Superinductor-based ultrastrong coupling in a superconducting circuit

We present an ultrastrong superinductor-based coupling consisting of a flux qubit galvanically coupled to a resonator. The coupling inductor is fabricated in granular Aluminum, a superinductor material able to provide large surface inductances. Spectroscopy measurements on the qubit-resonator system reveal a Bloch-Siegert shift of {23}{} and a coupling fraction of g/ωᵣ 0.13, entering the perturbative ultrastrong coupling (USC) regime. We estimate the inductance of the coupler independently by low-temperature resistance measurements providing Lc = (0.74±0.14)\,nH, which is compatible with g/ω 0.1. Our results show that superinductors are a promising tool to study USC physics in high-coherence circuits using flux qubits with small loop areas and low persistent currents.