Collective effects with interacting Superconducting Qubits in waveguide QED

In this talk, I want to present the research activities of the Superconducting Quantum Circuits  group at the Institute for Quantum Optics and Quantum Information in Innsbruck. In the first part, I will introduce circuit quantum electrodynamics and the 3D  architecture. I will show how we are using this architecture to realize a platform for quantum  many body simulations. Our basic building blocks are 3D Transmon qubits where we use the  naturally occurring dipolar interactions to realize interacting spin systems. The ability to arrange  the qubits on essentially arbitrary geometries allows us to design spin models with more than  nearest-neighbor interaction in various geometries. 

Combining these ideas with our waveguide architecture, allows us to study open system  dynamics with interacting spin systems. The collective behavior of coupled qubits in a  waveguide leads to sub-radiant and super-radiant states. The collective dark states can be used  to generate an effective two- level system that has a 500 times longer coherence time than a  single qubit in the same physical configuration. We engineered a system that creates sub radiant and super-radiant states by using a direct dipole-dipole, as well as a waveguide mediated interaction. We show experimentally that this four-qubit dark-state can be used as a  qubit state even though it is embedded in an open system. Furthermore, we use the bright state  to read out the qubit encoded in the dark state.