Dynamical polaron ansatz: a theoretical tool for the ultra-strong coupling regime of circuit QED

TL;DR

This paper introduces a semi-analytical variational ansatz called the dynamical polaron ansatz, enabling the study of ultra-strong coupling effects in circuit QED systems beyond the rotating wave approximation, with accurate static and dynamic predictions.

Contribution

The work presents a novel variational approach that accurately models ultra-strong coupling regimes in circuit QED, surpassing previous methods in both scope and precision.

Findings

Accurately predicts static and dynamic properties of superconducting qubits in strong coupling regimes.

Provides good qualitative descriptions beyond the Markovian approximation.

Shows quantitative agreement with Matrix-Product-State numerical results.

Abstract

In this work we develop a semi-analytical variational ansatz to study the properties of few photon excitations interacting with a collection of quantum emitters in regimes that go beyond the rotating wave approximation. This method can be used to approximate both the static and dynamical properties of a superconducting qubit in an open transmission line, including the spontaneous emission spectrum and the resonances in scattering experiments. The approximations are quantitatively accurate for rather strong couplings, as shown by a direct comparison to Matrix-Product-State numerical methods, and provide also a good qualitative description for stronger couplings well beyond the Markovian regime.