Quantum magnonics: The magnon meets the superconducting qubit

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.     

腔光子-自旋波量子耦合系统中各向异性奇异点的实验研究

通过耦合三维微波腔中光子和腔内钇铁石榴石单晶小球中的自旋波量子形成腔-自旋波量子的耦合系统,并通过精确调节系统参数在该实验系统中观测到各向异性奇异点.奇异点对应于非厄米系统中一种特殊状态,在奇异点处,耦合系统的本征值和本征矢均简并,并且往往伴随着非平庸的物理性质.以往大量研究主要集中在各向同性奇异点的范畴,它的特征是在系统参数空间中沿着不同参数坐标趋近该奇异点时具有相同的函数关系.在这篇文章中,主要介绍实验上在腔光子-自旋波量子耦合系统中通过调节系统的耦合强度和腔的耗散衰减系数两条趋近奇异点的路径而实现了各向异性奇异点,具体分别对应于在趋近奇异点时,本征值的虚部的变化与耦合强度和腔的衰减系数的变化会有线性和平方根不同的行为.各向异性奇异点的实现有助于基于腔光子-自旋波量子耦合系统的量子信息处理和精密探测器件的进一步研究.     

Nonreciprocal microwave transmission under the joint mechanism of phase modulation and magnon Kerr nonlinearity effect

Nonreciprocal microwave devices, in which the transmission of waves is non-symmetric between two ports, are indispensable for the manipulation of information processing and communication. In this work, we show the nonreciprocal microwave transmission in a cavity magnonic system under the joint mechanism of phase modulation and magnon Kerr nonlinearity effect. In contrast to the schemes based on the standard phase modulation or magnon Kerr nonlinearity, we find that the joint mechanism enables the nonreciprocal transmission even at low power and makes us obtain a high nonreciprocal isolation ratio. Moreover, when two microwave modes are coupled to the magnon mode via a different coupling strength, the presented strong nonreciprocal response occurs, and it makes the nonreciprocal transmission manipulating by the magnetic field within a large adjustable range possible, which overcomes narrow operating bandwidths. This study may provide promising opportunities to realize nonreciprocal structures for wave transmission.     

Observation of nonlinearity and heating-induced frequency shifts in cavity magnonics

When there is a certain amount of field inhomogeneity, the biased ferrimagnetic crystal can exhibit the higher-order magnetostatic (HMS) mode in addition to the uniform-precession Kittel mode. In cavity magnonics, we show the nonlinearity and heating-induced frequency shifts of the Kittel mode and HMS mode in a yttrium-iron-garnet (YIG) sphere. When the Kittel mode is driven to generate a certain number of excitations, the temperature of the whole YIG sample rises and the HMS mode can display an induced frequency shift, and vice versa. This cross effect provides a new method to study the magnetization dynamics and paves a way for novel cavity magnonic devices by including the heating effect as an operational degree of freedom.     

Enhancement of the Kerr effect for a quantum dot in a cavity

Nonlinear susceptibility of a quantum dot (QD) embedded in a two-sided cavity, is studied theoretically from a weak-coupling to a strong-coupling regime. In the relevance of a quantum logic gate, the corresponding nonlinear phase shifts (Kerr effect) are estimated for coherent wavepackets including one photon on average. In the weak-coupling regime, the phase shift enhances strongly as a function of a coupling constant between the cavity photon and QD, and eventually saturates in the strong-coupling regime. We also show transmission spectra to evaluate the efficiency of the phase shift. Although the efficiency decreases monotonically in the weak-coupling regime, it rises in the strong-coupling regime.     

Vibration and the Kerr effect

The theory of the Kerr effect (electric birefringence) is developed with explicit inclusion of molecular vibration. When the theory is applied to CHCl₃ and CHF₃, it is found that terms with a vibrational origin are an important component of the β-hyperpolarizability extracted from temperature-dependent Kerr studies. It is concluded that these vibrational terms partly account for the difference between β obtained from second-harmonic-generation experiments and from the Kerr constant.     

Saturation of the all-optical Kerr effect

Saturation of the intensity dependence of the refractive index is directly computed from ionization rates via a Kramers-Kronig transform. The linear intensity dependence and its dispersion are found to be in excellent agreement with complete quantum mechanical orbital computations. Higher-order terms concur with solutions of the time-dependent Schr?dinger equation. Expanding the formalism to all orders up to the ionization potential of the atom, we derive a model for saturation of the Kerr effect. This model widely confirms recently published and controversially discussed experimental data and corroborates the importance of higher-order Kerr terms for filamentation.     

Saturation of the all-optical Kerr effect in solids

We discuss the influence of the higher-order Kerr effect (HOKE) in wide bandgap solids at extreme intensities below the onset of optically induced damage. Using different theoretical models, we employ multiphoton absorption rates to compute the nonlinear refractive index by a Kramers-Kronig transform. Within this theoretical framework we provide an estimate for the appearance of significant deviations from the standard optical Kerr effect predicting a linear index change with intensity. We discuss the role of the observed saturation behavior in practically relevant situations, including Kerr lens mode-locking and supercontinuum generation in photonic crystal fibers. Furthermore, we present experimental data from a multiwave mixing experiment in BaF2, which can be explained by the appearance of the HOKE.     

Diffractional enhancement of the Kerr magnetooptic effect

The results of investigations of the transverse Kerr effect on an array of thin magnetic strips deposited on a silicon substrate are reported. The periodic structure of the sample gives rise to diffraction. It is observed that under certain experimental conditions the magnitude of the effect measured in diffracted beams is much greater than the maximum value for a sample with a uniform surface. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 466–469 (10 October 1997)     

Magnetooptical Kerr effect of Fe-gratings

The magnetooptical Kerr effect in longitudinal configuration is used to measure hysteresis loops of ferromagnetic Fe-gratings grown on Al₂O₃ at different orders of diffraction. At even order of diffraction the hysteresis loops exhibit anomalies which can be attributed to the interference of the magnetic and non-magnetic parts of the grating. The Kerr angle in saturation increases linearly with the order of diffraction.     

Magneto-optical Kerr effect in bilayer structures

This paper describes the magneto-optical effects and the reflectivity behaviors of bilayers based on magnetic and isotropic ()/anisotropic () layers under the condition of total internal reflection. In the framework of Green's dyadic technique, we show accurately the optical properties of anisotropic layers deposited on a substrate. We present numerical simulations which account for the variation of angle of incidence at the HeNe laser wavelength. The Kerr rotation is found to increase significantly around the optical modes in total reflection. We also discuss the importance of anisotropic effects due to the crystallization of the dielectric material () which occur in the reflectivity and Kerr rotation spectra. Received 26 January 2000     

Kerr effect in quantum dot structure

A three-level ladder QD system is used to study Kerr effect in QD structures. Inhomogenous broadening is included where it is shown to be critical in calculating Kerr effect in QDs. Signal detuning is shown to control Kerr dispersion.     

Nonlinear Kerr effect in magnetic crystals

The reflection of an intense light wave from the boundary of a semiinfinite magnetic crystal is investigated theoretically. Expressions are obtained for the rotation angle of the polarization ellipse and the degree of ellipticity of the reflected wave as a function of the polarization of the incident radiation. The physical wave-interaction mechanisms that give rise to this effect are established. Fiz. Tverd. Tela (St. Petersburg) 41, 656–659 (April 1999)     

On the higher-order Kerr effect in femtosecond filaments

A recent experiment probing the electronic nonlinearity in the femtosecond filament indicated that the optical Kerr effect not only saturates but even changes its sign at high intensities and thus switches from self-focusing to a strongly defocusing regime. Here we examine, through simulations and experiment, some implications of such a behavior. We perform comparative simulations based on the standard model on one hand and on a model implementing the intensity-dependent Kerr effect on the other. Comparison with an experiment provides a strong indication that of these two Kerr-effect models the standard model is better in capturing the observed length of the filament. However, neither of the models can reproduce length and filament radius. Possible implications are discussed.     

Optical Kerr effect in supercooled water

We present molecular dynamics simulations of the optical Kerr effect in liquid and supercooled water and compare with recent time-resolved Kerr spectroscopy measurements [R. Torre, Nature (London) 428, 296 (2004)]. The short time features of the Kerr response, characterized by peaks near 15, 60, and 160 fs, are weakly temperature dependent. The long-time decay is well described by a stretched exponential with a nearly constant stretch parameter and relaxation times that follow a power law approximately (T-T(S))(-gamma), with T(S)=198.3 K and gamma=2.35. Our findings are discussed in the light of the spectroscopy data and previous simulation analyzes of the structural relaxation in supercooled water.     

High frequency magnon generation by nonequilibrium electrons and the instability of the magnon system

The high frequency magnon generation by nonequilibrium electrons polarized opposite the magnetization vector is investigated. At some conditions the avalanche-type generation of magnons with wavevectors along the magnetization takes place and the magnon system becomes unstable.     

Additivity of the Kerr effect in thin-film magnetic systems

A universal approach is developed for calculating magneto-optic coefficients in multimedia systems. Three configurations are described: (i) a two-media, one-boundary system, (ii) a film sandwiched between two media and (iii) a multilayer system, such as a superlattice or even a randomly stacked sequence. It is proven that in the thin-film limit, the Kerr effect for a multilayer system obeys an additivity law: it is equal to the algebraic sum of the Kerr signals of the individual magnetic films in the system. The prediction is verified experimentally and in numerical calculations on an Fe/Cu/Fe 3-layer stack grown on a Pd-substrate.     

Second magnon in antiferromagnets

Hydrodynamic equations are derived microscopically for Heisenberg antiferromagnets with inclusion of quasimomentum as an almost conserved quantity. For finite external and anisotropy fields, the hydrodynamic quantities are magnetization, energy and momentum density. At small fields, the propagating mode is essentially an oscillation of the local temperature. For high fields it is a wave in the longitudinal magnetization. In the absence of both fields, the staggered magnetization and not the magnetization itself is coupled to momentum and energy. The propagating mode is mainly an energy wave. The propagating part in the staggered magnetization is inversely proportional to the wavelength.