Towards the ultra-strong coupling limit of QED with high-impedance superconducting circuits
In quantum electrodynamics (QED), the interaction between atoms and photons is typically weak due to the smallness of the fine structure constant. This interaction is enhanced in superconducting circuit QED, because microwave fields are typically confined to small volumes and dipole moments of artificial atoms can be made large. Interestingly, even further increase of interaction is possible, when the characteristic impedance of the superconducting cavity approaches the value of the resistance quantum. This is equivalent to increasing the effective fine structure constant above unity. Such high-impedance quantum circuits are possible when either arrays of Josephson tunnel junctions or highly disordered superconducting films, such as TiN, are used as circuit elements. As a result, one can achieve the coupling strength comparable or even larger than the photon energy, the so-called ultra-strong coupling limit of QED. In this talks, we will present the current situation with experiments on high-impedance quantum circuits and discuss the interesting new questions that can be addressed in this new regime of quantum physics.