The vacuum-gap transmon qubit: ultra-strong light matter coupling and new insights into the physics of the Lamb Shift
Aalto Quantum Physics Seminar (Nanotalo) Speaker: Prof. Gary Steele (Delft University of Technology, The Netherlands)
In this talk, I will tell you about our implementation of ultra-strong light-matter coupling using a compact vacuum-gap transmon qubit design. Using a transmon qubit whose capacitance is massively dominated by the coupling to the cavity it hangs above, we observe a vacuum Rabi splitting of 1.2 GHz and achieve ratios of g/w = 0.19, well into the ultra-strong coupling regime to a multimode electromagnetic cavity. 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. Using the new insights from our theoretical formulation, we are albe to answer interesting questions such as how the multimode Lamb shift converges, and how “quantum” is the Lamb shift in circuit QED (and beyond!).