Koji Usami

Hybridizing ferromagnetic magnons and microwave photons in the quantum limit.

We demonstrate large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode. Strong coupling is achieved in the quantum regime where the average number of thermally or externally excited magnons and photons is less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. A nonmonotonic temperature dependence of the Kittel-mode linewidth is observed below 1 K and is attributed to the dissipation due to the coupling with a bath of two-level systems.

Coherent coupling between a ferromagnetic magnon and a superconducting qubit

Making hybrid quantum information systems Different physical implementations of qubits—quantum bits—each have their pros and cons. An appealing idea is to combine them into hybrid architectures, taking advantage of their respective strengths. Tabuchi et al. placed a ferromagnetic sphere and a superconducting qubit in a cavity and used an electromagnetic mode of the cavity as the mediator between the two. They achieved strong coupling between a collective magnetic mode of the sphere and the qubit. Viennot et al. coupled a single spin in a double quantum dot to photons in a cavity. Both approaches hold promise for future applications. Science, this issue pp. 405 and 408 A collective mode of a ferromagnetic sphere is strongly coupled to a qubit in a cavity. Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending to macroscopic dimensions. A magnon is a quantum of such collective excitation modes in ordered spin systems. Here, we demonstrate the coherent coupling between a single-magnon excitation in a millimeter-sized ferromagnetic sphere and a superconducting qubit, with the interaction mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we use a parametric drive to realize a tunable magnon-qubit coupling scheme. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and may lead to advances in quantum information processing.

Bidirectional conversion between microwave and light via ferromagnetic magnons

Coherent conversion of microwave and optical photons in the single quantum level can significantly expand our ability to process signals in various fields. Efficient up-conversion of a feeble signal in the microwave domain to the optical domain will lead to quantum-noise-limited microwave amplifiers. Coherent exchange between optical photons and microwave photons will also be a stepping stone to realize long-distance quantum communication. Here we demonstrate bidirectional and coherent conversion between microwave and light using collective spin excitations in a ferromagnet. The converter consists of two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode called the Kittel mode, where microwave photons and magnons in the respective modes are strongly coupled and hybridized. An itinerant microwave field and a traveling optical field can be coupled through the hybrid system, where the microwave field is coupled to the hybrid system through the cavity mode, while the optical field addresses the hybrid system through the Kittel mode via Faraday and inverse Faraday effects. The conversion efficiency is theoretically analyzed and experimentally evaluated. The possible schemes for improving the efficiency are also discussed.

Cavity Optomagnonics with Spin-Orbit Coupled Photons.

We experimentally implement a system of cavity optomagnonics, where a sphere of ferromagnetic material supports whispering gallery modes (WGMs) for photons and the magnetostatic mode for magnons. We observe pronounced nonreciprocity and asymmetry in the sideband signals generated by the magnon-induced Brillouin scattering of light. The spin-orbit coupled nature of the WGM photons, their geometrical birefringence, and the time-reversal symmetry breaking in the magnon dynamics impose the angular-momentum selection rules in the scattering process and account for the observed phenomena. The unique features of the system may find interesting applications at the crossroad between quantum optics and spintronics.

Quantum magnonics: The magnon meets the superconducting qubit

Abstract The techniques of microwave quantum optics are applied to collective spin excitations in a macroscopic sphere of a ferromagnetic insulator. We demonstrate, in the single-magnon limit, strong coupling between a magnetostatic mode in the sphere and a microwave cavity mode. Moreover, we introduce a superconducting qubit in the cavity and couple the qubit with the magnon excitation via the virtual photon excitation. We observe the magnon–vacuum-induced Rabi splitting. The hybrid quantum system enables generation and characterization of non-classical quantum states of magnons.

Ground state cooling of a quantum electromechanical system with a silicon nitride membrane in a 3D loop-gap cavity

Cavity electro-(opto-)mechanics gives us a quantum tool to access mechanical modes in a massive object. Here we develop a quantum electromechanical system in which a vibrational mode of a SiN x membrane are coupled to a three-dimensional loop-gap superconducting microwave cavity. The tight confinement of the electric field across a mechanically compliant narrow-gap capacitor realizes the quantum strong coupling regime under a red-sideband pump field and the quantum ground state cooling of the mechanical mode. We also demonstrate strong coupling between two mechanical modes, which is induced by two-tone parametric drives and mediated by a virtual photon in the cavity.

Electro-mechano-optical detection of nuclear magnetic resonance

Signal reception of nuclear magnetic resonance (NMR) usually relies on electrical amplification of the electromotive force caused by nuclear induction. Here, we report up-conversion of a radio-frequency NMR signal to an optical regime using a high-stress silicon nitride membrane that interfaces the electrical detection circuit and an optical cavity through the electro-mechanical and the opto-mechanical couplings. This enables optical NMR detection without sacrificing the versatility of the traditional nuclear induction approach. While the signal-to-noise ratio is currently limited by the Brownian motion of the membrane as well as additional technical noise, we find it can exceed that of the conventional electrical schemes by increasing the electro-mechanical coupling strength. The electro-mechano-optical NMR detection presented here opens the possibility of mechanical parametric amplification of NMR signals. Moreover, it can potentially be combined with the laser cooling technique applied to nuclear spins.

Nonclassical photon number distribution in a superconducting cavity under a squeezed drive

A superconducting qubit in the strong dispersive regime of circuit quantum electrodynamics is a powerful probe for microwave photons in a cavity mode. In this regime, a qubit excitation spectrum is split into multiple peaks, with each peak corresponding to an individual photon number in the cavity (discrete ac Stark shift). Here, we measure the qubit spectrum in a cavity that is driven continuously with a squeezed vacuum generated by a Josephson parametric amplifier. By fitting the obtained spectrum with a model which takes into account the finite qubit excitation power, we determine the photon number distribution, which reveals an even-odd photon number oscillation and quantitatively fulfills Klyshkos criterion for nonclassicality.

Orbital angular momentum conservation in Brillouin light scattering within a ferromagnetic sphere

Magnetostatic modes supported by a ferromagnetic sphere have been known as the Walker modes, each of which possesses an orbital angular momentum as well as a spin angular momentum along a static magnetic field. The Walker modes with non-zero orbital angular momenta exhibit topologically non-trivial spin textures, which we call magnetic quasi-vortices. Photons in optical whispering gallery modes supported by a dielectric sphere possess orbital and spin angular momenta forming optical vortices. Within a ferromagnetic, as well as dielectric, sphere, two forms of vortices interact in the process of Brillouin light scattering. We argue that in the scattering there is a selection rule that dictates the exchange of orbital angular momenta between the vortices. The selection rule is shown to be responsible for the experimentally observed nonreciprocal Brillouin light scattering.

Experimental mapping of magnetostatic mode structures of a ferrimagnet spheroid

The exploration of the interaction of light with spin waves in ferromagnets within an optical cavity might lead to new chiral photonic devices and be a stepping stone towards the coherent optical manipulation of magnons in the quantum regime [1]. The developments made so far in cavity optomagnonics have been focused on the fundamental magnetostatic mode of an yttrium iron garnet (YIG) sphere, so-called ‘Kittel mode’ [2, 3] and lead to the observation of a magnon-induced Brillouin scattering between optical whispering gallery modes following selection rules dictated by the conservation of the orbital momentum. Higher-order magnetostatic modes, with reduced mode volume and different orbital angular momentum, could couple more efficiently with the optical whispering gallery modes hosted by the YIG sphere, lying in the vicinity of the sphere surface equatorial plane. Though the resonance frequencies of these modes can be predicted theoretically [4], small deviations due to the actual sample properties or environment, as well as possible hybridization of the modes, could cause strong misinterpretations. Hence, unambiguously experimentally identifying higher-order magnon modes in a spheroid is required to properly scrutinize their interaction with light.