Nonlinear dynamical Casimir effect and Unruh entanglement in waveguide QED with parametrically modulated coupling

TL;DR

This paper develops a comprehensive theoretical framework to analyze nonlinear dynamical Casimir and Unruh effects in waveguide QED with moving or modulated qubits, revealing photon generation, entanglement, and phase transitions.

Contribution

It introduces a novel combined perturbative and master-equation approach to study complex quantum nonlinearities and nonequilibrium states in waveguide QED systems with modulated coupling.

Findings

Directional dynamical Casimir effect with correlated photon pairs

Waveguide-mediated collective Unruh effect inducing entanglement and phase transitions

Back-action effects leading to hybrid phonon-biphoton modes

Abstract

We study theoretically an array of two-level qubits moving relative to a one-dimensional waveguide. This motion can be implemented mechanically or simulated via the modulation of the couplings between the qubits and the waveguide. When the frequency of this motion approaches twice the qubit resonance frequency, it induces parametric generation of photons and excitation of the qubits. The proposed quantum optomechanical system offers a plethora of possibilities for exploring various quantum electrodynamics phenomena. However, their theoretical analysis is challenging due to the presence of quantum nonlinearity, a continuum of propagating photonic modes, and the excitation of strongly nonequilibrium qubit states, which make many conventional analytical tools inapplicable. To address these challenges, we develop a comprehensive general theoretical framework that incorporates both perturbative diagrammatic techniques and a rigorous master-equation approach. Our calculations reveal several intriguing effects, including the directional dynamical Casimir effect, where momenta of emitted photon pairs are correlated, and the waveguide-mediated collective Unruh effect, where motion drives the qubits to a nontrivial steady state that can be entangled and exhibit phase transitions. Additionally, we examine the radiation back-action on the qubit motion, which becomes particularly pronounced when subradiant modes in the qubit array are excited. The back-action can significantly alter the mechanical spectra, potentially leading to the formation of hybrid phonon-biphoton modes.