Comparison of the Hanbury Brown-Twiss effect for bosons and fermions

Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source, highlighting the importance of two-photon correlations and stimulating the development of modern quantum optics. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms. By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: (3)He (a fermion) and 4He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom-atom correlation measurements can be used to reveal details in the spatial density or momentum correlations in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations.     

Observation of the radiative decay mode of the free neutron

The theory of quantum electrodynamics (QED) predicts that beta decay of the neutron into a proton, electron and antineutrino should be accompanied by a continuous spectrum of soft photons. While this inner bremsstrahlung branch has been previously measured in nuclear beta and electron capture decay, it has never been observed in free neutron decay. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework and heavy baryon chiral perturbation theory (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined a branching ratio of (3.13 +/- 0.34) x 10(-3) (68 per cent level of confidence) in the energy region between 15 and 340 keV, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; the characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. This result may provide opportunities for more detailed investigations of the weak interaction processes involved in neutron beta decay.     

反硝化过程中硝酸盐与亚硝酸盐之间对电子的竞争

分别测试硝酸盐和亚硝酸盐在反硝化过程中的降解速率,由于亚硝酸盐的降解速率低于硝酸盐,在此过程中会有亚硝酸盐的积累.后续的实验表明,这一降解速率的差别是因为两者反硝化菌对电子亲和力不同导致电子流的分布不同造成的.在反硝化初期,硝酸盐获得电子较多,因而其降解速率较快.当硝酸盐的质量浓度低于亚硝酸盐后,亚硝酸盐的降解速率较快,这表明电子流主要流向亚硝酸盐.     

确定同科电子原子态的一种方法

根据原子矢量模型提出确定同科电子原子态的一种方法．用该方法计算的原子和离子状态与其它方法的结果相同．     

Detection of coaxial backscattered electrons in SEM

We present a coaxial detection of the backscattered electrons in SEM. The lens-aperture has been used to filter in energy and focus the backscattered electrons. This particular geometry allows us to eliminate the low energy backscattered electrons and collect the backscattered electrons, which are backscattered close to the incident beam orientation. The main advantage of this geometry is adapted to topographic contrast attenuation and atomic number contrast enhancement. Thus this new SEM is very suitable to analyze the material composition. Foundation item: Supported by the National Natural Science Foundation of China (10045001) Biography: JIANG Chang-zhong (1962-), male, Ph. D, Associate professor.     

Extremely slow Drude relaxation of correlated electrons

The electrical conduction of metals is governed by how freely mobile electrons can move throughout the material. This movement is hampered by scattering with other electrons, as well as with impurities or thermal excitations (phonons). Experimentally, the scattering processes of single electrons are not observed, but rather the overall response of all mobile charge carriers within a sample. The ensemble dynamics can be described by the relaxation rates, which express how fast the system approaches equilibrium after an external perturbation. Here we measure the frequency-dependent microwave conductivity of the heavy-fermion metal UPd2Al3 (ref. 4), finding that it is accurately described by the prediction for a single relaxation rate (the so-called Drude response). This is notable, as UPd2Al3 has strong interactions among the electrons that might be expected to lead to more complex behaviour. Furthermore, the relaxation rate of just a few gigahertz is extremely low--this is several orders of magnitude below those of conventional metals (which are typically around 10 THz), and at least one order of magnitude lower than previous estimates for comparable metals. These observations are directly related to the high effective mass of the charge carriers in this material and reveal the dynamics of interacting electrons.     

Cerenkov自由电子激光的理论研究

本文利用等离子体的磁流体理论对相对论电子束穿过充满介质的园柱形波导的空腔时所引起的切伦柯夫辐射进行理论分析，并对其频率特性、不稳性增长率以及电磁场分布等给以定量讨论，给出了ＴＭｏｌ模自由电子激光的一些重要的数据结果     

电子与锂原子的弹性碰撞

采用处理慢电子被原子散射的等效势模型,计算能量在0.1～15eV范围内电子与锂原子系统的弹性散射总截面和微分截面,并把计算结果与一些实验和理论值进行对比,结果吻合较好.另讨论了Hammerling交换势中比例系数γ取不同数值时对结果的影响.     

非对称多势垒异质结构中电子的磁共振隧穿

以厄密函数作为包络波函数，运用传递矩阵方法系统地研究了具有抛物量子阱的非对称多势垒异质结构中电子隧穿的横向磁场效应。结果表明：对于不同的双势垒结构，传输系数的峰值随磁场的增强有的增大有的减小。对于三势垒结构，由于双量子阱准束缚态之间的耦合，共振时传输系数的峰值随磁场的增强呈现不同于双势垒结构的行为。第一峰值随磁场的增强单调减小，而第二峰值随磁场的增强先是减小而后增大，当峰值增大到几近于１时又开始减小。对于某些非对称多势垒异质结构具有比对称结构更好的横向磁场特性。考虑这一效应和电场效应对共振隧穿结构的设计具有指导意义     

Current measurement by real-time counting of single electrons

The fact that electrical current is carried by individual charges has been known for over 100 years, yet this discreteness has not been directly observed so far. Almost all current measurements involve measuring the voltage drop across a resistor, using Ohm's law, in which the discrete nature of charge does not come into play. However, by sending a direct current through a microelectronic circuit with a chain of islands connected by small tunnel junctions, the individual electrons can be observed one by one. The quantum mechanical tunnelling of single charges in this one-dimensional array is time correlated, and consequently the detected signal has the average frequency f = I/e, where I is the current and e is the electron charge. Here we report a direct observation of these time-correlated single-electron tunnelling oscillations, and show electron counting in the range 5 fA-1 pA. This represents a fundamentally new way to measure extremely small currents, without offset or drift. Moreover, our current measurement, which is based on electron counting, is self-calibrated, as the measured frequency is related to the current only by a natural constant.