This seminar has been financially supported by the European Union-NextGenerationEU through the "Quantum Busses for Coherent Energy Transfer (QUBERT)" project, in the framework of Curiosity Driven 2021 of the University of Genova.

ABSTRACT

Quadratic light-matter interactions are nonlinear couplings such that quantum emitters interact with photonic modes exclusively via the exchange of photon pairs. These two-photon interactions have mostly been considered as second- or higherorder effects, and so limited to extremely small coupling strengths. However, in the last few years it has been shown that a variety of novel physical phenomena emerges when the quadratic-coupling strength becomes comparable to dissipation rates (strong coupling) or to the system bare frequencies (ultrastrong coupling). In this talk, we will first discuss how strong or even ultrastrong quadratic interactions could be implemented with current atomic[1] or solid-state[2,3] quantum technologies. Then, we will present recent theoretical analyses of the optical properties of quantum emitters quadratically coupled either to single-mode quantum resonators[4] (cavity-QED), or to a continuum of propagating modes[5] (waveguideQED). We will focus on fundamental differences with respect to conventional lightmatter interactions, and we will discuss possible applications in quantum information tasks. As a striking example, we show that a single quadratically-coupled emitter can implement a two-photon logic gate with unit fidelity[5], circumventing a no-go theorem derived for conventional waveguide-QED interactions. 

[1] Spectral collapse via two-phonon interactions in trapped ions, Phys. Rev. A 92, 033817 (2015). 

[2] Two-photon quantum Rabi model with superconducting circuits, Phys. Rev. A 97, 013851 (2018). 

[3] Ultrastrong coupling regime of non-dipolar light-matter interactions, Phys. Rev. A 98, 053859 (2018). 

[4] Two-photon-interaction effects in the bad-cavity limit, Phys. Rev. A 105, L011702 (2022). 

[5] Waveguide QED with Quadratic Light-Matter Interactions, PRX Quantum 4, 030326 (2023).  

SHORT BIO

Simone Felicetti is a theoretical physicist working in the field of Quantum Science and Technology. His research activity follows two main research axes: the study of quantum light-matter interactions and the development of quantum-information protocols, both from a fundamental and an applied perspective. B.Sc. and M.Sc. degrees at Sapienza University (Rome), PhD degree at the University of the Basque Country (Bilbao). Two postdoctoral experiences, at Laboratoire MPQ (Paris) and at the IFIMAC center, UAM University (Madrid). Tenured researcher at CNR-IFN (Milan), then Senior Researcher at CNR-ISC (Rome).