On-chip Dicke-type magnon polaritons in the ultrastrong coupling regime via spatially separated nanomagnets

TL;DR

This paper demonstrates an on-chip hybrid quantum system with ultrastrong magnetic interactions between spatially separated nanomagnets and a superconducting resonator, enabling exploration of Dicke physics and quantum collective phenomena.

Contribution

It introduces a novel on-chip architecture for Dicke-type magnon polaritons with spatial separation, overcoming self-interaction issues and enabling the study of critical Dicke physics.

Findings

Observation of Bloch-Siegert shift confirming counter-rotating effects

Suppression of self-excitation terms enabling Dicke physics

Enhanced coupling strength without increased self-interaction energy

Abstract

Quantum electrodynamics lies at the heart of hybrid quantum systems essential for future technologies. The thermodynamic limit of the Dicke model, a fundamental model describing these systems, predicts exotic quantum phenomena, such as equilibrium superradiant phase transitions and ground-state two-mode squeezing. However, the experimental realization of genuine Dicke systems has remained elusive due to the inevitable existence of gauge-invariant self-interaction terms that hinder such phenomena. Here, we report on the on-chip realization of a Dicke-type system utilizing ultrastrong magnetic-dipole interactions between collective excitations in a spatially separated ferromagnetic array and a superconducting resonator, resulting in creation of magnon polaritons. Crucially, this spatially separated architecture allows the cooperative enhancement of the coupling strength without increasing the self-interaction energy. We experimentally confirm the Bloch-Siegert shift, originating from the counter-rotating terms, alongside the suppression of self-excitation terms required to observe critical Dicke physics. Our results establish a versatile platform, which provides the playground to explore quantum collective coupling physics and open pathways towards integrated quantum devices harnessing Dicke physics.