Ground state of a resonant two-qubit Rabi model in the ultrastrong coupling regime

TL;DR

This paper investigates the ground state properties of a two-qubit Rabi model in the ultrastrong coupling regime, using variational, transformation, and perturbation methods, and analyzes the entanglement between qubits.

Contribution

It introduces approximate analytical methods to determine the ground state of a two-qubit Rabi model in the ultrastrong regime and compares their accuracy with exact numerical results.

Findings

Good agreement between approximate and exact ground states in the ultrastrong regime.

Ground state entanglement increases with coupling strength.

Perturbation theory can further improve approximation accuracy.

Abstract

We consider a generalized Rabi model formed by two identical qubits interacting with a common oscillator mode. In the near resonance configuration where the oscillator frequency is close to the transition frequency of the qubit, we determine the ground state of the model approximately by using a variational method and a transformation method. Both methods are shown to provide good agreement with the exact numerical ground state for a range of coupling strength in the ultrastrong regime. In addition, we indicate how the accuracy of the approximation can be further improved by using perturbation theory. We also examine the ground state entanglement between the two qubits. By calculating the negativity, we quantify the degree of entanglement as a function of coupling strength.