栅极电势对强光场下石墨烯场效应管中电子隧穿的影响

利用时域有限差分方法研究了强光场下石墨烯场效应管中栅极电势对电子隧穿的影响. 在强光场下由于光学stark效应,石墨烯场效应管的完美手征透射被抑制.这种抑制除了 可以利用光场来调控外,也可以通过改变栅极电势的宽度、势垒高度等来调控. 研究了非方势垒中电子的隧穿. 研究发现,当电势的倾斜较小时,电子隧穿概率变化不大.而当电势倾斜很大时,电子隧穿概率急剧改变.     

氢原子的固有电偶极矩

用经典力学和双波量子力学计算了氢原子的固有电偶极矩。双波量子理论算得的结果在经典极限下与经典力学的结果一致。普通量子力学对氢原子Stark效应中表现出来的电偶极矩难以做出很好的解释，因为一个波函数描述的是系综而不是单个粒子。经典力学和双波量子力学可描述单个粒子的行为，对永久电偶极矩的计算和解释显得自然而合理。     

Relative entropy of entanglement of two-qubit ’X’ states

Two closest single-qubit states could be diagonalised by the same unitary matrix,which helps to find the relative entropy of entanglement of a two-qubit ’X’ state.We formulate two binary equations for the relative entropy of entanglement and the corresponding closest separable state of a given two-qubit ’X’ state.This approach can be applied to get the relative entropy of entanglement of many widely-discussed two-qubit states,such as pure states,Werner states,and so on.     

哈密顿量诱导的量子演化速度

在研究量子态的演化问题时,量子演化速率往往定义为量子初态与其演化态之间的态距离随时间的变化率.本文将量子演化的基本理论与线性代数方法相结合,通过量子态演化的路径距离来研究量子系统的演化.量子幺正演化系统中,量子演化算符包含了量子演化的路径信息,路径距离的大小则决定于演化算符本征值的幅角主值分布.由量子态演化的路径距离随时间的变化率而得到的量子瞬时演化速率则正比于系统哈密顿量的最大与最小本征值之差.作为应用之一,利用量子演化的路径距离及哈密顿量诱导的瞬时演化速率,可以给出量子演化新的时间下限.此时间下限只与系统的演化算符及哈密顿量有关,而与量子初态的具体形式无关,这与量子系统真实演化时间所具有的性质一致.严格的理论证明以及两个演化实例的数值结果均表明,在[0,π/(2ω_H)]时间范围内,本文给出的演化时间下限与真实演化时间重合,是真实演化时间的准确预测.通过量子演化的路径距离及相应演化速率来研究量子系统的演化,为相关问题的解答提供了新的思路和方法.     

Reversible waveform conversion between microwave and optical fields in a hybrid opto-electromechanical system

We present a scheme of reversible waveform conversion between microwave and optical fields in the hybrid opto-electromechanical system. As an intermediate interface, nanomechanical resonator optomechanically couples both optomechanical cavities in the optical and microwave frequency domains. We find the double-optomechanically induced transparency and achieve coherent signal waveform bi-directional transfer between microwave and optical fields based on quantum interference. In addition, we give an analytical expression of one-to-one correspondence between the microwave field and the optical output field, which intuitively shows the reversible waveform conversion relationship. In particular, by numerical simulations and approximate expression, we demonstrate the conversion effects of the three waveforms and discuss the bi-directional conversion efficiency and the bandwidth. such a hybrid opto- and electro-mechanical device has significant potential functions for electro-optic modulation and waveform conversion of quantum microwave-optical field in optical communications and further quantum networks.     

The duality of a single particle with an n-dimensional internal degree of freedom

The wave-particle duality of a single particle with an n-dimensional internal degree of freedom is re-examined theo- retically in a Mach-Zehnder interferometer. The famous duality relation D2 ＋ V2 〈 1 is always valid in this situation, where D is the distinguishability and V is the visibility. However, the sum of the particle information and the wave information, D2 V2, can be smaller than one for the input of a pure state if this initial pure state includes the internal degree of freedom of the particle, while the quantity D2~ V2 is always equal to one when the internal degree of freedom of the particle is excluded.     

光束分离器算符的分解特性与纠缠功能

光束分离器是量子光学中的基本线性器件之一, 它在量子纠缠态的制备与测量上起着重要作用. 基于光束分离器(BS)对算符的矩阵变换关系, 本文导出了BS算符在若干表象中的自然表示. 利用这个自然表示(而非SU(2)李代数关系)及有序算符内的积分技术, 可直接导出BS算符的正规乘积、紧指数表示及多种分解形式. 此外, 可直接导出一种纠缠态表象及其Schmidt分解. 这对于讨论连续变量量子隐形传输是十分方便的. 关键词： 光束分离器算符 纠缠态表象 有序算符内的积分技术 Schmidt分解     

Quantum speed limit for mixed states in a unitary system

Since the evolution of a mixed state in a unitary system is equivalent to the joint evolution of the eigenvectors contained in it, we could use the tool of instantaneous angular velocity for pure states to study the quantum speed limit (QSL) of a mixed state. We derive a lower bound for the evolution time of a mixed state to a target state in a unitary system, which automatically reduces to the quantum speed limit induced by the Fubini-Study metric for pure states. The computation of the QSL of a degenerate mixed state is more complicated than that of a non-degenerate mixed state, where we have to make a singular value decomposition (SVD) on the inner product between the two eigenvector matrices of the initial and target states. By combing these results, a lower bound for the evolution time of a general mixed state is presented. In order to compare the tightness among the lower bound proposed here and lower bounds reported in the references, two examples in a single-qubit system and in a single-qutrit system are studied analytically and numerically, respectively. All conclusions derived in this work are independent of the eigenvalues of the mixed state, which is in accord with the evolution properties of a quantum unitary system.     

New fractional entangling transform and its quantum mechanical correspondence

In this Letter, a new fractional entangling transformation(Fr ET) is proposed, which is generated in the entangled state representation by a unitary operator expfiθeabt abTg where aebT is the Bosonic annihilate operator. The operator is actually an entangled one in quantum optics and differs evidently from the separable operator, expfiθeaa t bbTg, of complex fractional Fourier transformation. The additivity property is proved by employing the entangled state representation and quantum mechanical version of the Fr ET. As an application, the Fr ET of a two-mode number state is derived directly by using the quantum version of the Fr ET, which is related to Hermite polynomials.     

Simultaneous Measurement of Fringe Visibility and Path Predictability of Wave-Particle Duality

An experimental scheme to simultaneously obtain the information of fringe visibility and path predictability is designed. In a modified Young's double-slit experiment, two density filters rotating at different frequencies are placed before the two pineholes to encode path information. The spatial and temporal distributions of the output provide us with the wave and particle information of the single photons, respectively. The simultaneous measurement of the wave and particle information inevitably disturbs the system and thus causes some loss of the duality information, which is equal to the mixedness of the photonic state behind the density filters.