基于色散位移光纤的高宣布式窄带单光子源

文章利用脉冲激光抽运色散位移光纤,通过自发四波混频过程产生的关联光子对,实验演示了一种光通讯波段的窄带宣布式单光子源. 实验测得-3 dB带宽为1.1 nm单光子场的二阶关联函数g⁽²⁾(0)=0.09±0.01,宣布效率可达0.5以上. 这种单光子源在量子通信中有潜在的应用前景. 关键词： 宣布式单光子 光纤 自发四波混频 量子通信     

Manipulating many-body quantum states via single-photon resonance

For an atomic Bose-Hubbard dimer quantum control via multiphoton processes have been investigated widely. We here explore how to manipulate the many-body quantum states via single-photon resonance by treating the periodic driving as a weak perturbation. The transition probabilities up to second-order approximation are given as functions of the driving parameters, which are considerable only for the single-photon resonance case. Due to some transition matrix elements vanishing, the first-order quantum transition obeys a selection rule. The non-forbidden transitions involve states of different entanglement entropies and all (part) of the forbidden transitions relate to the entropy balances between two states for odd (even) number of particles. The results provide a new route for manipulating many-body quantum states and entanglement entropies, and controlling the atomic tunnelings of the Bose-Hubbard dimer.     

Characterization of disorder in semiconductors via single-photon interferometry

The method of angular photonic correlations of spontaneous emission is introduced as an experimental, purely optical scheme to characterize disorder in semiconductor nanostructures. The theoretical expression for the angular correlations is derived and numerically evaluated for a model system. The results demonstrate how the proposed experimental method yields direct information about the spatial distribution of the relevant states and thus on the disorder present in the system.     

Pulse shaping via modulation of resonant absorption

Propagation of monochromatic linearly polarized plane electromagnetic wave through resonantly absorbing anisotropic medium with frequency-modulated response is studied analytically and numerically. Frequency modulation is assumed to be provided by means of either mechanical vibration of a solid sample or modulation of quantum transition frequency by a driving low-frequency electromagnetic field. Possibility of generation of train of pulses with polarization of the incident field as well as with orthogonal polarization is shown. For each polarization, optimal combinations of values of four parameters that provide maximal ratio of peak pulse intensity to the average output intensity is found numerically. Possible realization of this resonant method of pulse shaping in laser crystal Dy²⁺:CaF₂ is discussed.     

Controlled wake field acceleration via laser pulse shaping

We consider the interaction of high-intensity laser pulses with underdense plasmas and address the problem of the excitation of strong and stable wake plasma waves with regular electric fields to provide effective acceleration of charged particles over appreciably long distances. It is known that a relativistically strong laser pulse longer than the wavelength of plasma waves, propagating in a plasma is subject to self-modulation. This may result in a nonstationary behavior of the produced plasma wake field/particle dephasing, and reduced net acceleration. In this paper we present the results of 1(2/2)-D and 2(1/2)-D particle in cell (PIC) simulations which demonstrate that regular wake electric fields may be obtained by a properly shaped laser pulse (sharp steepening of its leading front). These results are relevant to the design of the 100 MeV laser wake field electron acceleration experiment that uses a terawatt picosecond CO₂ laser and is under construction at the Brookhaven Accelerator Test Facility     

Wideband all-order polarization mode dispersion compensation via pulse shaping

We demonstrate the application of ultrafast pulse-shaping techniques for experimental wideband all-order polarization mode dispersion (PMD) compensation, for the first time to our knowledge. PMD is treated as arbitrary variations of state of polarization and phase versus wavelength, in an all-order sense. Consequently, two pulse shapers are implemented in a serial manner to compensate for the polarization and the phase spectra independently. We report compensation of subpicosecond pulses (14 nm bandwidth around 1550 nm) that are anomalously spread to more than 2 ps as a result of PMD. This PMD compensation scheme can potentially be a powerful and cost-effective solution for fiber optic telecommunication networks.     

Terahertz pulse shaping via birefringence in lithium niobate crystal

Terahertz time-domain spectroscopy (THz-TDS) is employed to study the spectral response of lithium niobate crystal. A giant birefringence occurs in lithium niobate crystal in the THz frequency region. We demonstrate that the waveform of the THz wave is controllable by use of the birefringence property of the crystal. We also show experimentally that one THz pulse can be separated into two coherent sub-pulses; the separation as well as the relative intensity of the two sub-pulses are adjustable by changing the crystal orientation with respect to the polarization of the THz beam.     

Analysis of polarization state in quantum key distribution via single-photon two-qubit states

An improved quantum key distribution scheme via single-photon two-qubit states is proposed. The input–output model of the polarization state is established. And the influence of the interferometers to the polarization state is analyzed. Quantum bit error rate of polarization coding caused by birefringent and coordinate system difference between incident light and the fast and slow axes in fiber interferometer is simulated. Furthermore, maintaining conditions of polarization state are given on this basis.     

Lossless light projection

A new technique for energy-preserving phase-only light projection is demonstrated. The phase-only encoding is based on an extension of the Zernike phase-contrast method into the domain of full-range [0; 2pi ] phase modulation, breaking the usual small-phase-angle limitation. Controlling the spatial average value of the input-phase pattern and choosing appropriate phase retardation at the phase-contrast filter yield pure-phase-based image formation. Experimental results demonstrate close to 90% energy efficiency. Output intensity levels with magnitudes more than 3.5times that of the input intensity level were measured in the brightest regions of the projected images.     

Mid-infrared single-photon counting

We report a procedure to detect mid-infrared single photons at 4.65 microm by means of a two-stage scheme based on sum-frequency generation, by using a periodically poled lithium niobate nonlinear crystal and a silicon avalanche photodiode. An experimental investigation shows that, in addition to a high timing resolution, this technique yields a detection sensitivity of 1.24 pW with 63 mW of net pump power.     

Lossless strip-to-slot waveguide transformer

We present a highly efficient integratable waveguide transformer that is capable of converting Gaussian-like waveguide modes to much more complicated non-Gaussian-like slot-waveguide modes, and vice versa. The structure consists of several pairs of complementary tapers capable of making this mode conversion virtually lossless. The capability of extremely low-loss mode transformation between these two classes of waveguides has been demonstrated by means of single- and double-slot transformers. Our simulation has shown that the total transformation losses are less than 0.01 and 0.02 dB per transformer, respectively, and can be easily achieved, with a total device length of less than 100 microm.     

Multitap microwave photonic filters with programmable phase response via optical frequency comb shaping

We present a programmable multitap microwave photonic filter with an arbitrary phase response operating over a broad bandwidth. Complex coefficient taps are achieved by optical line-by-line pulse shaping on a 10 GHz flat optical frequency comb using a novel interferometric scheme. Through high-speed real-time measurements, we demonstrate programmable chirp control of a waveform via phase filtering. This achievement enables us to compress broadband microwave signals to their corresponding bandwidth-limited pulse duration.     

实时数字图像无损压缩的研究

针对实时数字图像的无损压缩方法进行了分析,采用三级5-3整数提升小波变换,小波变换的高频和低频系数分别采用不同的编码方法。进行了基于DSP和FPGA的压缩系统结构设计,提出了软、硬件并行的方法。进行三级小波变换时,硬件结构能够实现在对当前级小波变换的同时进行前一级小波变换的高频系数编码,提高了压缩系统的实时性。并对压缩系统的关键技术进行了研究。可以对80MB/s的数字图像进行无损压缩,压缩比可以达到1.68~1.8左右。     

Multiplexed time-correlated single-photon counting

We review the technique of multiplexed time-correlated single-photon counting whereby multiple fluorescence decay curves are recorded in parallel by statistically time-sharing a single time-to-amplitude converter. Application of the multiplexing technique to measuring the fluorescence lifetime topography of a self-absorbing sample is demonstrated. Further possibilities are discussed for multiplexed optical fiber sensor networks with built-in intelligence for detecting and discriminating between different metal ions in solution.     

Generation of pure single-photon states via spontaneous four-wave mixing in nanofibers with a variable cross section

A theory of single-photon sources of light is developed on the basis of spontaneous four-wave mixing in optical nanofibers. The spectral amplitude of the biphoton field is calculated with allowance for the real geometry of a nanofiber manufactured from standard optical fiber by heating and stretching. It is shown that by selecting pump parameters, we can simultaneously obtain the high spectral brightness and low frequency correlation of generated biphoton states in a nondegenerate mode of four-wave mixing that corresponds to single-photon states with the high purity.     

Lossless equalization of frequency combs

Frequency combs obtained by sinusoidal phase modulation of narrowband cw lasers are widely used in the field of optical communications. However, the resulting spectral envelope of the comb is not flat. We propose a general and efficient approach to achieve flat frequency combs with tunable bandwidth. The idea is based on a two-step process. First, efficient generation of a train with a temporal flat-top-pulse profile is required. Second, we use large parabolic phase modulation in every train period to map the temporal intensity shape into the spectral domain. In this way the resulting spectral envelope is flat, and the size is tunable with the chirping rate. Two different schemes are proposed and verified through numerical simulations.     

近红外单光子探测

随着以单个光子作为信息载体的量子通信和量子加密技术的兴起,近红外单光子探测技术受到了广泛关注.近红外单光子探测系统具有极高的灵敏度,所以它还可以胜任探测其它近红外波段微弱光信号的任务.半导体雪崩光电二极管是当前最成熟的近红外单光子探测系统的核心元器件;文章阐明了雪崩光电二极管的暗电流和击穿电压对单光子探测的影响,同时还讨论了工作温度、直流偏置、门信号性质和计数阈值等系统参数之间相互制约的关系.     

Passively stabilized single-photon interferometer

A single-photon interferometer is a fundamental element in quantum information science. In most previously reported works, single-photon interferometers use an active feedback locking system to stabilize the relative phase between two arms of the interferometer. Here, we use a pair of beam displacers to construct a passively stable single-photon interferometer. The relative phase stabilization between the two arms is achieved by stabilizing the temperature of the beam displacers. A purely polarized single-photon-level pulse is directed into the interferometer input port. By analyzing and measuring the polarization states of the single-photon pulse at the output port, the achieved polarization fidelity of the interferometer is about 99.1 ±0.1%. Our passively stabilized single-photon interferometer provides a key element for generating high-fidelity entanglement between a photon and atomic memory.