量子自旋波本征值及易轴型各向异性对其的影响

本文考虑体单轴各向异性利用界面重参数化方法从理论上严格推导了两层异质铁磁质中量子自旋波的本征值,并模拟计算了简立方晶格结构 (100) 第一布里渊区的能带结构,特别研究了易轴型体单轴各向异性对本征值的影响.结果发现在这样的体系中存在体模、禁闭模和界面模;各种本征模与体交换作用、界面交换作用、晶格自旋、原子层数有关,而且各向异性直接影响各种本征模的存在条件. 关键词： 铁磁薄膜 自旋波 各向异性     

量子点操控的光子探测和圆偏振光子发射

半导体量子点是研究光子与电子态相互作用的优选固态体系,并在光子探测和发射两个方向上展现出独特的技术机遇.其中基于量子点的共振隧穿结构被认为在单光子探测方面综合性能最佳,但受到光子数识别、工作温度两个关键性能的制约.利用腔模激子态外场耦合效应,有望获得圆偏振态可控的高频单光子发射.本文介绍作者提出的量子点耦合共振隧穿（QD-cRTD）的光子探测机理,利用量子点量子阱复合电子态的隧穿放大,将QD-cRTD光子探测的工作温度由液氦提高至液氮条件,光电响应的增益达到10⁷以上,并具备双光子识别能力;同时,由量子点能级的直接吸收,原型器件获得了近红外的光子响应.在量子点光子发射机理的研究方面,作者实现了量子点激子跃迁和微腔腔模共振耦合的磁场调控,在Purcell效应的作用下增强激子自旋态的自发辐射速率,从而增强量子点中左旋或右旋圆偏振光的发射强度,圆偏度达到90%以上,形成一种光子自旋可控发射的新途径.     

两模光机械系统中超冷原子的量子相变

Dicke模型(DM)用于描述单模玻色光场与多个全同二能级原子相互作用。本文利用自旋相干态变分法得到两模光机械系统中基态能量的精确解,并通过变分法求得相变点并画出基态相图,并在此基础上研究原子-场耦合强度等系统参数对基态稳定性的影响。通过稳定性讨论,我们发现:原子-光子耦合常数g和光子-声子耦合常量ζ都会对光机械系统的基态特性产生影响。当双模光腔变成单模光腔时,机械振子能诱导超辐射相的塌缩;而且当光子-声子耦合强度大时,超辐射相被完全压制,而直接出现两原子能级之间的转移;存在不稳定的非零光子态,类似于超辐射态。光机械腔中光子-声子耦合诱导的超辐射态的塌缩和复苏是不同于光腔内囚禁的BEC系统,即机械振子不存在时的情况,而双模光腔对量子相变点和相图预期也会有影响。可见,分析机械振子的对多稳性和相关的量子相变的影响是非常有意义的课题。     

具有三体相互作用海森堡自旋链模型的纠缠与几何失协

近年来有研究发现量子失协可以度量非纠缠的量子关联,而且非纠缠的量子关联在量子通信和量子计算中起到很重要的作用.本文研究了磁场,两种三体相互作用,各向异性参数,耦合常数,温度等参数对同时具有两种三体相互作用海森堡XXZ自旋链系统的量子纠缠,几何失协的影响以及与量子相变的关系.研究表明:量子纠缠和几何失协都可以清晰的表征本模型系统的量子相变现象;随着XZX+YZY型三体相互作用的增加量子纠缠和几何失协即使在高温时也可达到最大值;几何失协比量子纠缠更全面地描述了量子关联;XZY-YZX型三体相互作用的增加对量子纠缠有抑制作用;XZY-YZX型三体相互作用在一定区域内的增加对几何失协有抑制作用,在另一区域的增加可使几何失协增大到一个稳定的非零值.增大磁场和自旋耦合常数,减小各向异性参数会使纠缠的临界温度变大;调节自旋耦合常数可更有效的使量子纠缠和几何失协在高温时仍有一个较大的值.同时发现,在磁场0B5和各向异性参数-1J_Z10的区域两种量子关联都可以维持在最大值.     

冷原子系综中光纤腔增强且高保真度的光学存储

利用原子系综中的Duan-Lukin-Cirac-Zoller (DLCZ)过程可产生光与原子记忆(自旋波)量子纠缠,该纠缠可作为量子中继的重要元件.随着量子信息研究的深入发展,人们对量子信息存储其灵活多样性、可控性等方面提出更高的要求.本文在冷原子系综中演示了一种基于DLCZ过程的光纤腔增强且高保真度的光学存储方案,即将87Rb原子系综放于设计的光纤腔中,通过光纤腔增强“写出”和“读出”光子与原子系综的耦合实现自旋波量子信息的有效恢复,同时具有较高的保真度.观察到有腔且锁定的情况下斯托克斯光子产生概率比无腔时增加4.6倍,原子自旋波读出效率增加1.6倍,实验实现22%的读出效率并具有92%的量子态保真度,该读出效率对应一个40%的本质读出效率.这种高度可恢复、高量子态保真度的原子-光子纠缠源,可为未来长距离量子通信及广域大规模量子网络构建的实现提供另一种有效的途径.     

两层A～B铁磁耦合薄膜中的自旋波的色散关系

考虑表面各向异性场,在非周期性边界条件下应用界面重参数化方法精确求解了两层铁磁耦合薄膜中的低能自旋波本征值,结果表明:表面各向异性场不会影响薄膜整体的色散关系的变化趋势,但会影响本征值及其数目.     

界面各向异性对双层铁磁薄膜自旋波性质的影响

在海森堡模型的基础上,采用界面参数化方法,将双层铁磁薄膜中自旋波本征值问题归结为联立求解能量约束方程和界面参数化方程.重点研究了界面各向异性对薄膜中自旋波本征问题的影响.结果表明:界面各向异性使对称模的波形在界面处呈现明显的钉扎现象,且界面模的能量随各向异性场增强而增大.     

具有三体相互作用海森堡自旋链模型的纠缠与几何失协

近年来有研究发现量子失协可以度量非纠缠的量子关联,而且非纠缠的量子关联在量子通信和量子计算中起到很重要的作用.研究了磁场,两种三体相互作用,各向异性参数,耦合常数,温度等参数对同时具有两种三体相互作用海森堡XXZ自旋链系统的量子纠缠,几何失协的影响以及与量子相变的关系.研究表明:量子纠缠和几何失协都可以清晰的表征本模型系统的量子相变现象; 随着XZX+YZY型三体相互作用的增加量子纠缠和几何失协即使在高温时也可达到最大值;几何失协比量子纠缠更全面地描述了量子关联; XZY-YZX型三体相互作用的增加对量子纠缠有抑制作用;XZY-YZX型三体相互作用在一定区域内的增加对几何失协有抑制作用,在另一区域的增加可使几何失协增大到一个稳定的非零值。增大磁场和自旋耦合常数,减小各向异性参数会使纠缠的临界温度变大; 调节自旋耦合常数可更有效的使量子纠缠和几何失协在高温时仍有一个较大的值.同时发现,在磁场 和各向异性参数 的区域两种量子关联都可以维持在最大值.     

旁耦合于介观环与铁磁电极量子点系统中的自旋极化输运

借助单杂质Anderson模型哈密顿量,及利用格林函数和运动方程等理论,研究旁耦合于介观环和铁磁电极的量子点系统中的极化输运特性.结果表明,通过调节点-环耦合强度、铁磁电极中的极化强度、磁矩相对取向及温度等,均能实现控制体系中自旋极化电流的目的,达到自旋阀效应.为此系统作为一种新的自旋电子材料提供理论依据.     

Dzyaloshinsky-Moriya作用影响下的qubit-qutrit系统关联动力学

本文分析了与各自自旋链环境相互耦合的qubit和qutrit中的negativity和几何量子失协动力学,讨论了系统和环境间的耦合强度g、各向异性参数γ、粒子数N、初始参数p以及Dzyaloshinsky-Moriya相互作用对系统演化的影响,得到一些重要结论。另外发现,依赖于各参数的选取,negativity不总是大于几何量子失协,且几何量子失协存在突然改变点。     

Theory of the magnon Kerr effect in cavity magnonics

We develop a theory for the magnon Kerr effect in a cavity magnonics system, consisting of magnons in a small yttrium iron garnet(YIG) sphere strongly coupled to cavity photons, and use it to study the bistability in this hybrid system. To have a complete picture of the bistability phenomenon, we analyze two different cases in driving the cavity magnonics system, i.e.,directly pumping the YIG sphere and the cavity, respectively. In both cases, the magnon frequency shifts due to the Kerr effect exhibit a similar bistable behavior but the corresponding critical powers are different. Moreover, we show how the bistability of the system can be demonstrated using the transmission spectrum of the cavity. Our results are valid in a wide parameter regime and generalize the theory of bistability in a cavity magnonics system.     

Experimental observation of the topological structure of exceptional points

We report on a microwave cavity experiment where exceptional points (EPs), which are square root singularities of the eigenvalues as function of a complex interaction parameter, are encircled in the laboratory. The real and imaginary parts of an eigenvalue are given by the frequency and width of a resonance and the eigenvectors by the field distributions. Repulsion of eigenvalues--always associated with EPs--implies frequency anticrossing (crossing) whenever width crossing (anticrossing) is present. The eigenvalues and eigenvectors are interchanged while encircling an EP, but one of the eigenvectors undergoes a sign change which can be discerned in the field patterns.     

Chiral State Conversion in a Levitated Micromechanical Oscillator with In Situ Control of Parameter Loops

Physical systems with gain and loss can be described by a non-Hermitian Hamiltonian, which is degenerated at the exceptional points(EPs).Many new and unexpected features have been explored in the non-Hermitian systems with a great deal of recent interest.One of the most fascinating features is that chiral state conversion appears when one EP is encircled dynamically.Here, we propose an easy-controllable levitated microparticle system that carries a pair of EPs and realize slow evolution of the Hamiltonian along loops in the parameter plane.Utilizing the controllable rotation angle, gain and loss coefficients, we can control the structure, size and location of the loops in situ.We demonstrate that, under the joint action of topological structure of energy surfaces and nonadiabatic transitions, the chiral behavior emerges both along a loop encircling an EP and even along a straight path away from the EP.This work broadens the range of parameter space for the chiral state conversion, and proposes a useful platform to explore the interesting properties of exceptional points physics.     

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.     

Stationary Entanglement between Light and Microwave via Ferromagnetic Magnons

It is shown how to generate stationary entanglement between light and microwave in a hybrid opto-electro-magnonical system which mainly consists of a microwave cavity, a yttrium iron garnet (YIG) sphere, and a nanofiber. The optical modes in nanofiber can evanescently be coupled to whispering gallery modes, that are able to interact with magnon mode via spin–orbit interaction, in YIG sphere, while the microwave cavity photons and magnons are coupled through magnetic dipole interaction simultaneously. Under reasonable parameter regimes, pretty amount of entanglement can be generated, and it also shows persistence against temperature. The present work is expected to provide a new perspective for building more advanced and comprehensive quantum networks along with magnons for fast-developing quantum technologies and for studying the macroscopic quantum phenomena.     

Rabi oscillation and quantum decoherence of an optomechanical system with a three-level V-type atom trapped in a two-mode cavity

The European Physical Journal D - In this paper, we study a system consisting of two spatially separated cavities, where each cavity contains a magnon mode of YIG sphere coupled to a microwave...     

Observation of the exceptional point in superconducting qubit with dissipation controlled by parametric modulation

Open physical systems described by the non-Hermitian Hamiltonian with parity-time-reversal(PT) symmetry show peculiar phenomena, such as the presence of an exceptional point(EP) at which the PT symmetry is broken and two resonant modes of the Hamiltonian become degenerate. Near the EP, the system could be more sensitive to external perturbations and this may lead to enhanced sensing. In this paper, we present experimental results on the observation of PT symmetry broken transition and the EP using a tunable superconducting qubit. The quantum system of investigation is formed by the two levels of the qubit and the energy loss of the system to the environment is controlled by a method of parametric modulation of the qubit frequency. This method is simple with no requirements for additional elements or qubit device modifications. We believe it can be easily implemented on multi-qubit devices that would be suitable for further exploration of non-Hermitian physics in more complex and diverse systems.     

Proximity-Encirclement of Multiple Exceptional Points in Non-Hermitian Photonic Waveguides

Conventionally, dynamical encirclement of exceptional points in non-Hermitian systems is known to manifest a counterintuitive chiral state conversion. However, the prerequisite of such traits enclosing an exceptional point is broken when only encircling its proximity, preserving a still chiral switching. Research on the proximity-encirclement in multistate systems is lacking. In this paper, a photonic-waveguide-array non-Hermitian system is proposed to investigate the dynamics by encircling two exceptional points or their proximity. A series of encircling trajectories defined by the parametric equations are designed to steer the evolution of photonic modes in waveguides. The wave propagating along the waveguides is also simulated to capture this non-Hermitian physics. The chiral behavior in proximity-encirclement contrasts with the familiar encirclement of one exceptional point and exhibits the unexpected occurrence of nonadiabatic transitions. Furthermore, if two exceptional points are sufficiently encircled, the system will evolve to a stable final state earlier, as a symbol of the occurrence of the nonadiabatic transition. Such novel chiral conversion is maintained only if the encircling trajectories are located at adequate proximity.     

Two-body exceptional points in open dissipative systems

We study two-body non-Hermitian physics in the context of an open dissipative system depicted by the Lindblad master equation.Adopting a minimal lattice model of a handful of interacting fermions with single-particle dissipation,we show that the non-Hermitian effective Hamiltonian of the master equation gives rise to two-body scattering states with state-and interaction-dependent parity-time transition.The resulting two-body exceptional points can be extracted from the trace-preserving density-matrix dynamics of the same dissipative system with three atoms.Our results not only demonstrate the interplay of parity-time symmetry and interaction on the exact few-body level,but also serve as a minimal illustration on how key features of non-Hermitian few-body physics can be probed in an open dissipative many-body system.