Non-orthogonal qubit states expansion for the asymmetric quantum Rabi model

TL;DR

This paper introduces a variational wave function for the asymmetric quantum Rabi model's ground state, using non-orthogonal qubit states and squeezed coherent states, significantly improving accuracy over previous methods.

Contribution

The authors develop a new variational expansion involving non-orthogonal qubit states that accurately describes the ground state of the AQRM across various regimes.

Findings

Accurately describes the ground state in most parameter regimes.

Outperforms existing approximations.

Provides reliable calculations of physical observables.

Abstract

We present a physically motivated variational wave function for the ground state of the asymmetric quantum Rabi model (AQRM). The wave function is a weighted superposition of squeezed coherent states entangled with non-orthogonal qubit states, and relies only on three variational parameters in the regimes of interest where the squeezing effect becomes negligible. The variational expansion describes the ground state remarkably well in almost all parameter regimes, especially with arbitrary bias. We use the variational result to calculate various relevant physical observables of the ground state, and make a comparison with existing approximations and the exact solution. The results show that the variational expansion is a significant improvement over the existing approximations for the AQRM.