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

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 I, we introduce the tunable coupler design and discuss the characterization of the device including the coupling strength α.