Title:Quantum phase transition in the anisotropic Rabi model
Speaker: Stefano Chesi
Time：25th March 2019, 10:30am
Location：818 Conference Room, Communication Building
The quantum Rabi model (QRM) describes a two-level system interacting with a bosonic mode and is one of the simplest models of light-matter interactions. If the coupling strength is small, the QRM can be approximated by the even simpler Jaynes-Cummings (JC) model, where counter-rotating terms are neglected. Although these models have only few degrees of freedom, recent work has shown that a suitable limit of both the QRM  and JC model  displays a quantum phase transition similar to Dicke superradiance. In this talk we extend these results to the anisotropic QRM, where rotating- and counter-rotating terms have different weights . This question is important since typical realizations of strong light-matter interactions (e.g., in circuit QED) have quite naturally a finite anisotropy. Furthermore, the quantum phase transition has different universal properties in the known JC and QRM limits. Through a combination of analytic and numerical approaches we compute phase diagram, scaling functions, and critical exponents, which allow us to establish that the universality class at finite anisotropy is the same of the QRM. We also find other interesting features, like a superradiance-induced freezing of the effective mass and discontinuous scaling functions in the JC limit. Finally, by extending our study to a finite number of two-level systems, we demonstrate the full equivalence between the few-body quantum phase transition and regular Dicke superradiance occurring in the thermodynamic limit .
 M.-J. Hwang, R. Puebla, and M. B. Plenio, Phys. Rev. Lett. 115, 180404 (2015).
 M.-J. Hwang and M. B. Plenio, Phys. Rev. Lett. 117, 123602 (2016).
 M. Liu, S. Chesi, Z.-J. Ying, X. Chen, H.-G. Luo, and H.-Q. Lin, Phys. Rev. Lett. 119, 220601 (2017).
Stefano Chesi received his Ph.D. (2007) from Purdue University. He subsequently held postdoctoral positions in the University of Basel (2007-2011) and McGill University (2011-2013). In 2013 he became Research Scientist in RIKEN, Japan. In 2014, he joined Beijing CSRC as an Assistant Professor.
Theoretical Condensed Matter Physics
Coherent spin phenomena in nanostructures
Quantum Information based on semiconductor systems
Electron correlations in the solid-state