon state preparation, allowing a deeper investigation into tunneling rates. In the future, fast tuning of the barrier, through fast flux control of the Josephson energy, could offer further insights into the tunneling dynamics while also enabling full qubit control of decay-protected fluxon states.

[1] Hassani, Farid, et al.  Nature Communications 14.1 (2023): 3968.