Switchable ultrastrong coupling in circuit QED

Phys Rev Lett. 2010 Jul 9;105(2):023601. doi: 10.1103/PhysRevLett.105.023601. Epub 2010 Jul 6.

Abstract

We propose different designs of switchable coupling between a superconducting flux qubit and a microwave transmission line. They are based on two or more loops of Josephson junctions which are directly connected to a closed (cavity) or open transmission line. In both cases the circuit induces a coupling that can be modulated in strength, reaching the so-called ultrastrong coupling regime in which the coupling is comparable to the qubit and photon frequencies. Furthermore, we suggest a wide set of applications for the introduced architectures.