Photon Number Resolving Detector at Telecom Wavelengths: Charge Integration Photon Detector (CIPD)

A photon number resolving detector (PNRD) is the basic device for many optics applications, especially for developing optical quantum computer. The implementation of universal nonlinear quantum gates typically requires high quantum efficiency and a wide dynamic range up to a few tens of photons. We have developed a charge integration photon detector (CIPD) as a PNRD with a quantum efficiency of 80% and multiphoton measuring over ten photons at telecom wavelengths. This detector has a signal-to-noise ratio of 3 and operates at 40 Hz at present. In this paper, we introduce the principle of our CIPD and report the capability of detecting different types of photon statistics with a high linearity to incident photon numbers.     

Single‐photon quantum filtering with multiple measurements

The single‐photon quantum filtering problems have been investigated recently with applications in quantum computing. In practice, the detector responds with a quantum efficiency of less than unity since there exists some mode mismatch between the detector and the system and the single‐photon signal may be corrupted by quantum white noise. Consequently, quantum filters based on multiple measurements are designed in this paper to improve estimation performance. More specifically, the filtering equations for a 2‐level quantum system driven by a single‐photon input state and under multiple measurements are presented in this paper. Four scenarios, ie, (1) 2 diffusive measurements with Q‐P quadrature form, (2) 2 diffusive measurements with Q‐Q quadrature form, (3) diffusive plus Poissonian measurements, and (4) 2 Poissonian measurements, are considered. It is natural to compare the filtering results, ie, measuring a single channel or both channels, which one is better? By the simulation where we use a single photon to excite an atom, it seems that multiple measurements enable us to excite the atom with higher probability than only measuring a single channel. In addition, a measurement back‐action phenomenon is revealed by the simulation results.     

Homodyne In-Phase and Quadrature Detection of Weak Coherent States With Carrier Phase Tracking

We present a homodyne receiver structure for the detection of weak coherent states that uses sequential in-phase and quadrature measurements on the received optical signal. This receiver performs the optical carrier phase tracking requiring only a single balanced homodyne detector, by including a postdetection Costas-loop-type feedback, which additionally allows the use of suppressed carrier modulations in the received field, for efficient transmission. We report an experimental interferometric self-homodyne setup for the sequential detection of low photon number, binary phase-modulated optical signals that consist of strongly attenuated laser pulses by using a reference field as the local oscillator with an alternatively switched phase. A Costas loop postdetection subsystem is implemented in discrete time to perform fast real-time optical phase tracking. We also present the experimental results of the homodyne postdetection statistics for received BPSK signals with very low photon numbers, and compare them with the theoretical uncertainty limit. Finally, we conduct bit error rate measurements over a wide range of signal level, as well as a comparison with the standard quantum limit.      

Practical Quantum Cryptosystem for Metro Area Applications

A practical quantum cryptosystem prototype has been developed for metro area applications. It is of the size of a desktop and incorporates a temperature-independent optical interferometer, a highly reliable photon detector module, and random number generators. It supports various functions, including bit/frame synchronization, BB84 bit reconciliation, error correction and privacy amplification for quantum key generation, and encrypted communication. Two weeks of testing using a commercial fiber in the field demonstrated its ability to continuously generate final keys and to perform crypto communication using the keys.     

Implementing a Multiplexed System of Detectors for Higher Photon Counting Rates

Photon counting applications are typically limited by detector deadtime to operate at count rates of a few megahertz, at best, and often at significantly lower levels. This limitation is becoming more critical with the advance of photon counting applications such as photon-based quantum information. We present a first experimental proof of principle, and review the theoretical foundation of a multiplexed detection scheme that allows photons to be counted at higher rates than is possible with individual detectors or simple detector trees. In addition to this deadtime improvement, we discuss the impact of this scheme on other relevant characteristics such as afterpulsing and dark count rates.     

紫外脉冲法在特高压放电检测中的应用

目前的超高压系统一般通过红外成像仪、紫外成像仪等检测放电,但都未实现定量检测,而特高压系统的绝缘要求更高、设备对地距离更大,尤其特高压输电线路对检测距离的要求很高。为此,提出了基于紫外脉冲法的特高压输电系统放电非接触式检测方法,该法能实现对特高压输电系统放电程度的远距离精确对位的定量检测,可准确判定特高压电气设备的外绝缘状况;还研制了相应的检测器并在西北电网公司750kV官亭变电站进行了放电实测,验证了该方法的检测能力,其最大测量距离>32m。     

New research trends on high-precision time transfer technology

High-precision time transfer plays an important role in the areas of fundamental research and applications. Accompanying with the remarkable improvements in the ability of generating and measuring high-accuracy time-frequency signal, seeking for new time-transfer techniques between distant clocks with much further improved accuracy attracts attentions world-widely. The time-transfer technique based on optical pulses has the highest precision presently, and the further improvement in the accuracy is heavily dependent on the time-domain properties of the pulse as well as the sensitivity of the applied measurement on the exchanged pulse. The application of optical frequency comb in time transfer for a precision up to femtosecond level are currently the focus of much interest, and has recently achieved many breakthroughs. Further investigations show that, utilizing quantum techniques, i.e. quantum measurement technique and quantum optical pulse source, can lead to a new limit on the measured timing information. Furthermore, it can be immune from atmospheric parameters, such as pressure, temperature, humidity and so on. Such quantum improvements on time-transfer have a bright prospect in the future applications requiring extremely high-accuracy timing and ranging. The potential achievements will form a technical basis for the future realization of sub-femtosecond time transfer system.     

Generalized maximum correntropy detector for non‐Gaussian environments

This paper addresses the problem of multiple‐hypothesis detection. In many applications, assuming the Gaussian distribution for undesirable disturbances does not yield a sufficient model. On the other hand, under the non‐Gaussian noise/interference assumption, the optimal detector will be impractically complex. Therewith, inspired by the optimal maximum likelihood detector, a suboptimal detector is designed. In particular, a novel detector based on the generalized correntropy, which adopts the generalized Gaussian density function as the kernel, is proposed. Simulations demonstrate that, in non‐Gaussian noise models, the generalized correntropy detector significantly outperforms other commonly used detectors. The efficient and robust performance of the proposed detection method is illustrated in both light‐tailed and heavy‐tailed noise distributions.     

Long-distance QKD transmission using single-sideband detection scheme With WDM synchronization

We report a long-distance, polarization insensitive, quantum key distribution scheme using single-sideband (SSB) detection. The method uses an amplitude modulation to transmit photon in sidebands. The system acts as a fainted laser diode whose information quantum state is encoded in the phase difference between the main peak and its sidebands. Using the dispersion properties of the fiber, the relative phase between interacting quantum states can be reliably controlled by sending a reference signal over the physical link that transmits the quantum signal. Therefore, photon detection can be performed very accurately over long distances. Stability of the synchronization technique with respect to optical path variation is also discussed. The reconciliation process uses a LAN connection as public channel. Experiments were carried out in the C-band telecom window (1550 nm), over a 40-km standard single-mode fiber spool as well as over a 10-km deployed fiber.     

Progress in Silicon Single-Photon Avalanche Diodes

Silicon single-photon avalanche diodes (SPADs) are nowadays a solid-state alternative to photomultiplier tubes (PMTs) in single-photon counting (SPC) and time-correlated single-photon counting (TCSPC) over the visible spectral range up to 1-mum wavelength. SPADs implemented in planar technology compatible with CMOS circuits offer typical advantages of microelectronic devices (small size, ruggedness, low voltage, low power, etc.). Furthermore, they have inherently higher photon detection efficiency, since they do not rely on electron emission in vacuum from a photocathode as do PMTs, but instead on the internal photoelectric effect. However, PMTs offer much wider sensitive area, which greatly simplifies the design of optical systems; they also attain remarkable performance at high counting rate, and offer picosecond timing resolution with microchannel plate models. In order to make SPAD detectors more competitive in a broader range of SPC and TCSPC applications, it is necessary to face several issues in the semiconductor device design and technology. Such issues will be discussed in the context of the two possible approaches to such a challenge: employing a standard industrial high-voltage CMOS technology or developing a dedicated CMOS-compatible technology. Advances recently attained in the development of SPAD detectors will be outlined and discussed with reference to both single-element detectors and integrated detector arrays.     

基于后选择的连续变量量子密钥分发协议

连续变量量子密钥分发（CVQKD）是利用连续变化的物理量承载密钥的量子密钥分发协议,与电力系统普遍采用的离散变量量子密钥分发协议相比,其具有设备简单、探测效率高、兼容性好等优点,成为量子密钥分发的研究热点。然而,量子态在量子信道中的传输衰减严重限制CVQKD传输距离。提出了一种基于后选择方法的CVQKD协议,且不需要额外的单光子源。研究结果表明,该后选择方法中测量值越大的物理量被保留下的概率越高。该方法可以简化并降低连续量子密钥分发协议实现的成本,为探索出电力网络的量子保密通信方案提供研究思路。     

导线接头不同故障类型的紫外成像检测

导线接头在电网中起着机械和电气的连接作用,运行过程中由于导线接头松动、脱落等故障严重威胁电网安全可靠运行,并导致能量损耗,利用合理的手段检测导线接头运行状况一直是运行部门关注的焦点。该文模拟导线接头螺帽松动、螺杆松动、螺栓错位、扭力弯曲和剪切断股这几种典型故障缺陷,利用紫外检测仪检测其作用电压下紫外放电光子数差异。试验结果表明：随着作用电压的提高,导线接头各种故障下的紫外放电光子数随之增加;运行电压下,导线接头各种典型故障下测得的紫外光子数存在明显差异,利用紫外成像检测可以有效用于导线接头的故障检测。研究结果为输电线路的运行状态检测提供了参考。     

GIS局部放电检测方法的分析研究

为了研究脉冲电流法、超声波法和超高频法检测GIS设备内部局部放电的优缺点,利用220 kV GIS试验段建立了一套GIS局部放电检测平台。应用脉冲电流法、超声波法以及超高频法对高压导体尖端、地电极尖端和悬浮尖端典型缺陷进行了测试,获取大量的实验数据和图谱。试验结果表明:脉冲电流法对缺陷普遍具有较高的灵敏度,但抗干扰能力较差,对于现场应用有一定局限性;超声波法对于局部放电信号灵敏度较低,但是抗电磁干扰性强;超高频法抗干扰能力强且灵敏度高,适合用于现场检测。总体来看,多种检测方法需要配合来综合确认。     

Counting Constituents in Molecular Complexes by Fluorescence Photon Antibunching

Modern single molecule fluorescence microscopy offers new, highly quantitative ways for studying the systems biology of cells while keeping the cells healthy and alive in their natural environment. In this context, a quantum optical technique, photon antibunching, has found a small niche in the continuously growing applications of single molecule techniques to characterize small molecular complexes. Here, we review some of the most recent applications of photon antibunching in biophotonics research, and we provide a guide for how to conduct photon antibunching experiments at the single molecule level by applying techniques borrowed from time-correlated single photon counting (TCSPC). We provide a number of new examples for applications of photon antibunching to the study of multichromophoric molecules and small molecular complexes.     

X‐ray micrographic imaging system based on COTS CMOS sensors

This paper presents the use of commercial off the shelf CMOS image sensors for the acquisition of X‐ray images with high spatial resolution. The X‐ray images, with application in biology, electronic components inspection, and paleontology research, are obtained with 8‐keV photons from a Cu tube. The quantum efficiency of the detector is estimated using attenuation lengths of photons in the sensor and compared to traditional scintillator conversion layers. The spatial resolution observed with the sensor is limited by the charge redistribution produced after photon interaction with Si.     

Analysis of a low‐complexity change detection scheme

In many applications, for example in fault detection, it is important to discriminate between changes in system dynamics and abrupt changes in the disturbance level. A new low‐complexity change detection method based on the average behaviour of the estimated impulse response parameters of the normalized least mean‐square (NLMS) algorithm is presented. The solution includes second‐order Kalman filters based on exponential transient models for parameter convergence. Explicit formulas for time‐varying state covariances and Kalman gains are given. The receiver operating characteristics (ROC) is also computed and used for performance evaluation. The effects of the approximations in the averaging analysis that occur for high adaptation gains are handled with an experimental ROC analysis. Copyright © 2000 John Wiley & Sons, Ltd.     

Entangled photons

Based on the quantum behavior of entangled photon pairs generated via spontaneous parametric down conversion, this paper reviews the concept of effective two-photon wavefunction or biphoton wavepacket and emphasize the very different physics associated with the entangled two-photon system and with its "individual" subsystems. Experimental approaches for Bell state preparation, pumped by continuous wave and ultrashort pulse, and the propagation of biphoton wavepacket in dispersive media are discussed.     

Distribution analysis of the photon correlation spectroscopy of discrete numbers of dye molecules conjugated to DNA

The formation of protein complexes with other proteins and nucleic acids is critical to biological function. Although it is relatively easy to identify the components present in these complexes, it is often difficult to determine their exact stoichiometry and obtain information about the homogeneity of the sample from bulk measurements. We demonstrate the use of single molecule photon-pair correlation spectroscopy to distinguish between discrete numbers of molecules in biological complexes. Fluorescence photon antibunching is observed from a single molecule by employing time-correlated single photon counting in combination with a Hanbury-Brown and Twiss coincidence setup. In addition, pulsed laser excitation and time-tagged time-resolved data collection allow for the measurement of photon arrival times with nanosecond time resolution. The interphoton time distribution between consecutively arriving photons can be calculated and provides a measure of the second-order temporal correlation function. Analysis of this function yields an absolute measure of the number of molecules, N, present in a given complex. It is this ability to measure N that renders this technique powerful for determining stoichiometries in complex biological systems at the single molecule level. We investigate the counting efficiency and statistics of photon antibunching of specifically designed biological samples labeled with multiple copies of the same fluorescent dye and derive conclusions about its use in the analytical evaluation of complex biological samples.     

Compact CMOS active quenching/recharge circuit for SPAD arrays

Avalanche diodes operating in Geiger mode are able to detect single photon events. They can be employed to photon counting and time‐of‐flight estimation. In order to ensure proper operation of these devices, the avalanche current must be rapidly quenched, and, later on, the initial equilibrium must be restored. In this paper, we present an active quenching/recharge circuit specially designed to be integrated in the form of an array of single‐photon avalanche diode (SPAD) detectors. Active quenching and recharge provide benefits like an accurately controllable pulse width and afterpulsing reduction. In addition, this circuit yields one of the lowest reported area occupations and power consumptions. The quenching mechanism employed is based on a positive feedback loop that accelerates quenching right after sensing the avalanche current. We have employed a current starved inverter for the regulation of the hold‐off time, which is more compact than other reported controllable delay implementations. This circuit has been fabricated in a standard 0.18 µm complementary metal‐oxide‐semiconductor (CMOS) technology. The SPAD has a quasi‐circular shape of 12 µm diameter active area. The fill factor is about 11%. The measured time resolution of the detector is 187 ps. The photon‐detection efficiency (PDE) at 540 nm wavelength is about 5% at an excess voltage of 900 mV. The break‐down voltage is 10.3 V. A dark count rate of 19 kHz is measured at room temperature. Worst case post‐layout simulations show a 117 ps quenching and 280 ps restoring times. The dead time can be accurately tuned from 5 to 500 ns. The pulse‐width jitter is below 1.8 ns when dead time is set to 40 ns. Copyright © 2015 John Wiley & Sons, Ltd.     

Selenium-based amorphous silicon flat-panel digital X-ray imager for protein crystallography

This work proposes a large-area detector for protein crystallography based on an amorphous silicon (a-Si:H) thin film transistor (TFT) pixel-array backplane and an overlying amorphous selenium (a-Se) photoconductor for direct conversion of incident X-rays into an image charge. To achieve high sensitivity, avalanche multiplication in a-Se is adopted to make the detector sensitive to each incident X-ray. The use of a-Si:H technology enables large-area imaging of protein diffraction patterns at less expense compared to existing charge coupled device (CCD) and imaging plate (IP) detectors. In addition, a theoretical analysis shows that the detector exhibits fast readout speed (readout time <1 s), high dynamic range (~10˚), high sensitivity (~1 X-ray photon), and high detective quantum efficiency (~.7), thus validating its suitability for protein crystallography.