Stabilization of product states and excited-state quantum phase transitions in a coupled qubit-field system

TL;DR

This paper explores complex quantum phase transitions in a coupled qubit-field system, revealing how certain states can be stabilized dynamically in the ultrastrong coupling regime, with implications for quantum state control.

Contribution

It introduces an extended Rabi model with parity interactions, uncovering new quantum phase transitions and stabilization phenomena in excited states.

Findings

Identification of a dynamical stabilization of factorized qubit-field states

Discovery of quantum phase transitions in both ground and excited states

Analysis of the role of field mode softness in stabilization

Abstract

We study a system of a single qubit (or a few qubits) interacting with a soft-mode bosonic field. Considering an extended version of the Rabi model with both parity-conserving and parity-violating interactions, we disclose a complex arrangement of quantum phase transitions in the ground- and excited-state domains. An experimentally testable signature of some of these transitions is a dynamical stabilization of a fully factorized qubit-field state involving the field vacuum. It happens in the ultrastrong coupling regime where the superradiant field equilibrium is far from the vacuum state. The degree of stabilization varies abruptly with interaction parameters and increases with the softness of the field mode. We analyze semiclassical origins of these effects and show their connection to various forms of excited-state quantum phase transitions.