Radiation properties of high-order harmonic generation for the measurements of femto- and attosecond X-ray pulses

Radiation properties of high-order harmonic generation （HHG） are calculated for atoms in a strong laser field. The laser-duration dependence and the carrier-envelope-phase （CEP） dependence of HHG radiation properties are presented. The CEP dependence of the pure single distribution pulse of HHG radiation properties shows interesting 180° periodic structures. The quantum enhancement of the laser-assisted photo-ionization by femtosecond （1 fs=10^-15 s） and attosecond （1 as=10^-18 s） X-ray pulses and the interference patterns of photo-electron energy spectra are theoretically investigated. Transfer equations are presented for pulse reconstructions. The theoretical root-mean-square time （energy） differences of attosecond pulse reconstructions with different durations are less than 2 as （0,8 eV）. These methods may be developed as basic techniques to access ultra-fast measurements and molecular movie.     

中红外激光场中分子高次谐波生成的单轨道和多轨道效应

采用非微扰电动力学的方法研究了中红外激光场驱动分子产生的高次谐波能谱,并分析了不同分子轨道的贡献.研究发现,单个分子轨道产生的高次谐波能谱呈现出分子结构导致的干涉极小值.长脉冲中干涉极小值的位置随着激光强度增加向高能端移动,短脉冲中其位置固定.这种差别是由于不同脉宽的激光场中电子抖动动能的变化引起的.干涉极小值的最大移动量等于激光场中电子的抖动能.多个分子轨道辐射的谐波会产生干涉,这种干涉效应主要体现在两个方面:一是掩盖了单个轨道谐波谱的干涉极小值,从而使得总能谱中极小值不明显;二是通过总能谱中干涉结构的变化,体现出单轨道谐波谱中干涉极小值前后的相位突变.本研究解释了一系列的实验观测并预言了有待观测的现象.     

空间非均匀激光场下He原子辐射谐波的量子路径调控

理论研究了He原子在空间非均匀激光场下辐射谐波的量子路径调控.计算结果表明,随着空间非均匀激光场引入位置由负向-r0到正向-r0移动,谐波截止能量呈单调递增趋势,而且只有单一的短量子路径对最大谐波辐射过程起作用.通过分析谐波辐射时频分析图和电子含时波包演化图,对谐波辐射的特点给出了合理解释.随后适当引入一束太赫兹激光场,谐波强度被增强2个数量级,并且形成一个1208eV的超长平台区.最后,通过叠加谐波,可获得一系列持续时间在34as的超短脉冲,其波段覆盖为10~1nm.     

Fabry - Perot interference in a nanotube electron waveguide

The behaviour of traditional electronic devices can be understood in terms of the classical diffusive motion of electrons. As the size of a device becomes comparable to the electron coherence length, however, quantum interference between electron waves becomes increasingly important, leading to dramatic changes in device properties. This classical-to-quantum transition in device behaviour suggests the possibility for nanometer-sized electronic elements that make use of quantum coherence. Molecular electronic devices are promising candidates for realizing such device elements because the electronic motion in molecules is inherently quantum mechanical and it can be modified by well defined chemistry. Here we describe an example of a coherent molecular electronic device whose behaviour is explicitly dependent on quantum interference between propagating electron waves-a Fabry-Perot electron resonator based on individual single-walled carbon nanotubes with near-perfect ohmic contacts to electrodes. In these devices, the nanotubes act as coherent electron waveguides, with the resonant cavity formed between the two nanotube-electrode interfaces. We use a theoretical model based on the multichannel Landauer-Büttiker formalism to analyse the device characteristics and find that coupling between the two propagating modes of the nanotubes caused by electron scattering at the nanotube-electrode interfaces is important.     

Coherent dynamics of a flux qubit coupled to a harmonic oscillator

In the emerging field of quantum computation and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits (qubits). Single-qubit operations, direct coupling between two qubits and the realization of a quantum gate have been reported. However, complex manipulation of entangled states-such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments and cavity quantum electrodynamics-has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.     

超短脉冲高次谐波谱的连续性

研究超短脉冲（3个光学周期）高次谐波发射功率谱的性状及其成因. 结果表明, 短脉冲谐波谱与长脉冲（10个光学周期）相比, 其平台区末端的谐波谱变得伪连续. 这是由于3个原因导致该现象: (1) 在短脉冲情形下, 谐波的生成仅经历1~2次辐射, 在长脉冲情形下, 谐波的生成经历多次辐射的累加; (2) 对于相同的谐波级次, 短脉冲比长脉冲的谐波线宽宽; (3) 短脉冲比长脉冲的激光频谱宽.      

Isotope-induced partial localization of core electrons in the homonuclear molecule N2

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction ('gerade' and 'ungerade') are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N(2) through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species--a phenomenon analogous to the acquisition of partial 'which-way' information in macroscopic double-slit experiments.     

联合双原子高次谐波发射的动力学研究

首先从经典理论出发,分析了一维普薛耳-特勒势(P-T势)模型原子在激光场作用下的动力学行为。进而设定联合双原子模型的核间距。然后利用一束特殊设定的双色激光脉冲辐照该模型,在保证原子有适当的电离和较大的复合效率的前提下,获得了截止位置远达Ip 8Up且效率高达10-9的高次谐波发射。最后给出了电离电子空间波包随时间的演化行为,并从波包动力学出发对谐波发射功率谱的形成机制给出了合理的解释。     

Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such 'single-molecule magnets' (SMMs) straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization and quantum phase interference. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.     

一维模型原子高次谐波特性研究

通过数值计算求解含时的薛定谔方程,研究了在隧道电离区域16个激光脉冲作用于不同势阱势函数对应的一维模型原子产生的高次谐波特性,结果表明：在隧道电离区域势函数势阱的形状对一维模型原子的高次谐波强度产生较大影响,同时谐波的平台宽度的高阶区域也受到势函数的影响.     

RecBCD enzyme is a DNA helicase with fast and slow motors of opposite polarity

Helicases are molecular motors that move along and unwind double-stranded nucleic acids. RecBCD enzyme is a complex helicase and nuclease, essential for the major pathway of homologous recombination and DNA repair in Escherichia coli. It has sets of helicase motifs in both RecB and RecD, two of its three subunits. This rapid, highly processive enzyme unwinds DNA in an unusual manner: the 5'-ended strand forms a long single-stranded tail, whereas the 3'-ended strand forms an ever-growing single-stranded loop and short single-stranded tail. Here we show by electron microscopy of individual molecules that RecD is a fast helicase acting on the 5'-ended strand and RecB is a slow helicase acting on the 3'-ended strand on which the single-stranded loop accumulates. Mutational inactivation of the helicase domain in RecB or in RecD, or removal of the RecD subunit, altered the rates of unwinding or the types of structure produced, or both. This dual-helicase mechanism explains how the looped recombination intermediates are generated and may serve as a general model for highly processive travelling machines with two active motors, such as other helicases and kinesins.     

Amplitude spectroscopy of a solid-state artificial atom

The energy-level structure of a quantum system, which has a fundamental role in its behaviour, can be observed as discrete lines and features in absorption and emission spectra. Conventionally, spectra are measured using frequency spectroscopy, whereby the frequency of a harmonic electromagnetic driving field is tuned into resonance with a particular separation between energy levels. Although this technique has been successfully employed in a variety of physical systems, including natural and artificial atoms and molecules, its application is not universally straightforward and becomes extremely challenging for frequencies in the range of tens to hundreds of gigahertz. Here we introduce a complementary approach, amplitude spectroscopy, whereby a harmonic driving field sweeps an artificial atom through the avoided crossings between energy levels at a fixed frequency. Spectroscopic information is obtained from the amplitude dependence of the system's response, thereby overcoming many of the limitations of a broadband-frequency-based approach. The resulting 'spectroscopy diamonds', the regions in parameter space where transitions between specific pairs of levels can occur, exhibit interference patterns and population inversion that serve to distinguish the atom's spectrum. Amplitude spectroscopy provides a means of manipulating and characterizing systems over an extremely broad bandwidth, using only a single driving frequency that may be orders of magnitude smaller than the energy scales being probed.     

Coherent manipulation of semiconductor quantum bits with terahertz radiation

Quantum bits (qubits) are the fundamental building blocks of quantum information processors, such as quantum computers. A qubit comprises a pair of well characterized quantum states that can in principle be manipulated quickly compared to the time it takes them to decohere by coupling to their environment. Much remains to be understood about the manipulation and decoherence of semiconductor qubits. Here we show that hydrogen-atom-like motional states of electrons bound to donor impurities in currently available semiconductors can serve as model qubits. We use intense pulses of terahertz radiation to induce coherent, damped Rabi oscillations in the population of two low-lying states of donor impurities in GaAs. Our observations demonstrate that a quantum-confined extrinsic electron in a semiconductor can be coherently manipulated like an atomic electron, even while sharing space with approximately 10(5) atoms in its semiconductor host. We anticipate that this model system will be useful for measuring intrinsic decoherence processes, and for testing both simple and complex manipulations of semiconductor qubits.     

提取强混沌干扰中谐波信号的方法

提出一种新的从强混沌干扰中提取谐波信号的方法. 根据混沌的几何性质, 应用相空间重构技术和离散小波变换, 建立一种新的相空间投影方法, 将谐波信号分离出来, 并对含噪谐波信号进行功率谱密度估计, 提取谐波信号的频率, 利用小波滤波和最小二乘估计, 提高谐波信号的提取精度. 通过计算机仿真实验, 表明该方法非常有效.      

功率谱估计在扫描隧道显微术中的应用

扫描隧道显微镜(简称STM)所采集到的隧道电流信号,不仅包含着表征样品的表面形貌和分子结构的有效信号,而且还隐含着整个扫描隧道显微系统的动态特性.针对STM采集的信号,运用短时傅立叶变换对其波形数据进行功率谱密度估计,对信号在频率域中的特征向量加以提取.结果表明:基于谱空间的特征向量,准确地分辨出了工频在信号中的干扰,也能有效地解释和分析STM系统的动态特性,进而认为存在一小阻尼系统对信号施以影响;此外,分析的结果可以为STM扫描过程的控制,诊断以及扫描图像的重建和解释提供有力的理论指导.     

Single-pulse coherently controlled nonlinear Raman spectroscopy and microscopy

Molecular vibrations have oscillation periods that reflect the molecular structure, and are hence being used as a spectroscopic fingerprint for detection and identification. At present, all nonlinear spectroscopy schemes use two or more laser beams to measure such vibrations. The availability of ultrashort (femtosecond) optical pulses with durations shorter than typical molecular vibration periods has enabled the coherent excitation of molecular vibrations using a single pulse. Here we perform single-pulse vibrational spectroscopy on several molecules in the liquid phase, where both the excitation and the readout processes are performed by the same pulse. The main difficulty with single-pulse spectroscopy is that all vibrational levels with energies within the pulse bandwidth are excited. We achieve high spectral resolution, nearly two orders of magnitude better than the pulse bandwidth, by using quantum coherent control techniques. By appropriately modulating the spectral phase of the pulse we are able to exploit the quantum interference between multiple paths to selectively populate a given vibrational level, and to probe this population using the same pulse. This scheme, using a single broadband laser source, is particularly attractive for nonlinear microscopy applications, as we demonstrate by constructing a coherent anti-Stokes Raman (CARS) microscope operating with a single laser beam.     

Kondo physics in carbon nanotubes

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.     

子空间分解法在声目标特征提取中的应用

研究用于识别直升机目标声信号的特征提取方法,方法通过对直升机信号频特征分析,采用基于子空间分解的多重信号分类法算法提取信号谐波频率作为目标特征,利用子空产妥将观测数据分解为信号子这僮与噪声子空间特点,抑制噪声干扰,提高识别能力。     

Quantum computing in molecular magnets

Shor and Grover demonstrated that a quantum computer can outperform any classical computer in factoring numbers and in searching a database by exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm requires both superposition and entanglement of a many-particle system, the superposition of single-particle quantum states is sufficient for Grover's algorithm. Recently, the latter has been successfully implemented using Rydberg atoms. Here we propose an implementation of Grover's algorithm that uses molecular magnets, which are solid-state systems with a large spin; their spin eigenstates make them natural candidates for single-particle systems. We show theoretically that molecular magnets can be used to build dense and efficient memory devices based on the Grover algorithm. In particular, one single crystal can serve as a storage unit of a dynamic random access memory device. Fast electron spin resonance pulses can be used to decode and read out stored numbers of up to 105, with access times as short as 10-10 seconds. We show that our proposal should be feasible using the molecular magnets Fe8 and Mn12.