Superradiant Phase Transitions and the Standard Description of Circuit QED

TL;DR

This paper demonstrates that the standard effective model for circuit QED inaccurately predicts superradiant phase transitions, and provides a generalized no-go theorem confirming their absence in realistic systems.

Contribution

It reveals the failure of the standard model in predicting superradiant transitions and extends the no-go theorem to multi-level artificial atoms in circuit QED.

Findings

Standard model predicts superradiant phase transition erroneously.

Full microscopic treatment confirms no superradiant transition occurs.

Generalized no-go theorem applies to multi-level artificial atoms.

Abstract

We investigate the equilibrium behaviour of a superconducting circuit QED system containing a large number of artificial atoms. It is shown that the currently accepted standard description of circuit QED via an effective model fails in an important aspect: it predicts the possibility of a superradiant quantum phase transition, even though a full microscopic treatment reveals that a no-go theorem for such phase transitions known from cavity QED applies to circuit QED systems as well. We generalize the no-go theorem to the case of (artificial) atoms with many energy levels and thus make it more applicable for realistic cavity or circuit QED systems.