Fast-switching coupler for measuring the interaction dynamics of a system formed of a flux qubit coupled to a waveguide in the ultra-strong coupling regime

The ultra-strong coupling (USC) regime of light-matter interactions, realized when the interaction rate approaches the atomic frequency, is to this day relatively unexplored experimentally. In particular, studying the time dynamics of coupled atom-field systems in the USC regime is challenging due to stringent requirements related to the short interaction times. Previously, we demonstrated a tunable coupler that controls the interaction strength between a superconducting flux qubit and a co-planar waveguide transmission line. In this talk, we discuss our theoretical and experimental work towards observation of time-domain dynamics in the USC regime with this device.

We present results of a theoretical analysis of the system, that includes the role of higher levels of the qubit circuit and a realistic model for qubit measurement. Next, we present our experimental work in progress. In our design, using control pulses sent through bias lines near the coupler, the coupling strength can be fast-tuned at sub nanosecond time scales. Measurements can be done by decoupling the qubit from the waveguide, followed by state readout using a coupled DC-SQUID. This work opens new avenues for experiments on dynamics in the spin-boson model at strong coupling and entanglement harvesting.