Ultrastrong coupling phenomena beyond the Dicke model

TL;DR

This paper investigates ultrastrong light-matter interactions in circuit QED systems with multiple qubits, revealing that direct qubit-qubit interactions alter ground state properties and prevent superradiant phase transitions, contrasting standard models.

Contribution

It introduces a minimal circuit model showing that direct qubit-qubit interactions significantly modify the ultrastrong coupling regime in circuit QED, challenging previous assumptions about superradiant phases.

Findings

Direct qubit-qubit interactions are present in the Hamiltonian.

Strong interactions lead to decoupling of the photon mode.

The ground state becomes highly entangled, preventing superradiant phase transition.

Abstract

We study effective light-matter interactions in a circuit QED system consisting of a single $LC$ resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground and excited state properties of such circuits in the ultrastrong coupling regime. In contrast to a superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve 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.