Nonequilibrium and nonperturbative dynamics of ultrastrong coupling in open lines

TL;DR

This paper investigates the complex dynamics of a qubit coupled to 1D photons in the ultrastrong regime, revealing ground state excitations, equilibration behavior, and resonance properties through nonperturbative simulations and experimental feasibility.

Contribution

It provides a nonperturbative analysis of ultrastrong qubit-photon coupling dynamics using Matrix Product States, extending understanding beyond perturbative approaches.

Findings

Ground state contains excitations of qubit and field

Qubit equilibrates via spontaneous emission to a near-ground state

Resonances match adiabatic renormalization predictions

Abstract

We study the time and space resolved dynamics of a qubit with an Ohmic coupling to propagating 1D photons, from weak coupling to the ultrastrong coupling regime. A nonperturbative study based on Matrix Product States (MPS) shows the following results: (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions, is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalization [A. J. Leggett et al., Rev. Mod. Phys. 59, 1, (1987)]. Finally, a non-perturbative ab-initio calculations show that this physics can be studied using a flux qubit galvanically coupled to a superconducting transmission line.