Cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet sphere

• Published in: npj quantum information Vol. 1; no. 1; p. 15014
• Main Authors: Zhang, Dengke, Wang, Xin-Ming, Li, Tie-Fu, Luo, Xiao-Qing, Wu, Weidong, Nori, Franco, You, JQ
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 24.11.2015; Nature Publishing Group
• Subjects: 639/766/483/3925; 639/766/483/481; Classical and Quantum Gravitation; Coupling; Damping; Ferromagnetism; Holes; Magnons; Microwaves; Physics; Quantum Computing; Quantum electrodynamics; Quantum Field Theories; Quantum Information Technology; Quantum Physics; Relativity Theory; Spin waves; Spintronics; String Theory; Yttrium-iron garnet
• ISSN: 2056-6387; 2056-6387
• DOI: 10.1038/npjqi.2015.14

Hybridizing collective spin excitations and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can also have potential applications to quantum information. Here we report an experimental study of cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room temperatures. We observe for the first time a strong coupling of the same cavity mode to both a ferromagnetic-resonance (FMR) mode and a magnetostatic (MS) mode near FMR in the quantum limit. This is achieved at a temperature ~22 mK, where the average microwave photon number in the cavity is less than one. At room temperature, we also observe strong coupling of the cavity mode to the FMR mode in the same YIG sphere and find a slight increase of the damping rate of the FMR mode. These observations reveal the extraordinary robustness of the FMR mode against temperature. However, the MS mode becomes unobservable at room temperature in the measured transmission spectrum of the microwave cavity containing the YIG sphere. Our numerical simulations show that this is due to a drastic increase of the damping rate of the MS mode. Quantum electrodynamics: Physics with a crystal ball New research unveils quantum-coherence properties of ferromagnetic magnons in a magnetic sphere at both cryogenic and room temperatures. Tie-Fu Li and J. Q. You from the Beijing Computational Science Research Center, along with collaborators in China, Japan and the USA, placed a submillimeter yttrium-iron-garnet (YIG) sphere within a three-dimensional microwave cavity. They observed a strong interaction between the ferromagnetic resonances of the small magnetic sphere and photons in the surrounding cavity, and confirmed that single photons in the cavity showed strong and robust coupling with the collective spin excitations of magnetic YIG. The coupling extended from cryogenic temperatures up to room temperature, emphasizing the considerable practical potential of this system. These findings remind us that the interaction of different quantum systems can lead to properties unknown to classical systems, revealing potential practical applications.