Quantum non-linearities of a qubit ultra-strongly coupled to a waveguide

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, thus turning into a highly intricate light-matter object.

In the main part of this talk, I investigate theoretically the scattering of low-power coherent signals on this complex system, a common experimental protocol performed routinely in laboratories. 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. By using the second-order correlation function in the scattered signal, I will then present certain signatures of this phenomenon that can be expected in experiments.

In the final part of this talk, I will present an ongoing experimental attempt to measure the second-order correlation function, this time in the moderate regime of ‘strong’ coupling. I will explain the strategy implemented to perform this observation, the problems that have been encountered, and the new tools recently developed that are our disposal to overcome these difficulties.