Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime

TL;DR

This paper theoretically explores the quantum vacuum properties of a chain of superconducting Josephson atoms coupled to a resonator, revealing a quantum phase transition with vacuum degeneracy in the ultrastrong coupling regime.

Contribution

It derives the Hamiltonian for the system and analytically characterizes the vacuum degeneracy and its protection in the ultrastrong coupling regime.

Findings

Quantum phase transition with degenerate vacuum occurs above a critical coupling.

Degeneracy is exponentially lifted with finite size, decreasing with increasing coupling.

Vacuum degeneracy is protected against fluctuations in the ultrastrong coupling regime.

Abstract

We investigate theoretically the quantum vacuum properties of a chain of $N$ superconducting Josephson atoms inductively coupled to a transmission line resonator. We derive the quantum field Hamiltonian for such circuit-QED system, showing that, due to the type and strength of the interaction, a quantum phase transition can occur with a twice degenerate quantum vacuum above a critical coupling. In the finite-size case, the degeneracy is lifted, with an energy splitting decreasing exponentially with increasing values of $g^2 N^2$, where $g$ is the dimensionless vacuum Rabi coupling per artificial atom. We determine analytically the ultrastrong coupling asymptotic expression of the two degenerate vacua for an arbitrary number of artificial atoms and of resonator modes. In the ultrastrong coupling regime the degeneracy is protected with respect to random fluctuations of the transition energies of the Josephson elements.