Superinductor-based ultrastrong coupling in a superconducting circuit

We present an ultrastrong superinductor-based coupling mechanism in a circuit consisting of a flux qubit galvanically coupled to an LC resonator. The coupling inductor is fabricated with granular aluminum, a superinductor material able to provide large surface inductances. Despite the low persistent current exhibited by the qubit, Ip=11.6 nA, spectroscopy measurements reveal a Bloch–Siegert shift of 23 MHz and a coupling fraction of g/ωr≃0.13, entering the perturbative ultrastrong coupling regime. An independent estimate of the coupler inductance by low-temperature resistance measurements leads to Lc=(0.74±0.14) nH, which is compatible with g/ωr≳0.1. Our results show that superinductors are a promising resource to study ultrastrong coupling physics in high-coherence circuits using flux qubits with small loop areas and low persistent currents.