Cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet sphere
Update: 2016-02-02 13:51:00      Author:

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. Among them, a hybrid system consisting of a coplanar waveguide resonator and a spin ensemble was proposed [1] and experimentally utilized [2] 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 and rare-earth ions doped in a crystal. 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. Indeed, it is a challenging task to realize both good quantum coherence of the spin ensemble and its strong coupling to a cavity.

Recently, collective spins in a yttrium-iron-garnet (YIG) ferromagnetic material were explored to achieve their strong [3] and even ultrastrong couplings [4] to a microwave cavity. In contrast to spin ensembles based on dilute paramagnetic impurities, these spins are strongly exchange-coupled and have a much higher density (~ 4.2×1021 cm-3). Because of this high spin density, a strong coupling of the spin excitations to the cavity can be easily realized using a YIG sample as small as sub-milimeter in size. Moreover, the contribution of magnetic dipole interactions to the linewidth of spin excitations, which can have a dominant role among paramagnetic impurities, is suppressed by the strong exchange coupling between the ferromagnetic electrons. Thus, when the same spin density is involved, the spin excitations in YIG can exhibit much better quantum coherence than those of the paramagnetic impurities.

Very recently, Dengke Zhang et al. in the CSRC Quantum Physics and Quantum Information Division reported an experimental study of cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room temperatures. They observed 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 was achieved at a temperature ~22 mK, where the average microwave photon number in the cavity is less than one. At room temperature, they also observed strong coupling of the cavity mode to the FMR mode in the same YIG sphere and found a slight increase of the damping rate of the FMR mode. These observations reveal the extraordinary robustness of the FMR mode against temperature. The MS mode becomes unobservable at room temperature in their measured transmission spectrum of the microwave cavity containing the YIG sphere. Their numerical simulations showed that this is due to a drastic increase of the damping rate of the MS mode



Fig. 1. The transmission spectrum of the 3D cavity with a small YIG sphere is measured as a function of the static magnetic field. Strong magnon–photon coupling achieved at cryogenic temperature (top) and room temperature (below) with different frequencies around (a) TE101 mode and (b) TE102 mode.

This work is supported by the NSAF Grant No. U1330201, the NSFC Grant No. 91421102, and the MOST 973 Program Grant Nos. 2014CB848700 and 2014CB921401.

For more information, please see the paper: Dengke Zhang, Xin-Ming Wang, Tie-Fu Li, Xiao-Qing Luo, Weidong Wu, Franco Nori and JQ You, “Cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet sphere”, npj Quantum Information 1, 15014 (2015). doi: 10.1038/npjqi.2015.14.


[1] Imamoğlu, A. Cavity QED based on collective magnetic dipole coupling: Spin ensembles as hybrid two-level systems. Phys. Rev. Lett. 102, 083602 (2009).

[2] Kubo, Y. et al. Strong coupling of a spin ensemble to a superconducting resonator. Phys. Rev. Lett. 105, 140502 (2010).

[3] Tabuchi, Y. et al. Hybridizing ferromagnetic magnons and microwave photons in the quantum limit. Phys. Rev. Lett. 113, 083603 (2014).

[4] Goryachev, M. et al. High-cooperativity cavity QED with magnons at microwave frequencies. Phys. Rev. Appl. 2, 054002 (2014).



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