Non-linear light-matter interaction at ultra-strong coupling by Dr Daniel Braak

21 Oct 2025 02.00 PM - 03.00 PM MSE Meeting Room (N4.1-01-28) Alumni, Current Students

NTU MSE Seminar Hosted by Associate Professor Zhao Yang

Abstract

The interaction between a two-level system and a single quantized light mode can often be described faithfully with the quantum Rabi model (QRM) where the spin-flip operator couples linearly to the mode. There are generalizations of the QRM with non-linear [1] or additional Stark-like coupling [2] which exhibit qualitatively novel physics in the ultra-strong coupling regime, e.g. the "spectral collapse" phenomenon. They are associated with the appearance of a continuous spectrum without normalizable eigenstates and normalizable (bound) states embedded into the continuum. A numerical approach to these effects of very strong non-linear coupling is not feasible due to truncation of the bosonic state space. I shall give an overview of analytical methods applicable in these cases.

[1] D. Braak, "Spectral determinant of the two-photon quantum Rabi model", Ann. Phys. (Berlin) vol.535, p.202200519 (2023)
[2] D. Braak, L. Cong, H.-P. Eckle, H. Johannesson and E. K. Twyeffort, "Spectral continuum in the Rabi-Stark model", J. Opt. Soc. Am. B vol. 41, p.C97 (2024)

Biography
Dr Daniel Braak
Institute of Physics
Augsburg University

Daniel Braak is a Senior Researcher in the Institute of Physics at Augsburg University. He graduated from the Department of Physics at the University Bonn and obtained a Ph.D. in the Department of Physics at the Technical University Karlsruhe. Afterwards, he worked as post-doctoral fellow in the Department for Physics and Astronomy at Rutgers University. He is interested in exactly solvable models with bosonic degrees of freedom and the mathematical structure of integrable systems in quantum mechanics. 

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