Particle production in ultrastrong-coupling waveguide QED

In recent years, the field of light-matter interaction has made a further stride forward with the advent of superconducting qubits ultra-strongly coupled to open waveguides. In this setting, the qubit becomes simultaneously coupled to many different modes of the waveguide, leading to a wealth of non-linear dynamical phenomena.

First, I will show how one can tackle the time-evolution of such a non-trivial system using a novel numerical technique based on an expansion of the full state vector in terms of multi-mode coherent states. Inspired by earlier semi-classical approaches, this numerically exact method provides an important advance compared to the state-of-the-art techniques that have been used so far to study the many-mode ultra-strong coupling regime.

I will then move on to present the central prediction of my work concerning the scattering of low-power coherent signals on a qubit. Most remarkably, I will show that the qubit non-linearity, transferred to the waveguide through the ultra-strong light-matter interaction, is able to split photons from the incoming beam into several lower-energy photons. This splitting leads to the emergence of a low-frequency continuum in the scattered power spectrum that dominates the inelastic signal.