Asymmetric polaron picture for the quantum Rabi model

TL;DR

This paper introduces an improved asymmetric polaron variational method for the quantum Rabi model, revealing richer physics of the quantum phase transition and enhancing understanding of light-matter interactions in ultra-strong coupling regimes.

Contribution

The authors develop an asymmetric polaron picture (APP) that improves accuracy and uncovers new physics in the quantum Rabi model's phase transition analysis.

Findings

Asymmetrically deformed polarons and antipolarons in the ground state lead to a richer phase diagram.

Reversal of asymmetry direction in polarons occurs in the first excited state.

Polaron asymmetry significantly enhances quantum resources in metrology and influences the quantum phase transition.

Abstract

The experimental access to ultra-strong couplings in light-matter interactions has made the quantum phase transition (QPT) in the quantum Rabi model practically relevant, while the physics of the QPT has not yet been fully explored. The polaron picture is a method capable of analyzing in the entire coupling regime and extracting the essential physics behind the QPT. However, the asymmetric deformation of polarons is missing in the current polaron picture. In the present work we propose an improved variational method in asymmetric polaron picture (APP). Our APP not only increases the method accuracy but also reveals more underlying physics concerning the QPT. We find that in the ground state both the polarons and antipolarons are asymmetrically deformed to a large extent, which leads to a richer phase diagram. We also analyze the first excited state in which we unveil an asymmetry direction reversal for the polarons and an attraction/replusion transition differently from the ground state. Finally, we apply the APP in quantum Fisher information analysis and critical coupling extraction, the improvements indicate that the polaron asymmetry makes a considerable contribution to the quantum resource in quantum metrology and plays an unnegligible role in the QPT. Our results and mechanism clarifications expose more subtle energy competitions and abundant physics, and the method potentially might have broader applications in light-matter interactions.