Superinductor-based non-perturbative ultrastrong coupling in a superconducting quantum circuit

Superconducting quantum circuits are a versatile platform to study light-matter interaction and the Quantum Rabi model. The freedom in qubit and resonator designs and coupling engineering allows to study regimes of interaction beyond the bare frequencies of the system, entering the so-called ultrastrong coupling (USC) regime (0.1 < g/ω_r < 1) [1].

Non-perturbative ultrastrong couplings (0.3 < g/ω_r < 1) between a flux qubit and a resonator have been typically achieved using shared Josephson junctions. Recently, granular aluminum has been proposed as an alternative to reach ultrastrong couplings while keeping small persistent currents, and small qubit perimeters, which is advantageous for qubit coherence [2]. In this work, we present measurements of a flux qubit galvanically coupled to an LC oscillator by a shared wire of granular aluminum. The coupling is estimated to fall in the non-perturbative USC regime with g/ω_r ~0.3 and a persistent current below 40nA. This new approach opens the door to new designs with novel superinductors and the possibility to reach high coherence in ultrastrongly coupled flux qubit-resonator devices.