Toyofumi Ishikawa

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.

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.

Resolving quanta of collective spin excitations in a millimeter-sized ferromagnet

Dany Lachance-Quirion, 2, ∗ Yutaka Tabuchi, Seiichiro Ishino, Atsushi Noguchi, Toyofumi Ishikawa, Rekishu Yamazaki, and Yasunobu Nakamura 3, † Institut quantique and Département de Physique, Université de Sherbrooke, J1K 2R1, Sherbrooke, Québec, Canada Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama 351-0198, Japan (Dated: October 4 2016)Quanta of collective spin excitations are observed in a ferromagnet by measurements of a superconducting quantum bit. Combining different physical systems in hybrid quantum circuits opens up novel possibilities for quantum technologies. In quantum magnonics, quanta of collective excitation modes in a ferromagnet, called magnons, interact coherently with qubits to access quantum phenomena of magnonics. We use this architecture to probe the quanta of collective spin excitations in a millimeter-sized ferromagnetic crystal. More specifically, we resolve magnon number states through spectroscopic measurements of a superconducting qubit with the hybrid system in the strong dispersive regime. This enables us to detect a change in the magnetic moment of the ferromagnet equivalent to a single spin flipped among more than 1019 spins. Our demonstration highlights the strength of hybrid quantum systems to provide powerful tools for quantum sensing and quantum information processing.

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.

New aspects and new tools in hypernuclear studies: Experiments with a superconducting toroidal spectrometer

SummaryThe experimental program with a new superconducting toroidal spectrometer system which will be installed in a new low-momentum K-meson beam line at KEK is discussed based on the recent development of hypernuclear studies using stopped-K− reactions. Current topics, such as bound Σ hypernuclei, weak pionic decays of Λ hypernuclei and production of Λ hypernuclei via direct and indirect reactions, are described.RiassuntoSi discute il programma sperimentale con un nuovo sistema a spettrometro toroidale superconduttore che sarà installato in una nuova linea di fascio di mesoni K a basso impulso a KEK basandosi sugli sviluppi recenti degli studi sugli ipernuclei che utilizzano le reazioni con K− assorbito a riposo. Si descrivono gli argomenti correnti, come gli ipernuclei legati Σ, is decadimenti pionici deboli e la produzione di ipernuclei Λ mediante reazioni dirette e indirette.РеэюмеОбсуждается зкспериментальная программа на новой системе с сверхпроводяшим тороидальным спектрометром, которая установлена в новой линии на пучке K-меэонов с малым импульсом в KEK. Программа преднаэначена для исследования гиперядер, обраэованных в реакциях остановивщихся K−-меэонов. Обсуждаются свяэанные Σ-гиперядра, слабые пионные распады Λ-гиперядер и обраэование Λ-гиперядер в реэультате прямых и непрямых реакций.

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.

Anisotropic Charge Dynamics in Layered Manganite Crystals

Anisotropic charge dynamics has been investigated for single crystals of layered manganites, La2−2x Sr1+2x Mn2O7 (0.3≤x≤0.5). Remarkable variation in the magnetic structure as well as in the charge-transport properties is observed with changing doping-level x. A crystal with x=0.3 behaves like a 2-dimensional ferromagnetic metal at the temperature region between ~90 K and ~270 K, and shows the interplane tunneling magnetoresistance at lower temperatures which is sensitive to the interplane magnetic coupling between the adjacent MnO2 bilayers. Optical probe for these layered manganites has also clarified highly anisotropic and incoherent charge dynamics.