由杂质自旋散射引起的纠缠

研究了自由电子与固定杂质自旋的散射过程中的纠缠行为.研究发现,固定自旋的杂质能起到纠缠过滤器的作用,实现自由电子间的纠缠浓缩.当两无相互作用的电子先后经过同一Kondo杂质并与之发生散射时,可以使得原本不纠缠的电子发生纠缠.类似地,当运动的电子先后经过两无相互作用的Kondo杂质并与它们发生散射,可以使得原本不纠缠的K...     

读出效率对光与原子纠缠产生的影响

在光与原子纠缠态产生中,自旋波读出效率是影响纠缠质量的一个重要因素.本文在实验和理论上研究了读出效率与纠缠质量(Bell参量)的关系.实验上利用~(87)Rb冷原子系综中的自发Raman散射过程产生了光与原子量子纠缠.通过改变读光功率或OD (光学厚度),实现了读出效率的变化.在此基础上,研究了光与原子纠缠质量(Bell参量)随读出效率变化的关系.该实验将为高保真度的光与原子纠缠产生提供帮助.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

一种研究自旋翻转散射效应的新方法

金属中自旋翻转散射长度远长于电子平均自由程，近来关于自旋翻转散射效应的研究主要集中于扩散区域．文章作者提出了一种使用双势垒磁性隧道结来研究纳米尺度结构中弹道区域的自旋翻转散射效应的新方法．这种方法可以从磁电输运性质的测量，得出中间隔离层中的自旋翻转散射效应的温度和偏压关系，进一步可以得出诸如电子平均自由程和自旋翻转散射长度等自旋散射信息，以及中间层的态密度和量子阱信息．     

应用级联运动方程方法研究并联双量子点的量子纠缠（英文）

量子点体系是一类具有代表性的量子杂质体系.体系与环境之间的耗散过程和电子间的强相互作用对量子态的相干、纠缠有着极其重要的影响.多体效应和电子转移之间的竞争对局域自旋态之间的量子纠缠特性具有重要影响.本文应用级联运动方程方法对强关联双量子点系统的纠缠进行了定量研究.同时探究了总系统与子系统的信息熵之间的关系.     

光学腔增强光与冷原子耦合及自旋波读出效率的实验研究

在87Rb冷原子系综中,通过自发拉曼散射过程产生了光子与原子自旋波的关联对.利用光学腔增强光与原子的相互作用,使光子和原子自旋波关联对的产生率增加了3.8倍.同时,我们也研究了光学腔对原子自旋波读出效率的影响.结果表明,腔使自旋波的读出效率提高了1.5倍.本文的研究结果为下一步产生高恢复效率的光与原子量子纠缠提供了实验...     

基于量子费希尔信息的纠缠判据

对于几类常见的量子态, 对自旋压缩和量子Fisher信息进行了比较研究.得到量子Fisher信息的表达式,以Fisher信息为基础,给出了量子纠缠的判据,结果表明:比较于自旋压缩,该判据有明显的优势.(1).对于两个Qubit的对称态,该判据与自旋压缩,concurrence完全等价;(2).对于两个Qubit的非对称态的纠缠,自旋压缩不能检验其纠缠,但对于部分态,该判据也能检验;(3).对于Dicke态, 自旋压缩不能检验其纠缠,但该判据能检验且完全等价于concurrence.     

基于量子Fisher信息的纠缠判据

对于几类常见的量子态,对自旋压缩和量子Fisher信息进行了比较研究.得到量子Fisher信息的表达式,以Fisher信息为基础,给出了量子纠缠的判据,结果表明;比较于自旋压缩,该判据有明显的优势.(1).对于两个Qubit的对称态,该判据与自旋压缩,concurrence完全等价;(2).对于两个Qubit的非对称态的纠缠,自旋压缩不能检验其纠缠,但对于部分态,该判据也能检验;(3).对于Dicke态,自旋压缩不能检验其纠缠,但该判据能检验且完全等价于concurrence.     

耦合量子点中的纠缠测量

运用Gurvitz提出的求解薛定谔方程的方法并结合数值计算,分析了两个自旋相反的电子在耦合双量子点中的振荡和纠缠情况,以及如何利用量子点接触读出纠缠信息.结果显示两电子通过库伦相互作用形成量子纠缠态.强库伦作用下,它们黏在一起运动,类似于单电子在量子比特中的振荡.这种情况下,把量子点接触探测器靠近耦合双量子点,可以通过探测器的电流变化率读取电子的纠缠信息.     

基于量子费希尔信息的纠缠判据

对于几类常见的量子态, 对自旋压缩和量子Fisher信息进行了比较研究.得到量子Fisher信息的表达式,以Fisher信息为基础,给出了量子纠缠的判据,结果表明:比较于自旋压缩,该判据有明显的优势.(1).对于两个Qubit的对称态,该判据与自旋压缩,concurrence完全等价;(2).对于两个Qubit的非对称态的纠缠,自旋压缩不能检验其纠缠,但对于部分态,该判据也能检验;(3).对于Dicke态, 自旋压缩不能检验其纠缠,但该判据能检验且完全等价于concurrence.     

First order QED processes in a spatially flat FLRW space-time with a Milne-type scale factor

The quantum electrodynamics(QED)in a spatially flat(1+3)-dimensional Friedmann-Lema?tre-Robertson-Walker(FLRW)space-time with a Milne-type scale factor is outlined focusing on the amplitudes of the allowed processes in the first order perturbations.The definition of the transition rates is reconsidered such that an appropriate angular behavior of the probability for creation of an electron-positron pair from a photon is obtained,which has a similar rate as the creation of a photon and an electron-positron pair from vacuum.It is shown that these processes are allowed only in the first order perturbations,since the photon emission or absorption by an electron or positron are forbidden.     

Resonance two-photon pair production on nuclei and on electrons

Expressions are obtained for the cross section of photoproduction of an electron-positron pair on a nucleus and on an electron in the field of a weak electromagnetic wave in the resonance region corresponding to the virtual electron (positron) being on the mass shell. The magnitude of these cross sections relative to ordinary pair production by a photon on a nucleus and on an electron is estimated. It is shown that this mechanism can enhance pair production by a photon on a nucleus and on an electron in the resonance region with simultaneous detection of the electron (positron) at a large angle.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 12–15, February, 1981.We thank A. A. Sokolov for his interest in the work.     

Detecting entanglement using a double-quantum-dot turnstile

We propose a scheme based on using the singlet ground state of an electron spin pair in a double-quantum-dot nanostructure as a suitable setup for detecting entanglement between electron spins via the measurement of an optimal entanglement witness. Using time-dependent gate voltages and magnetic fields the entangled spins are separated and coherently rotated in the quantum dots and subsequently detected at spin-polarized quantum point contacts. We analyze the coherent time evolution of the entangled pair and show that by counting coincidences in the four exits an entanglement test can be done. This setup is close to present-day experimental possibilities and can be used to produce pairs of entangled electrons "on demand."     

北京正负电子对撞机中的束束作用偏转效应

 利用静电分离器对正负电子束团在垂直方向上的相对轨道偏差进行了扫描,并用数字示波器对束流轨道的变化进行监视,在北京正负电子对撞机(BEPC)上完成了垂直方向上的束束作用偏转效应的观察与测量。实验过程和实验结果可为北京正负电子对撞机改进工程(BEPCⅡ)的对撞调节提供参考依据。把束团之间相对位置偏差的测量转换为对束流偏转角度的测量是可行的,也是有效克服束流位置探测器分辨率不足的一种方法。     

Role of final state correlations in electron and positron impact ionization of H and He

Single ionization of H and double ionization of He are investigated using a multiple scattering approach and a correlated final state wavefunction, which satisfies the correct asymptotic condition for the electron and positron impact ionization of atoms. Analyses presented here show the influence of the Coulomb correlations in the final state on the three- and fivefold differential cross sections. Situations are discussed in which consideration of every pair of electron-electron and/or electron-positron correlation in the final state becomes important for physically satisfactory results.     

Photon bremsstrahlung from an extreme relativistic hydrogen plasma

The effect of the electron-positron pair creation on the intensity of radiation by an extreme relativistic plasma is studied. The degeneracy of the electron and positron gases is taken into account. It is found that the pair creation is of considerable importance for both the quantity and temperature dependence of the radiation intensity.Dedicated to Professor V. Votruba on his sixtieth birthday.     

利用信息流方法优化多激发自旋链中的量子态传输

研究了量子态在一条均匀耦合的反铁磁自旋链中传输时, 信道中自旋激发数变化对其传输性质的影响. 利用信息流方法分析输出端粒子的算符演化动力学, 获得了量子态传输的平均保真度与信道自旋初态之间的关系. 结果表明, 平均保真度的大小只依赖于信道中自旋激发数的奇偶性. 通过比较在奇偶激发信道中获得的最大平均保真度, 构建了优化信道来提升量子态在自旋链中的传输质量. 进一步分析了纠缠在激发信道中的传输情况, 发现纠缠的传输质量不仅和信道中自旋激发的具体个数有关, 还取决于激发自旋的初始排列. 特别地, 当信道中自旋无激发或全部激发时, 纠缠传输的大小和持续时间都优于其他的激发信道. 上述研究结果有助于在实际系统中搭建适合量子态和纠缠传输的量子信道.     

Entanglement of spin pairs in alternating open spin-1/2 chains with the XY Hamiltonian

We investigate entanglement of spin pairs in alternating open spin chains (s = 1/2) with spin-spin interactions (SSI) in the thermodynamic equilibrium state in an external magnetic field. The reduced density matrix of an arbitrarily chosen spin pair was calculated. The entanglement of a spin pair was evaluated with the Wootters criterion. The temperature at which the entangled state arises in the chosen pair was determined. Entanglement (concurrence) is shown to oscillate as a function of the position of a spin pair in the chain. The results demonstrate the dependence of the entanglement in arbitrarily chosen pairs of neighboring spins on the temperature, the position of the spin pair in the chain, chain length, and the ratio between the SSI constants. Qualitative explanation of these dependences is offered. The role of the terminal spins in the generation of entanglement is explained.     

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: I. Electron-positron pair production

The production of electron-positron pairs in a vacuum neutron star magnetosphere is investigated for both low (compared to the Schwinger one) and high magnetic fields. The case of a strong longitudinal electric field where the produced electrons and positrons acquire a stationary Lorentz factor in a short time is considered. The source of electron-positron pairs has been calculated with allowance made for the pair production by curvature and synchrotron photons. Synchrotron photons are shown to make a major contribution to the total pair production rate in a weak magnetic field. At the same time, the contribution from bremsstrahlung photons may be neglected. The existence of a time delay due to the finiteness of the electron and positron acceleration time leads to a great reduction in the electron-positron plasma generation rate compared to the case of a zero time delay. The effective local source of electron-positron pairs has been constructed. It can be used in the hydrodynamic equations that describe the development of a cascade after the absorption of a photon from the cosmic gamma-ray background in a neutron star magnetosphere.     

Experiments on the physics of charged particle beams at the VEPP-4M electron-positron collider

The VEPP-4M accelerator facility with a universal KEDR detector is designed to conduct experiments with colliding electron-positron beams. High-energy physics, nuclear physics, and studies using synchrotron radiation are the main directions of research with this facility. In addition, experiments on poorly explored issues in the physics of beams in the electron-positron storage ring and methodological studies to prepare an experiment aimed at testing corollaries of the CPT theorem for an electron and a positron are regularly conducted at the VEPP-4 facility. A number of works performed in recent years are described: studies to increase the accuracy of comparing the electron and positron spin precession frequencies by the resonant depolarization method; measurements of the count rate of Touschek electrons as a function of the beam energy in a wide range; comparison of the methods for measuring the beam energy spread; a study of the electron beam dynamics when a nonlinear betatron resonance is crossed.