000124362 001__ 124362 000124362 005__ 20240319081028.0 000124362 0247_ $$2doi$$a10.1103/PhysRevA.106.063717 000124362 0248_ $$2sideral$$a132777 000124362 037__ $$aART-2022-132777 000124362 041__ $$aeng 000124362 100__ $$0(orcid)0000-0002-6123-8165$$aTerradas-Briansó, Sergi 000124362 245__ $$aUltrastrong waveguide QED with giant atoms 000124362 260__ $$c2022 000124362 5060_ $$aAccess copy available to the general public$$fUnrestricted 000124362 5203_ $$aQuantum optics with giant emitters has shown a new route for the observation and manipulation of non-Markovian properties in waveguide QED. In this paper we extend the theory of giant atoms, hitherto restricted to the perturbative light-matter regime, to deal with the ultrastrong-coupling regime. Using static and dynamical polaron methods, we address the low-energy subspace of a giant atom coupled to an Ohmic waveguide beyond the standard rotating-wave approximation. We analyze the equilibrium properties of the system by computing the atomic frequency renormalization as a function of the coupling characterizing the localization-delocalization quantum phase transition for a giant atom. We show that virtual photons dressing the ground state are nonexponentially localized around the contact points but decay as a power law. The dynamics of an initially excited giant atom is studied, pointing out the effects of ultrastrong coupling on the Lamb shift and the spontaneous emission decay rate. Finally, we comment on the existence of the so-called oscillating bound states beyond the rotating-wave approximation 000124362 540__ $$9info:eu-repo/semantics/openAccess$$aby$$uhttp://creativecommons.org/licenses/by/3.0/es/ 000124362 590__ $$a2.9$$b2022 000124362 592__ $$a1.11$$b2022 000124362 591__ $$aPHYSICS, ATOMIC, MOLECULAR & CHEMICAL$$b12 / 35 = 0.343$$c2022$$dQ2$$eT2 000124362 593__ $$aAtomic and Molecular Physics, and Optics$$c2022$$dQ1 000124362 591__ $$aOPTICS$$b44 / 99 = 0.444$$c2022$$dQ2$$eT2 000124362 594__ $$a5.4$$b2022 000124362 655_4 $$ainfo:eu-repo/semantics/article$$vinfo:eu-repo/semantics/publishedVersion 000124362 700__ $$0(orcid)0000-0002-1734-1405$$aGonzález-Gutiérrez, Carlos A. 000124362 700__ $$aNori, Franco 000124362 700__ $$0(orcid)0000-0001-9273-8165$$aMartín-Moreno, Luis 000124362 700__ $$0(orcid)0000-0003-4478-1948$$aZueco, David 000124362 773__ $$g106 (2022), 063717 [13 pp.]$$pPhys. rev., A$$tPhysical Review A$$x2469-9926 000124362 8564_ $$s1715547$$uhttps://zaguan.unizar.es/record/124362/files/texto_completo.pdf$$yVersión publicada 000124362 8564_ $$s3074453$$uhttps://zaguan.unizar.es/record/124362/files/texto_completo.jpg?subformat=icon$$xicon$$yVersión publicada 000124362 909CO $$ooai:zaguan.unizar.es:124362$$particulos$$pdriver 000124362 951__ $$a2024-03-18-16:54:53 000124362 980__ $$aARTICLE