Non-perturbative ultrastrong couplings in superconducting quantum circuits*

Superconducting quantum circuits offer an excellent opportunity to study light-matter interactions. Unlike other platforms, these circuits can be tuned and designed to reach couplings beyond the strong coupling regime (g/ω > 0.1). Under these conditions, the standard approximations do not hold and new physics arise. 

In this work we present an experimental study of the physics of the non-perturbative ultrastrong coupling (USC) regime. Our device consists of a superconducting flux qubit galvanically coupled to an LC lumped element resonator. A large coupling inductance is provided by a thin and narrow aluminum strip that connects the qubit loop and the LC oscillator. The inductance is linear and large enough to provide a coupling in the range of the non-perturbative USC regime. We confirm the large coupling by spectroscopy measurements of our device. In particular, we observe the large and distinctive anticrossings of the USC regime and the unconventional associated physics.