Experimental demonstration of the treatment of time-dependent flux in circuit QED devices

The choice of allocation of external flux to inductive elements in circuit quantum-electrodynamics was long thought to be arbitrary, until recent theoretical work emphasized that this is not the case [1]. Without careful consideration there often arises an additional term in the time-dependent Hamiltonian proportional to $dPhi/dt$. Here we present an experimental demonstration of this theory using a fluxonium qubit and investigating the effects of fast flux ramps. Our data is consistent with theory which includes the additional time derivative term, validating the treatment of time-dependent flux in circuit quantization presented in [1].

[1] X. You, et. al, Phys. Rev. B (2019).