Quantum and classical binomial distributions for the charge transmitted through coherent conductor

We discuss controversial results for the statistics of charge transport through coherent conductors. Two distribution functions for the charge transmitted was obtained previously, one actually coincides with classical binomial distribution, the other is different, and we call it here quantum binomial distribution. We show that high-order charge correlators, determined by the either distribution functions, all can be measured in different setups. The high-order current correlators, starting with the third order, reveal (missed in previous studies) special oscillating frequency dependence on the scale of the inverted time flight from the obstacle to the measuring point. Depending on setup, the oscillating terms give substantially different contributions.     

Calculating particle correlators with the account of detector efficiency

The formulae for m-order correlators K _m of a given particle observable (e.g., energy, transverse momentum or a conserved discrete quantum number) accounting for the track reconstruction efficiencies in a real detector are presented. The calculation of second- to fourth-order correlators is considered in some detail. Similar to the case of an ideal detector, the correlators can be expressed through the event-by-event fluctuation measures of the observable single event mean, the pseudocorrelators (determined by the pseudo-central moments of the observable distribution) and their cross terms. It allows one to avoid the combinatorics and essentially reduce the computer time when calculating the higher-order correlators in high multiplicity events. Compared with the case of ideal detector, this reduction is somewhat smaller due to the increased number of pseudocorrelators and additional calculations of the moments of the distribution of the track weights. For a constant track reconstruction efficiency, the correlator formulae reduce to those for an ideal detector. However, in real experiments the efficiencies are usually essentially dependent on particle momenta and may lead to substantial corrections of momentum correlators on the level of tens of percent.     

On the detection of finite-frequency current fluctuations

We consider a measurement of finite-frequency current fluctuations, using a resonance circuit as a model for the detector. We arrive at an expression for the measurable response in terms of the current-current correlators which differs from the standard (symmetrized) formula. The possibility of detection of vacuum fluctuations is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 280–284 (10 February 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Spin-spin correlators in the majorana representation

In the Majorana representation of a spin 1/2 we find an identity which relates spin-spin correlators to one-particle fermionic correlators. This should be contrasted with the straightforward approach in which two-particle (four-fermion) correlators need to be calculated. We discuss applications to the analysis of the dynamics of a spin coupled to a dissipative environment and of a quantum detector performing a continuous measurement of a qubit's state.     

Enhancing photon entanglement in a three-mode optomechanical system via imperfect phonon measurements

By considering a 3-mode optomechanical system formed by two cavities interacting with a common mechanical mode, we demonstrate that phonon-counting measurements lead to a significant enhancement of entanglement in the output of the two cavities. This conclusion still holds for an inefficient detector, but the dependence on system parameters changes qualitatively from the ideal limit of perfect projective measurements. We find non-trivial optimal points for the entanglement as functions of detector efficiency, measurement outcome, and optical drive strengths. We characterize both the highest achievable entanglement as well as a ‘typical' value, obtained at the most likely measurement outcome. Numerical results are well understood within an approximate analytical approach based on perturbation theory around the ideal detector limit.     

Interferometric flaw detection with an increased sensitivity of measurements in study of reflecting composite periodic structures

Methods that allow an interferometric comparison of the elementary structures of a reflecting composite periodic structure and a reference structure are described. Specific features of the realization of these methods by interferometric correlators are considered in the case of studying specular and diffusely reflecting composite structures with an increase in the sensitivity of measurements. Experimental results of testing these methods in application to the study of plane metal surfaces with cylindrical depressions are presented. Interference patterns with a twofold increase in the measurement sensitivity are obtained.     

物体表面形貌的正弦相位调制实时干涉测量技术研究

表面形貌干涉测量技术是一种高精度的非接触式测量技术,在工业生产和科学研究中具有广泛的应用。提出一种实时测量表面形貌的正弦相位调制干涉测量新技术。该技术用激光二极管作光源,用自制的高速图像传感器探测干涉信号,通过信号处理电路实时解相得到被测表面所对应的相位分布,实时分析相位获得物体表面形貌。该技术消除了光强和部分外界干扰的影响,提高了系统的测量精度。楔形光学平板表面形貌的测量结果表明,测量点为60×60个的情况下,测量时间小于8.2 ms,重复测量精度(RMS)为4.3 nm。     

Detection of finite-frequency current moments with a dissipative resonant circuit

We consider the measurement of higher current moments with a dissipative resonant circuit, which is coupled inductively to a mesoscopic device in the coherent regime. Information about the higher current moments is coded in the histograms of the charge on the capacitor plates of the resonant circuit. The role of dissipation is shown to be essential for the measured noise to remain finite. We also identify which combination of current correlators enters the measurement of the third moment. The results are illustrated for a quantum point contact.     

Intensity correlations of thermal light

We demonstrate measurement of normalized Glauber’s intensity correlation functions of different orders using an array photodetector. As the light source, we use a laser beam scattered by a rotating ground-glass disc, which has statistics close to that of thermal light. We compare the measurements of the normalized correlation functions to that of the difference-intensity variance and show that they are in a certain sense complementary. The independence of the variance measurement on the number of temporal modes has been demonstrated for the first time. Different versions of high-order ghost imaging are also realized and characterized quantitatively.     

Experiment planning: Optimal measurement by the minimal sample dispersion criterion

The criteria of optimum measurements based on a minimum dispersion and the variational problem of optimum distribution of measurements are formulated. An analytical solution for the density of measurement distribution in the case of optimum measurement is obtained and the discrete equivalent of the optimal measurement is constructed. Measurements with optimal distribution have the most stable characteristics. They provide the minimum loss of accuracy when the numbers of measurements are both changed and redistributed. An optimal measurement of any portion of measurements (reading of any detector) has the same contribution to the determination of the accuracy of measurements, i.e., optimal measurements provide equivalence (homogeneity) of the measurements at different points. Optimal measurements provide the best measure of not only the values of the function, but also of the values of an arbitrary parameter of this function. In this case, the best of the quadratic forms to determine an arbitrary parameter of measured function is the expression for the distribution function χ². Continuous representation of discretely defined distribution function χ², corresponding exactly to optimal distribution of the measurements is proposed. An example of solving the problem of planning the experiment is given: measuring the spatial parameters of the laser beam with the lowest standard deviation of a limited number of photodetectors.     

Correlators of hadron currents: The model and the ALEPH data on τ-decay

The model with the meson spectrum, consisting of zero-width equidistant resonances, is considered with connection to current correlators in coordinate space. The comparison of the explicit expressions for the correlators, obtained in this model, with the experimental data of ALEPH Collaboration on τ-decay is made and good agreement is found.     

Leggett-Garg inequality with a kicked quantum pump

A kicked quantum nondemolition measurement is introduced, where a qubit is weakly measured by pumping current. Measurement statistics are derived for weak measurements combined with single-qubit unitary operations. These results are applied to violate a generalization of the Leggett-Garg inequality. The violation is related to the failure of the noninvasive detector assumption, and may be interpreted as either intrinsic detector backaction, or the qubit entangling the microscopic detector excitations. The results are discussed in terms of a quantum point contact kicked by a pulse generator, measuring a double quantum dot.     

Measuring fluence of fast neutrons with planar silicon detectors

The results of measurements of 1-MeV (Si) equivalent fast neutron fluence with silicon planar detectors are reported. The measurement method is based on the linear dependence of the reverse detector current increment on the neutron fluence: ΔI = α _I × Φ × V. This technique provides an opportunity to measure the equivalent fluence in a wide dynamic range from 108 to 10¹⁶ cm^–2 with an unknown neutron energy spectrum and without detector calibration. The proposed method was used for monitoring in radiation resistance tests of different detector types at channel no. 3 of IBR-2 and for determining the fluence of fission and leakage neutrons at the KVINTA setup.     

热光的高阶双缝干涉

干涉效应反映了光场的相干性,非相干光源中无序性会破坏干涉.然而对于高阶干涉来说这一看法并不成立.文章作者最近的理论和实验研究表明,横向传播方向无序的热光源可以实现高阶双缝干涉.尽管单个探测器的强度分布是均匀的,处于不同位置的两个探测器的联合强度关联却出现了干涉条纹.当两个探测器同步反向移动时,条纹间距减小为一阶干涉条纹的一半.实验结果同不久前报道的在自发参量下转换过程中产生的纠缠双光子对的双缝实验中所观察到的亚波长干涉效应十分类似.实验结果可以用多模热光场的二阶空间关联性质来解释.热光的高阶双缝干涉是著名的Hanbury-Brown和Twiss实验的空间干涉版本,因此也可称其为Hanbury-Brown和Twiss型双缝干涉。     

Influence of an electret on the sensitivity of CR-39 nuclear track detector in diffusion chamber

Cylindrical diffusion chamber for radon measurement equipped with nuclear track detector (CR-39) and electretis considered in this work. Electret is electrostatic analogous of permanent magnet which preserves constant and strong electric field for long time period. Electrets were used to attract radon progeny formed in a cup where CR-39 was placed for radon measurements. In this way the sensitivity of detector could be increased by bringing progeny just in front of detector in more convenient measuring geometry. Influence of distance between electret and detector on sensitivity of such equipment has been investigated theoretically. Different geometrical setups of detector and electret were tested in order to optimize this kind of device. Some results are presented in this report.      

Charge and current beats in T-shaped qubit–detector systems

The time evolution of a charge qubit coupled electrostatically with different detectors in the forms of single, double and triple quantum dot linear systems in the T-shaped configuration between two reservoirs is theoretically considered. The correspondence between the qubit quantum dot oscillations and the detector current is studied for different values of the inter-dot tunneling amplitudes and the qubit–detector interaction strength. We have found that even for a qubit coupled with a single QD detector, the coherent beat patterns appear in the oscillations of the qubit charge. This effect is more evident for a qubit coupled with double or triple-QD detectors. The beats can be also observed in both the detector current and the detector quantum dot occupations. Moreover, in the presence of beats the qubit oscillations hold longer in time in comparison with the beats-free systems with monotonously decaying oscillations. The dependence of the qubit dynamics on different initial occupations of the detector sites (memory effect) is also analyzed.     

Continuous‐scan capability at SSRL and applications to X‐ray diffraction

Typical X‐ray diffraction measurements are made by moving a detector to discrete positions in space and then measuring the signal at each stationary position. This step‐scanning method can be time‐consuming, and may induce vibrations in the measurement system when the motors are accelerated and decelerated at each position. Furthermore, diffraction information between the data points may be missed unless a fine step‐scanning is used, which further increases the total measurement time. To utilize beam time efficiently, the motor acceleration and deceleration time should be minimized, and the signal‐to‐noise ratio should be maximized. To accomplish this, an integrated continuous‐scan system was developed at the Stanford Synchrotron Radiation Lightsource (SSRL). The continuous‐scan system uses an in‐house integrated motor controller system and counter/timer electronics. SPEC software is used to control both the hardware and data acquisition systems. The time efficiency and repeatability of the continuous‐scan system were tested using X‐ray diffraction from a ZnO powder and compared with the step‐scan technique. Advantages and limitations of the continuous‐scan system and a demonstration of variable‐velocity continuous scan are discussed.     

Dispersion effect on the current voltage characteristic of AlGaN/GaN high electron mobility transistors

The current voltage (IV) characteristics are greatly influenced by the dispersion effects in AlGaN/GaN high electron mobility transistors. The direct current (DC) IV and pulsed IV measurements are performed to give a deep investigation into the dispersion effects, which are mainly related to the trap and self-heating mechanisms. The results show that traps play an important role in the kink effects, and high stress can introduce more traps and defects in the device. With the help of the pulsed IV measurements, the trapping effects and self-heating effects can be separated. The impact of time constants on the dispersion effects is also discussed. In order to achieve an accurate static DC IV measurement, the steady state of the bias points must be considered carefully to avoid the dispersion effects.     

基于复合结构的气体电子倍增器增益模拟和实验研究

气体电子倍增器(GEM)作为高性能的微结构气体探测器在高能物理相关领域内得到了广泛的研究和应用.其中增益是GEM探测器基本性能研究中的一个重要参数,该值的精确测量至关重要.增益的测量一般采用电流测量或者能谱测量方法,但均存在精度较低或者过程繁琐的问题,且无法精确测量低增益值.针对GEM探测器增益的精确测量,本文提出了一种由GEM探测器与微网结构气体探测器(MM)级联构成的复合结构探测器(GEM-MM).利用GEM-MM结构以相对方法实现GEM增益的精确测量.该方法既可以省去传统方法中复杂的电子学标定过程,同时不需要进行原初电离电子数的估算,保证了增益的精确测量,并且可以实现GEM低增益的测量.基于GEM-MM测量GEM增益的原理,本文首先对GEM-MM电荷输运过程进行了模拟研究,优化了合适的工作电压.比较了三种不同类型和配比工作气体下GEM增益模拟结果,并在Ar/iC_4H_(10)(95/5)气体中测量了单层GEM在3—24范围内的有效增益.不同Penning系数下GEM增益的模拟结果表明,Penning系数为0.32时GEM增益的模拟结果与实验测量结果符合得很好.由此可以确定一个大气压下的Ar/iC_4H_(10)(95/5)气体中,Penning系数为0.32±0.01.