Ultrastrong coupling regime of nondipolar light-matter interactions

Introduction: In cavity quantum electrodynamics, the interaction between photons and atoms is often described within the dipolar approximation, leading to linear (single-photon) interaction terms. However, a variety of... [ view full abstract ]

Introduction: In cavity quantum electrodynamics, the interaction between photons and atoms is often described within the dipolar approximation, leading to linear (single-photon) interaction terms. However, a variety of novel physical phenomena emerges when two-photon interactions (TPIs) become dominant. In particular, a collapse of the discrete energy spectrum into a continuous band has been predicted in the ultrastrong coupling regime of TPIs[1]. In the many-body limit TPIs introduce a rich interplay between the spectral collapse and the superradiant phase transition[2].

These considerations prompted various efforts to design quantum-simulation schemes of TPI-models in different atomic systems[3]. However, the implementation of a genuine TPI requires an interaction more complex than dipolar. As recently shown, superconducting circuits are promising platforms for the implementation of such nondipolar interactions[4]. 

Methods: We design a circuit-QED scheme which allows to reach the ultrastrong coupling regime of a nondipolar interaction between an artificial atom and a quantum resonator. We show that the system Hamiltonian is well approximated by the two-photon quantum Rabi model and we derive an input-output analysis based on Floquet theory[5].

Results: We show that highly-nonlinear quantum processes such as the two-photon blockade effect are amplified when the coupling strength is pushed into the ultrastrong coupling regime, until a critical value is reached, for which the statistics of the output field is abruptly modified. Furthermore, we show that a growing number of resonances is observable in the output fluoresce spectrum in the proximity of the collapse point.

Discussion: Our results unveil novel fundamental quantum-optical phenomena that could be observed in simple scattering experiments, paving the way to the observation of the onset of the spectral collapse in a solid-state device. 

[1] I. Travenec, Phys. Rev. A 85,043805 (2012).

[2] L. Garbe, I. L. Egusquiza, E. Solano, C. Ciuti, T. Coudreau, P. Milman, S. Felicetti, Phys. Rev. A 95, 053854 (2017).

[3] S. Felicetti, J. S. Pedernales, I. L. Egusquiza, G. Romero, L. Lamata, D. Braak, and E.Solano,Phys. Rev. A 92, 033817 (2015); R. Puebla, M. Hwang, J. Casanova, M. Plenio, Phys. Rev.A 95, 063844 (2017); P. Schneeweiss, A. Dareau, C. Sayrin, preprint at arXiv:1706.07781 (2017).

[4]  S. Felicetti, D. Z. Rossatto, E. Rico, E. Solano, and P. Forn-Díaz, Phys. Rev. A 97 013851 (2018).

[5] A. Ridolfo, M. Leib, S. Savasta, and M. J. Hartmann, Phys. Rev. Lett. 109, 193602 (2012); A. Le Boité, M.-J. Hwang, H. Nha, M. B. Plenio, Phys. Rev. A 94, 033827 (2016).