Quantum circuits: From ultra-strong light-matter coupling to ballistic transport in graphene

In this talk, I will present two results from our recent work with superconducting microwave circuits. In the first, I will tell you about our implementation of ultra-strong light-matter coupling using a compact vacuum-gap transmon qubit design. Motivated by a desire to analyze our data quantitatively, we were led to a new circuit-quantisation model for this ultra-strong regime with transmon qubits, with implications for understanding the problem of the convergence of the Lamb shift and the quantum (or non-quantum) nature of the Lamb shift itself.

In the second part of the talk, I will tell you about our recent work combining quantum circuits with ballistic graphene Josephson junctions. Using a unique technique that gives us simultaneous access to both the microwave and d.c. characteristics, we both analyze the potential of graphene devices for quantum microwave circuit applications, and use the microwave measurements to gain new insight into the current-phase relation of the induced mesoscopic proximity effect superconductivity.