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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

Physical Review B

We report the first observation of the magnon-polariton bistability in a cavity magnonics system consisting of cavity photons strongly interacting with the magnons in a small yttrium iron garnet (YIG) sphere. The bistable behaviors emerged as sharp frequency switchings of the cavity magnon polaritons (CMPs) and related to the transition between states with large and small numbers of polaritons. In our experiment, we align, respectively, the [100] and [110] crystallographic axes of the YIG sphere parallel to the static magnetic field and find very different bistable behaviors (e.g., clockwise and counter-clockwise hysteresis loops) in these two cases. The experimental results are well fitted and explained as being due to the Kerr nonlinearity with either a positive or negative coefficient. Moreover, when the magnetic field is tuned away from the anticrossing point of CMPs, we observe simultaneous bistability of both magnons and cavity photons by applying a drive field on the lower branch.

Bistability of Cavity Magnon Polaritons.

Magnon-polaritons are hybrid light-matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon-polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsical nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sphere in a microwave cavity and engineer the dissipations of photons and magnons to create cavity magnon-polaritons which have non-Hermitian spectral degeneracies. By tuning the magnon-photon coupling strength, we observe the polaritonic coherent perfect absorption and demonstrate the phase transition at the exceptional point. Our experiment offers a novel macroscopic quantum platform to explore the non-Hermitian physics of the cavity magnon-polaritons.

/pdf/observation-of-the-exceptional-point-in-cavity-magnon-10gnou1usl.pdf

Observation of the exceptional point in cavity magnon-polaritons

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. 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.

/pdf/cavity-quantum-electrodynamics-with-ferromagnetic-magnons-in-35e9mbylbf.pdf

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

We experimentally demonstrate magnon Kerr effect in a cavity-magnon system where magnons in a small yttrium iron garnet (YIG) sphere are strongly but dispersively coupled to the photons in a three-dimensional cavity. When the YIG sphere is pumped to generate considerable magnons, the Kerr effect yields a perceptible shift of the cavity's central frequency and more appreciable shifts of the magnon modes. We derive an analytical relation between the magnon frequency shift and the drive power for the uniformly magnetized YIG sphere and find that it agrees very well with the experimental results of the Kittel mode. Our study paves the way to explore nonlinear effects in the cavity-magnon system.

Magnon Kerr effect in a strongly coupled cavity-magnon system

Magnon–polaritons are hybrid light–matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon–polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsically nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sphere in a microwave cavity and engineer the dissipations of photons and magnons to create cavity magnon–polaritons which have non-Hermitian spectral degeneracies. By tuning the magnon–photon coupling strength, we observe the polaritonic coherent perfect absorption and demonstrate the phase transition at the exceptional point. Our experiment offers a novel macroscopic quantum platform to explore the non-Hermitian physics of the cavity magnon–polaritons. Strong coupling between magnons and photons allows coupling of magnongs to qubits, suggesting that magnon-polaritons could find applications in quantum information. Here, Zhang et al. observe an exceptional point and spontaneous symmetry breaking in a cavity magnon-polariton system.

/pdf/observation-of-the-exceptional-point-in-cavity-magnon-qt33l598wf.pdf

Tie-Fu Li

Papers

Bistability of Cavity Magnon Polaritons.

Observation of the exceptional point in cavity magnon-polaritons

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

Magnon Kerr effect in a strongly coupled cavity-magnon system

Observation of the exceptional point in cavity magnon-polaritons.