The dielectric constant of PbTe at 4.2 K and\tilde v=84.15 cm-1, 96.97cm−1, 103.60cm−1

The dielectric constant of a PbTe epitaxial layer has been measured by surface wave spectroscopy using an optically pumped far-infrared laser and the technique of attenuated total reflection.     

Giant violations of classical inequalities through conditional homodyne detection of the quadrature amplitudes of light

Conditional homodyne detection is proposed as an extension of the intensity correlation technique introduced by Hanbury-Brown and Twiss [Nature (London) 177, 27 (1956)]. It detects giant quadrature amplitude fluctuations for weakly squeezed light, violating a classical bound by orders of magnitude. Fluctuations of both quadrature amplitudes are anomalously large. The squeezed quadrature also exhibits an anomalous phase.     

Atomic spectroscopy with squeezed light for sensitivity beyond the vacuum-state limit

A frequency tunable source of squeezed light has been developed which is suitable for a variety of spectroscopic applications. In initial experiments continuous tunability over a range of 2 GHz has been achieved with a directly observed nonclassical noise reduction of 6 dB relative to the vacuum-state limit in a balanced homodyne detector. A process of light-induced absorption in the nonlinear crystal has been identified as the principal loss mechanism which prevents the observation of yet larger degrees of squeezing. Although our source is potentially broadly tunable over the range of wavelengths from 840 to 970 nm, the current research centers on the performance at 852 nm for spectroscopy of the D ₂ line of atomic cesium. For frequency-modulated (FM) saturation spectroscopy in a vapor cell, an improvement of 3.1 dB in sensitivity relative to the usual quantum limit is demonstrated for the detection of Doppler-free resonances. When corrected for the thermal noise of the detector, the enhancement in signal-to-noise ratio brought by the squeezed field is 3.8 dB relative to the shot-noise limit set by the vacuum fluctuations of the probe field.     

Adiabatic Brownian ratchets with the inclusion of inertia

Inertial corrections to the drift velocity of a Brownian particle have been calculated for two main classes of Brownian ratchets operating in the adiabatic regime of fluctuations of the potential energy: first, the stationary periodic potential and dichotomic fluctuations of an external force with zero average value (rocking ratchet) and, second, dichotomic fluctuations of the periodic potential itself. It has been shown that, in contrast to passive transport at which the inertial correction always reduces the effective mobility and diffusion coefficients, inertial corrections for Brownian ratchets can play a constructive role, increasing the drift velocity at least at high temperatures.     

Magnetic-field induced coupling of quasi-degenerate phonon modes in the rare-earth compounds TmVO4 and TbF3

In this work the effects of magnetoelastic interaction in Rare-Earth compounds between excited crystal field states and nondegenerate optical phonons have been studied in tetragonal TmVO₄ and in orthorhombic TbF₃, using Raman and far-infrared spectroscopy. In certain favourable cases a mutual energy renormalization of the phonons has been observed as a function of a magnetic field (TmVO₄) or depending on the direction of the spontaneous magnetization of the crystal (TbF₃).In magnetically ordered TbF₃ an effective phonon-phonon coupling between modes of pure transverse and longitudinal polarization has been studied. This interaction results in an elliptical polarization of the coupled polar modes and transfers transition intensity to the quasi-longitudinal modes.The observed phenomena have been treated theoretically in full agreement with the experiments as higher order effects using the well established theory of magnetoelastic effects.     

Observation of Radial Propagation of Electrostatic Fluctuations in Edge Plasma of the Sino United Spherical Tokamak

Radial propagation of electrostatic fluctuations in the edge plasma of Sino-United Spherical Tokamak (SUNIST) has been measured using Langmuir probes. The propagation characteristics of the floating potential fluctuations are analysed by the two-point correlation technique. The results show radially outward propagation of the turbulent fluctuations at all measured radial positions. The power-average wavenumber profile is approximately constant in plasma edge region and suddenly increases to the limiter. These results are in good agreement with the model predictions proposed by Mattor which suggests that the drift wave propagation may be a source of edge turbulence.     

FAR-INFRARED PHOTOCONDUCTIVITY MECHANISM ON SHALLOW IMPURITY IN GaAs

We report the far-infrared photoconductivity spectroscopy (FIRPCS) of the high purity n-GaAs grown by molecular beam epitaxial (MBE) technique. The current-voltage (I-V) measurement and FIRPCS under different electric field strength have been performed on high purity n-GaAs at 4.2K. Except for photothermal ionization process, a new far-infrared photoconductivity (FIRPC) mechanism is observed. This photo ionization process is assisted by impact ionization and depends on external electric field. This new mechanism originates from photo-field ionization.     

时域脉冲平衡零拍探测器的高精度自动平衡

时域脉冲平衡零拍探测器能够直接对光场量子态的正交分量进行测量, 是连续变量量子密钥分发系统的核心测量器件, 其测量精度与密钥分发系统的额外噪声及安全密钥速率密切相关. 本文理论分析了探测器双臂不平衡对测量精度的影响, 实验设计并研制了双臂可精确自动平衡的时域脉冲平衡零拍探测器. 通过高精度控制探测器一臂的光纤圆环曲率半径, 实现了光纤内部光场强度的精密衰减, 进而获得了探测器的精确自动平衡. 实验测试结果表明时域平衡零拍探测器双臂具有10^-5以上的平衡精度, 能够长时间稳定运行, 有效避免了探测器的输出电压进入非线性区或饱和区. 该探测装置可应用于连续变量量子密钥分发系统, 有效降低由于量子态信号探测过程引入的额外噪声, 提高系统的长期稳定性.     

The arrow of time

Dispersive Fourier transform spectroscopy (DFTS) is a technique for determining the optical constants of solids, liquids and gases from direct measurements of both the amplitude and phase of either their reyection or transmission coeficients. Measurements are usually made at normal incidence with the specimen in one arm of a Michelson interferometer. The technique has been widely used for measurements in the far-infrared region of the spectrum, where the phase measurement is comparatively straightforward. Recent developments have established the feasibility of extending the spectral range to the visible and ultraviolet. In this article the principles of DFTS are briefly reviewed, and a selection of results presented to illustrate the scope and potential of the technique.     

Theory of the effect of a dc current on surface polaritons in n-type silicon

A theoretical investigation has been made of the effect of a drift current on surface polaritons in n-type silicon. The current direction is taken to be the same as the propagation direction of the surface polaritons. Retardation is included, but damping is neglected. In obtaining the dispersion relation, the specular-reflection/mirror-image technique of Kliewer and Fuchs is used to handle the boundary conditions. The results indicate that an interaction takes place between the current and polaritons which gives rise to polariton instabilities for certain frequency ranges. These instabilities are a consequence of the presence of the surface.     

Optically detected resonance spectroscopy of interface fluctuation quantum dots

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.     

Measurement of density fluctuations on the JIPP T-IIU tokamak via an HCN laser scattering

Observation of density fluctuations in tokamak plasmas is important to study the plasma confinement and to perform high power heating of the plasma. We observed the density fluctuations by means of an HCN laser scattering method during rf heating in the ion-cyclotron range of frequency on a tokamak plasma.The density fluctuations at the drift wave frequency are not enhanced so much by the heating, but the frequency spectrum is shifted to higher frequency. The increase of the density fluctuation level during the heating has been observed only in low-frequency region owing to MHD activity.     

Phase-dependent fluctuations of resonance fluorescence beyond the squeezing regime

Phase-dependent quantum features of the light scattered by a two-level atom driven by a monochromatic laser were investigated theoretically using the method of conditional homodyne detection [Carmichael, Castro-Beltran, Foster, Orozco, Phys. Rev. Lett. 85 (2000) 1855]. The splitting of fluctuations into terms of second and third order correlations of the dipole noise is obtained analytically. For the out-of-phase quadrature and weak laser driving the former are known to be squeezed. The third order fluctuations, newly found in this paper, grow with the laser intensity, contaminate squeezing below saturation, and dominate above it. They are responsible for the non-classicality and non-Gaussianity of the fluorescence for moderate to strong driving. Conditional homodyne detection, in both time and frequency domains, illustrates more general phase-dependent effects than squeezing, and is much less restricted by finite collection and quantum detector efficiencies than standard homodyne detection schemes.     

Two-mode homodyne tomography of time-encoded single-photon ebits

We have developed a technique for dual-temporal-mode, ultrafast, time-domain, balanced homodyne detection. We have tested it by performing homodyne measurements on pairs of picosecond-duration temporal modes (or “time bins”) carrying a delocalized single photon. The experimental homodyne data have been used to reconstruct both the density matrix and the two-mode Wigner function of the delocalized state, thus showing the correlations existing between the pair of temporal modes carrying the single photon.     

Effect of the electric current on the Casimir force between graphene sheets

The dependence of the thermal component of the Casimir force and Casimir friction between graphene sheets on the drift velocity of charge carriers in one of the sheets has been analyzed. It has been shown that the drift motion results in the measurable change in the thermal Casimir force owing to the Doppler effect. The thermal Casimir force, as well as Casimir friction, increases strongly in the case of resonant photon tunneling, when the energy of an emitted photon coincides with the excitation energy of an electron-hole pair. In the case of resonant photon tunneling, the dominant contribution to the Casimir friction even at temperatures above room temperature comes from quantum friction caused by quantum fluctuations. Quantum friction can be detected in an experiment on the friction drag between graphene sheets in a high electric field.     

频域近红外光谱方法定量测量组织氧饱和度

通过精确测量光子密度波的相位延迟和幅度衰减,借助于一个光学参数已知的校准模型,可以定量检测组织的光学吸收参数和约化散射系数.通过测量多个波长下组织的吸收系数,就可以进一步求解出人体局部组织的血氧饱和度.血液－酵母溶液仿真模型和前臂动脉阻断实验的结果都表明该三波长的频域近红外光谱系统可以实时定量地检测组织氧饱和度的动态变化情况.     

平衡零拍探测器的改进

平衡零拍探测技术是连续变量量子信息科学研究中测量量子态量子噪声的最佳方法之一。在平衡零拍探测系统中,需要一对性能和结构尽量相同的光电探测器,以便很好地减去经典噪声,有效探测散粒噪声基准及光场噪声。根据基尔霍夫电流定律及光电二极管串联的办法,设计并制作出一种性能优良的低噪声平衡零拍探测器。该探测器从光电二极管串联节点处取出光电流相减信号,然后对信号进行放大,在对两探测器全同性要求降低的同时提高了平衡零拍探测的性能。实验结果表明,该探测器在2MHz处共模抑制比达39dB,能很好地满足量子信息对低噪声、线性增益及高共模抑制比的要求。     

Positron trapping measurements of high precision using a labelling technique

The small effects associated with positron trapping in metal defects are masked in practice not only by inherent statistical fluctuations, but also by electronic instabilities. the unique directional correlation of annihilation quanta allows us to use a very efficient labelling technique which facilitates simultaneous measurements of the annihilation energy spectrum in the sample and in a standard. The result is that the system becomes essentially free of drift, so that precision of energy determination is limited by the statistics of counting alone. We have used the efficient technique of Ge(Li) spectroscopy together with labelling to measure the annihilation lineshape. Lineshape parameters, including the first two moments, have been extracted from this data. The techniques are applied to the measurement of recovery at room temperature and to a determination of the binding energy of positrons to traps in aluminum. Paper B 25 presented at 3rd Internat'l Conf. Positron Annihilation, Otami, Finland (August 1973).     

Experimental study on methods minimizing the influence of velocity-changing collisions in Doppler-free saturation spectroscopy

A method based on homodyne mixing is described which facilitates the use of rf-modulation of the pump beam for laser saturation spectroscopy together with conventional kHz lock-in amplifiers yielding a substantial reduction of disturbing pedestals present due to collisions in an atomic vapour. The technique benefits also from the significant improvement in the signal to noise ratio when transferring the Doppler-free signal to a frequency domain of several MHz where the interfering laser noise is low. These experimental procedures allow measurements of high quality saturated absorption spectra in vapour cells of otherwise unfavourable high gas pressure such as in commercial see-through hollow-cathodes. In addition we report the detection of an optogalvanic signal at rf-modulation (3.3 MHz).