New paper on Arxiv on ultrastrong coupling in a qubit-waveguide system

A new paper is now on the arxiv

Ultrastrong coupling of a single artificial atom to an electromagnetic continuum

Pol Forn-Diaz et al. have observed signatures of ultrastrong coupling between a superconducting flux qubit and the electromagnetic continuum of a one-dimensional waveguide, a transmission line. This work opens the door to study a new regime of quantum optics of single emitters in free space.

The figure below shows the evolution of the qubit spectrum as coupling is increased, with the linewidth growing beyond the qubit frequency, a signature of ultrastrong coupling.