Hidden Single-Qubit Topological Phase Transition without Gap Closing in Anisotropic Light-Matter Interactions

TL;DR

This paper uncovers a hidden topological phase transition in a single-qubit light-matter system that occurs without the usual gap closing, revealing new unconventional transition mechanisms and their signatures.

Contribution

It demonstrates the existence of a gapless, hidden topological phase transition in the quantum Rabi model, expanding understanding of topological phenomena in few-body quantum systems.

Findings

Discovery of a hidden node-phase transition without gap closing or parity change.

Identification of hysteresis associated with the unconventional TPT.

Analysis of Wigner function signatures of the transition.

Abstract

Conventionally the occurrence of topological phase transitions (TPTs) requires gap closing, whereas there are also unconventional cases without need of gap closing. Although traditionally TPTs lie in many-body systems in condensed matter, both cases of TPTs may find analogs in few-body systems. Indeed, the ground-state node number provides a topological classification for single-qubit systems. While the no-node theorem of spinless systems is shown to also restrict the fundamental quantum Rabi model in light-matter interactions, it is demonstrated that the limitation of the no-node theorem can be broken not only in a small counter-rotating term (CRT) but also in the large-CRT regime, which striates a rich phase diagram with different TPTs. While these transitions are mostly accompanied with gap closing and parity reversal, a hidden node-phase transition is revealed that has neither gap closing nor parity change, which turns out to be an analog of the unconventional TPT in condensed matter. A hysteresis sign for the unconventional TPT is unveiled via the transition from amplitude squeezing to phase squeezing in the gapped phase. The imprints in the Wigner function are also addressed. The clarified mechanisms provide some special insights for the subtle role of the CRT.