Waveguide QED in the Dipole Gauge

In recent studies on ultrastrong coupling between matter and light in cavities, the significance of gauge choice when employing the widely-used two-level approximation has been highlighted. Expanding upon these investigations, we extend the analysis to waveguide QED, where we demonstrate that truncations performed in the dipole gauge also yield accurate results. To illustrate this point, we consider the case of a dipole coupled to a cavity array. Various numerical and analytical techniques have been employed to investigate the low-energy dynamics of the system. Leveraging these theoretical tools, we argue that single photon scattering is an ideal method for investigating gauge-related issues. Our findings reveal two novel effects in the scattering spectra, which cannot be reproduced in a truncated model using the Coulomb gauge. Firstly, the primary resonance is modified due to a Lamb shift contribution. Secondly, we observe asymmetric transmission amplitudes surrounding this resonance, reflecting the asymmetry of the spectral density in this model. Additionally, we explore other features in the scattering spectra resulting from ultrastrong couplings, such as the emergence of Fano resonances and inelastic channels. Finally, we propose an experimental test of our ideas in the context of circuit QED.Comment: 15 pages, 10 figure

Ultrastrong waveguide QED with giant atoms

Quantum optics with giant emitters has shown a new route for the observation and manipulation of non-Markovian properties in waveguide QED. In this paper we extend the theory of giant atoms, hitherto restricted to the perturbative light-matter regime, to deal with the ultrastrong-coupling regime. Using static and dynamical polaron methods, we address the low-energy subspace of a giant atom coupled to an Ohmic waveguide beyond the standard rotating-wave approximation. We analyze the equilibrium properties of the system by computing the atomic frequency renormalization as a function of the coupling characterizing the localization-delocalization quantum phase transition for a giant atom. We show that virtual photons dressing the ground state are nonexponentially localized around the contact points but decay as a power law. The dynamics of an initially excited giant atom is studied, pointing out the effects of ultrastrong coupling on the Lamb shift and the spontaneous emission decay rate. Finally, we comment on the existence of the so-called oscillating bound states beyond the rotating-wave approximatio

Electrodinámica cuántica en guías de onda con emisores distantes

La descripción física de la interacción de la luz y la materia ha sido uno de los problemas más importantes desde los inicios de la mecánica cuántica. Debido al gran éxito de la teoría cuántica y de los avances experimentales en los últimos años, hoy en día es posible controlar y manipular una gran variedad de sistemas atómicos y fotónicos con gran precisión. En este trabajo estamos interesados en el estudio de sistemas de dos niveles (qubits) en interacción con fotones que se propagan en un medio unidimensional o guía de onda. En particular,hemos caracterizado la dinámica de un sistema de dos niveles acoplado a una cadena de cavidades estudiando la emisión Markoviana y no Markoviana. Hemos aplicado la teoría de la emisión Markoviana de dos átomos acoplados a una guía al caso de una cadena de cavidades.Hemos analizado el rol de la distancia entre átomos en la emisión espontánea, haciendo énfasis en fenómenos de interacción y decaimiento colectivo. Finalmente, hemos incluído la dinámica de los estados ligados originados por los bordes de banda de la guía y aquellos formados por efectos colectivos y de distancia. <br /

Ultrastrong waveguide QED with giant atoms

Resumen del trabajo presentado a la XXXVIII Reunión Bienal de la Real Sociedad Española de Física, celebrada en Murcia del 11 al 15 de julio de 2022.N