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

Dengke Zhang,Jq You,Franco Nori,Tie-Fu Li,Xiao-Qing Luo,Weidong Wu,Xin-Ming Wang

doi:10.1038/npjqi.2015.14

Abstract

AbstractHybridizing 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.

Highlights

Hybrid quantum circuits combining two or more physical systems can harness the distinct advantages of different physical systems to better explore new phenomena and potentially bring about novel quantum technologies
The damping rate of the FMR mode in our YIG sphere does not show an appreciable contribution from this slow-relaxation process, because γFMR slightly increases when raising the temperature from 22 mK to room temperature (Table 1)
Our YIG sample is of a better quality than that in ref. 13, regarding the quantum coherence of the FMR mode

Summary

Introduction

Hybrid quantum circuits combining two or more physical systems can harness the distinct advantages of different physical systems to better explore new phenomena and potentially bring about novel quantum technologies (for a review, see ref. 1). A hybrid system consisting of a coplanar waveguide resonator and a spin ensemble was proposed[2] and experimentally utilized[3,4,5,6,7,8] to implement both on-chip cavity quantum electrodynamics and quantum information processing This spin ensemble is usually based on dilute paramagnetic impurities, such as nitrogen-vacancy centers in diamond[9,10] and rare-earth ions doped in a crystal.[11] By increasing the density of the paramagnetic impurities, strong and even ultrastrong couplings between the cavity and the spin ensemble can be achieved, but the coherence time of the spin excitations is drastically shortened. When the same spin density is involved, the spin excitations in YIG can exhibit much better quantum coherence than those of the paramagnetic impurities

Methods

Results

Conclusion