Tunable coupler for mediating interactions between a two-level system and a waveguide from a decoupled state to the ultra-strong coupling regime: part II

Coupling between a two-level system (TLS) and a waveguide is a fundamental paradigm of light-matter interactions. We introduce and experimentally demonstrate a tunable coupler between a TLS, implemented using a flux qubit, and the electromagnetic fields in a superconducting waveguide. The device uses on-chip flux biasing to tune the coupling strength from a decoupled state, to near the ultrastrong-coupling (USC) regime of light-matter interaction where the interaction rate approaches the gap frequency of the TLS. The normalized coupling strength, α, is measured to range from 6.2 × 10^-5 to 2.19 × 10^-2 and is predicted to have an even broader range by the fitted circuit model, creating an effective switch for the coupling. This wide range of coupling strengths presents a significant advantage over previous TLS-waveguide coupler designs for a range of applications. This work opens new research avenues in several areas including the development of photon sources, the investigation of the spin-boson model at strong coupling, and the experimental realization of entanglement harvesting. In part II, we discuss the theoretical modeling of the coupler and the decoherence sources affecting the qubit.