Ground state of three qubits coupled to a harmonic oscillator with ultrastrong coupling

TL;DR

This paper analytically investigates the ground state of a three-qubit Rabi model without the rotating-wave approximation, revealing entanglement properties and their dependence on coupling strength.

Contribution

It provides an analytical treatment of the ground state in a three-qubit Rabi model and explores entanglement behavior beyond previous approximations.

Findings

Analytical ground state matches numerical results across coupling strengths.

Pairwise entanglement depends quadratically on coupling strength.

No qubit-qubit entanglement at large coupling strengths.

Abstract

We study the Rabi model composed of three qubits coupled to a harmonic oscillator without involving the rotating-wave approximation. We show that the ground state of the three-qubit Rabi model can be analytically treated by using the transformation method, and the transformed ground state agrees well with the exactly numerical simulation under a wide range of qubit-oscillator coupling strengths for different detunings. We use the pairwise entanglement to characterize the ground-state entanglement between any two qubits and show that it has an approximately quadratic dependence on the qubit-oscillator coupling strength. Interestingly, we find that there is no qubit-qubit entanglement for the ground state if the qubit-oscillator coupling strength is large enough.