Kondo induced π-phase shift of microwave photons in a circuit quantum electrodynamics architecture

The Kondo effect refers to the screening of a spin-1/2 impurity by a cloud of conduction electrons, then forming a many-body Fermi liquid ground state. Theoretical calculations suggest that the Kondo resonance can interact with light and should give rise to a π-phase shift of the photon signal in the case where the ground state is a Fermi liquid. This π-phase shift of light is driven from the Korringa-Shiba relation of quantum impurity Fermi-liquid ground states. We report the first observation of such a π-phase shift in a graphene double quantum dot within a circuit quantum electrodynamics architecture where the microwave photons couple to the pseudospin or charge degrees of freedom. We study the evolution of the π-phase shift as a function of temperature and bias voltage. The observed Kondo temperature TK∼550mK is in agreement with dc conductance measurements. All our results support the formation of a Kondo resonance located above the Fermi level of the electronic reservoirs with the occurrence of an SU(4) Fermi-liquid ground state. We finally study how the Kondo-photon interaction can be tuned by inter-dot electron tunnel coupling strengths. Our findings may contribute to a better understanding of many-body physics in hybrid circuit systems, and open up applications in atomic thin materials from the light-matter interaction.