Crossover between classical and quantum shot noise in chaotic cavities

The discreteness of charge in units of e led Schottky in 1918 to predict that the electrical current in a vacuum tube fluctuates even if all spurious noise sources are eliminated carefully. This phenomenon is now widely known as shot noise. In recent years, shot noise in mesoscopic conductors, where charge motion is quantum-coherent over distances comparable to the system size, has been studied extensively. In those experiments, charge does not propagate as an isolated entity through free space, as for vacuum tubes, but is part of a degenerate and quantum-coherent Fermi sea of charges. It has been predicted that shot noise in mesoscopic conductors can disappear altogether when the system is tuned to a regime where electron motion becomes classically chaotic. Here we experimentally verify this prediction by using chaotic cavities where the time that electrons dwell inside can be tuned. Shot noise is present for large dwell times, where the electron motion through the cavity is 'smeared' by quantum scattering, and it disappears for short dwell times, when the motion becomes classically deterministic.     

Bunching of fractionally charged quasiparticles tunnelling through high-potential barriers

Shot noise measurements have been used to measure the charge of quasiparticles in the fractional quantum Hall (FQH) regime. To induce shot noise in an otherwise noiseless current of quasiparticles, a barrier is placed in its path to cause weak backscattering. The measured shot noise is proportional to the charge of the quasiparticles; for example, at filling factor v=1/3, noise corresponding to q=e/3 appears. For increasingly opaque barriers, the measured charge increases monotonically, approaching q=e asymptotically. It was therefore believed that only electrons, or alternatively, three bunched quasiparticles, can tunnel through high-potential barriers encountered by a noiseless current of quasiparticles. Here we investigate the interaction of e/3 quasiparticles with a strong barrier in FQH samples and find that bunching of quasiparticles in the strong backscattering limit depends on the average dilution of the quasiparticle current. For a very dilute current, bunching ceases altogether and the transferred charge approaches q=e/3. This surprising result demonstrates that quasiparticles can tunnel individually through high-potential barriers originally thought to be opaque for them.     

Coherent transfer of Cooper pairs by a movable grain

Superconducting circuits that incorporate Josephson junctions are of considerable experimental and theoretical interest, particularly in the context of quantum computing. A nanometre-sized superconducting grain (commonly referred to as a Cooper-pair box) connected to a reservoir by a Josephson junction is an important example of such a system. Although the grain contains a large number of electrons, it has been experimentally demonstrated that its states are given by a superposition of only two charge states (differing by 2e, where e is the electronic charge). Coupling between charge transfer and mechanical motion in nanometre-sized structures has also received considerable attention. Here we demonstrate theoretically that a movable Cooper-pair box oscillating periodically between two remote superconducting electrodes can serve as a mediator of Josephson coupling, leading to coherent transfer of Cooper pairs between the electrodes. Both the magnitude and the direction of the resulting Josephson current can be controlled by externally applied electrostatic fields.     

Δ-Σ频率合成器带外量化噪声抑制技术

基于对电荷泵电流不匹配引起的高阶Δ-Σ调制器（DSM）量化噪声建模,提出一种改进型小数分频频率合成器（Frac-N）模型,即：在传统小数分频频率合成器的反馈支路上嵌入一个不含分频器的宽频带锁相环（PLL）构成的噪声滤除器（NF）。另外,为了减小鉴频鉴相器（PFD）输入端相位误差,设计了一个线性度能达到89%的电荷泵。该频率合成器采用0.18μm CMOS电路实现,仿真结果表明采用该噪声滤除技术可以对高阶DSM产生的带外量化噪声进行有效抑制。     

基于Monte Carlo模拟的实际纳米MOSFET散粒噪声抑制研究

实验测量和理论结果均表明,随着MOS器件尺寸缩小到纳米尺度,其过剩噪声的主要成分将从以热噪声为主转变为以散粒噪声为主,且散粒噪声受费米抑制作用和库仑抑制作用．而目前对纳米MOSFET散粒噪声抑制的研究时,采取了完全不考虑其抑制,或者只是强调抑制的存在而并未给出具体的抑制分析．本文将采用蒙特卡罗（Monte Carlo）模拟方法对实际纳米MOSFET电流噪声进行数值模拟研究,并考虑费米作用和库仑作用对散粒噪声的抑制影响,分别得到费米抑制因子和费米与库仑共同作用时的抑制因子．在此基础上,重点考察栅极电压、源漏电压、温度和掺杂浓度对散粒噪声抑制的影响及其关系的理论分析,得到的模拟结果与文献给出的实验结果和介观理论结果相吻合．     

脉冲噪声驱动的分数阶调和振子的均方位移

利用光钳实验获取浸没于粘弹性介质中粒子的均方位移数据并为其建模是微观流变学中计算介质的局部响应函数的一种常用方法. 本文研究一类由单个脉冲噪声驱动的分数阶调和振子的均方位移. 利用Laplace变换及双Laplace变换技巧,本文得到了振子位移的均值、方差及双时相关函数,进而求得均方位移的解析表达式. 基于Mittag-Leffler函数的渐进性质,本文研究了振子在短时和长时情形下的不同扩散行为. 研究表明,在短时振子弹道扩散而在长时则幂律地趋近于一个均衡值,即被板扎.     

Interference between two indistinguishable electrons from independent sources

Very much like the ubiquitous quantum interference of a single particle with itself, quantum interference of two independent, but indistinguishable, particles is also possible. For a single particle, the interference is between the amplitudes of the particle's wavefunctions, whereas the interference between two particles is a direct result of quantum exchange statistics. Such interference is observed only in the joint probability of finding the particles in two separated detectors, after they were injected from two spatially separated and independent sources. Experimental realizations of two-particle interferometers have been proposed; in these proposals it was shown that such correlations are a direct signature of quantum entanglement between the spatial degrees of freedom of the two particles ('orbital entanglement'), even though they do not interact with each other. In optics, experiments using indistinguishable pairs of photons encountered difficulties in generating pairs of independent photons and synchronizing their arrival times; thus they have concentrated on detecting bunching of photons (bosons) by coincidence measurements. Similar experiments with electrons are rather scarce. Cross-correlation measurements between partitioned currents, emanating from one source, yielded similar information to that obtained from auto-correlation (shot noise) measurements. The proposal of ref. 3 is an electronic analogue to the historical Hanbury Brown and Twiss experiment with classical light. It is based on the electronic Mach-Zehnder interferometer that uses edge channels in the quantum Hall effect regime. Here we implement such an interferometer. We partitioned two independent and mutually incoherent electron beams into two trajectories, so that the combined four trajectories enclosed an Aharonov-Bohm flux. Although individual currents and their fluctuations (shot noise measured by auto-correlation) were found to be independent of the Aharonov-Bohm flux, the cross-correlation between current fluctuations at two opposite points across the device exhibited strong Aharonov-Bohm oscillations, suggesting orbital entanglement between the two electron beams.     

基于分数阶傅里叶变换的量化噪声抑制方法

针对A/D转换器在采样和量化过程中产生的量化噪声问题,提出一种基于分数阶傅里叶变换的量化噪声抑制方法.利用经典量化器的统计特性,分析出量化噪声在分数阶傅里叶域具有白噪声特性,利用过采样和分数阶傅里叶域数字滤波方法对输入信号的量化噪声进行抑制,提高信号质量.理论推导和仿真结果表明,该方法能够有效抑制量化噪声,提高A/D转换器的输出信噪比,而且针对线性调频等信号的量化噪声抑制效果优于传统傅里叶域的方法.      

双势垒异质结构中自旋相关的散粒噪声

利用Landauer-Büttiker散射理论和传递矩阵方法研究了两端具有铁磁接触的双势垒异质结构(F/DB/F)中自旋相关的散粒噪声。计算结果表明:电流和散粒噪声随阱宽的增加发生周期性的振荡,随着垒厚的增加产生了明显的相位差,与自旋向上电子相比,垒厚对自旋向下电子的电流和散粒噪声影响更大。Rashba自旋轨道耦合强度的增加加大了电流和散粒噪声的振荡频率。偏压的增加减小了电流和散粒噪声的振荡频率,增大了电流和散粒噪声的峰谷比和峰值。电流和散粒噪声随自旋轨道耦合强度和偏压的变化强烈依赖于两铁磁电极中磁化方向的夹角。     

正常金属—d波超导结中的粗糙界面散射和准粒子有限寿命效应对散粒噪声的影响

考虑到粗糙界面散射和准粒子的有限寿命效应，利用散射理论，计算正常金属-d波超导结中的散粒噪声，研究表明，粗糙界面散射和准粒子的有限寿命效应都可导致平均电流与散粒噪声功率降低，而噪声功率与平均电流的比值都增加。     

Observation of a quarter of an electron charge at the nu = 5/2 quantum Hall state

The fractional quantum Hall effect, where plateaus in the Hall resistance at values of h/nue2 coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. (Here h is Planck's constant, nu the filling factor and e the electron charge.) Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of nu, the observation of the even denominator nu = 5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are 'non-abelian'-in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the nu = 5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the non-abelian nature of the nu = 5/2 state, it is the first step towards a full understanding of these new fractional charges.     

正常金属—绝缘层—d波超导结中的隧道谱与散粒噪声

在正常金属-绝缘层-d波超导隧道结中，考虑到绝缘层的势垒散射效应，运用Bogoliubov-de Gennes方程和Blonder-Tinkham-Klapwijk理论模型，计算系统的微分电导和散粒噪声随偏压的变化关系，研究表明，绝缘层的势垒高度与绝缘层的厚度都能使：（1）零偏压电导峰变得尖锐；（2）微分散粒噪声的峰位置向零偏压处位移；（3）散粒噪声功率与平均电流的比值随绝缘层厚度与高度的增大在大偏压下趋于2e值。     

喷丸强化对40#车轴钢静态拉伸及疲劳寿命的影响

通过对喷丸工艺处理后的４０＃ 车轴钢静态拉伸和疲劳寿命的测定,对试验结果进行了分析,找出了因喷丸产生的压应力距表面深度的变化曲线,通过成组的对比性试验后发现喷丸强化作用在初期效果显著,当裂纹穿过了喷丸强化层后,其强化作用就逐渐减弱直至消失。     

光超导波中正常态声子行为研究:声子对束缚态

考虑了光超导波理论中光声相互作用的高阶项，研究了声子与双光子的相互作用，利用推广的光子库柏对算符，在平均场近似下得出两动量相反的横光学声子可以通过交换虚光子库柏对产生一个有效的吸引力而形成声子对束缚态，其准粒子是由声子和库柏对组成的复合准粒子，讨论了这种情况下的光超导波基态能．     

Real-time detection of electron tunnelling in a quantum dot

Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations. Although there is ample experimental evidence that such correlations exist, electron dynamics in engineered nanostructures have been observed directly only on long timescales. The faster dynamics associated with electrical currents or charge fluctuations are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise or higher statistical moments of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor. We use electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 micros or more) inelastic scattering times in nearly isolated dots.     

纳米器件中双势垒结构的散粒噪声分析及抑制

分析了纳米电子器件中散粒噪声的散射理论,通过对电导问题和散射问题相似性的讨论,得到一种基于双势垒结构纳米器件的散粒噪声抑制的方法。     

正常金属—绝缘层—无序层—s波超导结中的隧道谱与散粒噪声

考虑到无序层中的体杂质散射以及绝缘界面层的粗糙散射,运用Bogoliubov-de Gennes(BdG)方程和Blonder-Tinkham-Klapwijk(BTK)理论模型,计算正常金属-绝缘层-无序层-s波超导隧道结系统的微分电导、平均电流和散粒噪声功率,研究表明,无序层中的杂质散射和粗糙界面散射都能抑制系统的微分电导、平均电流和散粒噪声功率。     

Quantum suppression of superconductivity in ultrathin nanowires

It is of fundamental importance to establish whether there is a limit to how thin a superconducting wire can be, while retaining its superconducting character--and if there is a limit, to determine what sets it. This issue may also be of practical importance in defining the limit to miniaturization of superconducting electronic circuits. At high temperatures, the resistance of linear superconductors is caused by excitations called thermally activated phase slips. Quantum tunnelling of phase slips is another possible source of resistance that is still being debated. It has been theoretically predicted that such quantum phase slips can destroy superconductivity in very narrow wires. Here we report resistance measurements on ultrathin (< or = 10 nm) nanowires produced by coating carbon nanotubes with a superconducting Mo-Ge alloy. We find that nanowires can be superconducting or insulating depending on the ratio of their normal-state resistance (R(N)) to the quantum resistance for Cooper pairs (Rq). If R(N) < Rq, quantum tunnelling of phase slips is prohibited by strong damping, and so the wires stay superconducting. In contrast, we observe an insulating state for R(N) > Rq, which we explain in terms of proliferation of quantum phase slips and a corresponding localization of Cooper pairs.     

介观互感电容耦合双谐振电路在压缩真空态下的量子涨落

对介观互感电容耦合电路作双模耦合谐振子处理,将其量子化.通过三次幺正变换,将体系的哈密顿量对角化,给出了体系的本征能谱,研究了压缩真空态下回路中电荷和电流的量子涨落.     

Compressed Multi-image Reconstruction Based on Quantization Noise Distribution

Because of the quantization noise introducedduring the compression, super-resolution reconstruction(SRR) techniques are complicated for the compressedimages. This paper aims to incorporate the prior knowledgeof discrete cosine transform(DCT) coefficients intomodeling the quantization noise. The spatial covariancematrix of the quantization noise is estimated by utilizing theLaplacian distribution of the alternating current(AC)coefficients. After estimating the spatial joint covariance ofoverall noises for the imaging system, we propose a generalBayesian framework to enhance the resolution forcompressed images. Experiments demonstrate theeffectiveness of the proposed algorithm and show thesuperiority to previous methods in objective and subjectiveaspects.