RQC Seminar

202nd RQC Seminar

• Speaker

  Prof. Salvatore Savasta
  ( University of Messina, Italy )

• Date

  16:00-17:00 (4:00 p.m. - 5:00 p.m.), May 28, 2025 (Wednesday)

• Venue

  Hybrid(ZOOM・ Wako Main Research 3F 345-347 Seminar Room / 研究本館3階 セミナー室 (345-347) (C01))

• Title

  Low-Energy Effective Models in Cavity and Circuit
  Quantum Electrodynamics : Renormalization of the Quantum Rabi Model

• Inquiries

  norilab_rqc_assist[at]ml.riken.jp

Abstract
Effective low-energy models serve as foundational tools across diverse areas of physics, from describing many-body systems to characterizing light-matter interactions. By systematically eliminating terms that virtually excite particles into high-energy states through perturbative methods, they provide simplified yet accurate descriptions of low-energy physics while still incorporating the influence of high-energy states through renormalized coupling terms. This approach has proven to be remarkably successful in various contexts.
In the context of light-matter interactions, low-energy effective Hamiltonians serve as indispensable tools, particularly in cavity and circuit quantum electrodynamics. These models are typically derived by just truncating the matter Hilbert space to a few relevant states while considering only one or a few modes of the electromagnetic resonator. The quantum Rabi model (QRM) stands as the quintessential example, describing the interaction between a two-level quantum emitter and a single-mode resonator. Although these models effectively describe such systems, there are regimes where the two-level truncation fails to produce accurate, or even reliable results [1-4].
In the work presented here, we propose a different perspective to address these issues through a renormalization procedure of the QRM, which we name Renormalized QRM [5]. This procedure is inspired by the renormalization of the Hopfield model, necessary for the description of experimental data. While renormalizing models of interacting bosons is simple due to their harmonic nature, the renormalization of the QRM is not as straightforward and remains largely unexplored. Our approach incorporates the effective influence of higher atomic energy levels, providing a significantly more accurate representation of the system while still maintaining a two-level description. I also discuss possible generalizations beyond the QRM. This procedure opens new possibilities for precisely engineering and understanding cavity and circuit QED systems, which are highly sought-after, especially for quantum information processing.

[1] D. De Bernardis, P. Pilar, T. Jaako, S. De Liberato, and P. Rabl, Breakdown of gauge invariance in ultrastrong-coupling cavity QED, Phys. Rev. A 98, 053819 (2018)
[2] O. Di Stefano, A. Settineri, V. Macrì, L. Garziano, R. Stassi, S. Savasta, and F. Nori, Resolution of gauge ambiguities in ultrastrong-coupling cavity quantum electrodynamics, Nat. Phys. 15, 803 (2019)
[3] G. Arwas, V. E. Manucharyan, and C. Ciuti, Metrics and properties of optimal gauges in multimode cavity QED, Phys. Rev. A 108, 023714 (2023).
[4] A. Frisk Kockum, A. Miranowicz, S. De Liberato, S. Savasta, and F. Nori, Ultrastrong coupling between light and matter, Nature Reviews Physics 1, 19 (2019).
[5] D. Lamberto, A. Mercurio, O. Di Stefano, V. Savona, S. Savasta, Renormalization and Low-Energy Effective Models in Cavity and Circuit QED, arXiv:2503.01651 (2025).