On-chip magnon polaritons in the ultrastrong coupling regime

Light-matter interactions underpin the quantum technologies from quantum information processing to quantum sensing. When the coupling strength of light-matter interactions approaches the resonance frequencies of light and matter - the ultrastrong coupling regime - antiresonant (counter-rotating) processes, in which a photon and a matter excitation are simultaneously created or annihilated, induce non-negligible ground-state quantum entanglement between light and matter. Ultrastrong coupling thus provides a robust platform for noise-tolerant quantum entanglement, essential for reliable quantum technologies. However, the diamagnetic term typically counteracts antiresonant interactions, inhibiting intriguing phenomena such as thermal equibilium superradiant phase transitions. Here, we present an on-chip platform consisting of a superconducting resonator and thin ferromagnetic films achieving ultrastrong magnon-photon coupling system (magnon polaritons) via collective magnetic-dipole interactions, significantly circumventing the diamagnetic term. We experimentally demonstrate a pronounced Bloch-Siegert shift of about 60 MHz - direct evidence of antiresonant interactions - and observe cooperative enhancement of the effective coupling strength through increasing the number of remote magnon elements coupling with one photon mode. This scalable platform facilitates exploration of exotic quantum phenomena driven by antiresonant interactions, bridging spintronics and quantum optics, and enabling noise-tolerant quantum technological applications.