Quantum Optics Seminar by Peter Rabl

Ultrastrong coupling phenomena beyond the Dicke model

ABSTRACT In this talk I will discuss effective light-matter interactions in circuit QED systems in the so-called ultrastrong coupling regime, i.e., in the regime where the coupling between the cavity mode and the superconducting qubits is the dominant energy scale. Starting from a minimal circuit model, I will show that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian of such a circuit necessarily contains direct qubit-qubit interactions, which have a drastic effect on the ground and excited state properties. Most surprisingly, in contrast to a superradiant phase transition expected from the standard Dicke model, an opposite mechanism occurs, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve some of the previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two- or multi-atom cavity QED settings can differ significantly from what is commonly assumed.