RQC Seminar

167th RQC Seminar

• Speaker

  Prof. Stephen Hughes
  ( The Department of Physics, Engineering Physics ＆ Astronomy, Queen's University, Canada )

• Date

  4:00-5:00 p.m. (16:00-17:00), October 25, 2024 (Friday)

• Venue

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

• Title

  Reconciling quantum and classical spectral theories of ultrastrong coupling

• Inquiries

  rqc_info[at]ml.riken.jp

Abstract
Cavity quantum-electrodynamics (QED) is a rich area of optical physics, where extreme light–matter coupling can give rise to ultrastrong coupling. The ultrastrong coupling regime presents some fascinating and unique quantum mechanical effects, such as ground state virtual photons and vacuum squeezing. Recent papers predict that the spectral features allow one to quantify certain quantum attributes such as entanglement and entropy, yet these “numbers” have little meaning in ultrastrong coupling and are also gauge dependent, like most quantum theories in ultrastrong coupling.

Focusing on the widely adopted quantum Hopfield model with cavity dissipation, I will first show how the linear spectrum of an ultrastrongly coupled cavity and a dipole can be described either classically or quantum mechanically, but only when the quantum model includes: (i) corrections to maintain gauge invariance, and (ii) a specific type of cavity bath coupling, which has so far not been identified. I also show the impact of this cavity bath model on the quantum Rabi model, which has no classical analog in ultrastrong coupling (which will be explained). These results can be used to guide emerging experiments, and impact current models and interpretations of ultrastrong coupling between light and matter, including often ill-defined ones used, for example, to predict things like superradiant phase transitions.