Enhanced autocompensating quantum cryptography system

We have improved the hardware and software of our autocompensating system for quantum key distribution by replacing bulk optical components at the end stations with fiber-optic equivalents and implementing software that synchronizes end-station activities, communicates basis choices, corrects errors, and performs privacy amplification over a local area network. The all-fiber-optic arrangement provides stable, efficient, and high-contrast routing of the photons. The low-bit error rate leads to high error-correction efficiency and minimizes data sacrifice during privacy amplification. Characterization measurements made on a number of commercial avalanche photodiodes are presented that highlight the need for improved devices tailored specifically for quantum information applications. A scheme for frequency shifting the photons returning from Alice's station to allow them to be distinguished from backscattered noise photons is also described.     

Quantum cryptography using optical fibers

Quantum cryptography permits the transmission of secret information whose security is guaranteed by the uncertainty principle. An experimental system for quantum crytography is implemented based on the linear polarization of single photons transmitted by an optical fiber. Polarization-preserving optical fiber and a feedback loop are employed to maintain the state of polarization. Error rates of less than 0.5% are obtained.     

High-speed backup scheme for zone 2 of non-pilot distance relays

Distance relays use time-stepped protection zones for coordination purposes. The time delay introduced by zone 2 relays limits the use of these devices for extra high voltage (EHV) applications, since the fast fault clearance requirement is not achieved. Normally, transmission lines are protected by means of intercommunicated protection schemes making use of a dedicated channel in order to overcome that delay; however, this channel is also prone to failure. Hence, it could be really advantageous to provide the line with fast protection coverage without making use of communication channels if a secure enough protection algorithm were developed for this specific purpose. An algorithm that is able to detect remote breaker operation and thus to accelerate the time-delayed zone 2 trip of local relay by monitoring changes in two proposed signals is presented. The algorithm performs adequately in two- and three-terminal lines as it is shown by computer simulation results. It is proposed that this algorithm can be used as a fast backup scheme to protect transmission lines in case main communication channel degrades, fails or is not available.     

Eavesdropping on practical quantum cryptography

Abstract

Practical implementations of quantum cryptography use attenuated laser pulses as the signal source rather than single photons. The channels used to transmit are also lossy. Here we give a simple derivation of two beamsplitting attacks on quantum cryptographic systems using laser pulses, either coherent or mixed states with any mean photon number. We also give a simple derivation of a photon-number splitting attack, the most advanced, both in terms of performance and technology required. We find bounds on the maximum disturbance for a given mean photon number and observed channel transmission efficiency for which a secret key can be distilled. We start by reviewing two incoherent attacks that can be used on single photon quantum cryptographic systems. These results are then adapted to systems that use laser pulses and lossy channels.     

Quantum cryptography with perfect multiphoton entanglement

Multiphoton entanglement in the same polarization has been shown theoretically to be obtainable by type-I spontaneous parametric downconversion (SPDC), which can generate bright pulses more easily than type-II SPDC. A new quantum cryptographic protocol utilizing polarization pairs with the detected type-I entangled multiphotons is proposed as quantum key distribution. We calculate the information capacity versus photon number corresponding to polarization after considering the transmission loss inside the optical fiber, the detector efficiency, and intercept-resend attacks at the level of channel error. The result compares favorably with all other schemes employing entanglement.     

Security aspects of quantum cryptography with D-dimensional systems

Abstract

We analyse the security of quantum key distribution with three-dimensional systems, and show that this scheme is more advantageous against symmetric attacks than protocols using two-dimensional states. We generalize the resulting optimal eavesdropping transformation to cryptographic systems with arbitrary dimensions.     

Phase auto-compensating high-dimensional quantum cryptography in elliptical-core few-mode fibres

High-dimensional quantum cryptography through optical fibres with several spatial modes requires an efficient quantum key distribution (QKD). However, optical modes acquire different phases and lags due to modal dispersion and random fluctuations, and a modal crosstalk appears under propagation. At present, special optical fibres for spatial multiplexing are being proposed in order to reduce notably the modal crosstalk, however, arbitrary relative phases and lags between modes are always present, which prevents getting an efficient phase encoding QKD. In this work, we take advantage of elliptical-core few-mode optical fibres presenting a very low modal crosstalk and propose an exact phase auto-compensating method by making photons travel several times the path between Alice and Bob (rounds) and by using appropriate modal inversions in each round trip. In order to make clear the proposed phase auto-compensating method, we study in detail a four-dimensional BB84 QKD case with single photon states excited in both polarization and spatial LP modes.     

量子保密通信用单光子探测系统的设计初探

设计了以1310nm激光波长作为光源的单光子探测装置系统,对铟镓砷雪崩光电二极管(InGaAs-APD)作为量子保密通信单光子探测器件时的偏压生成、低温控制以及单光子信号检测进行了综合考虑,用脉冲发生器的单稳态电路等效地实现了直流电平叠加脉冲的“光子门”,用跨阻式前置放大器和精密的比较甄别器来改善系统的探测灵敏度。初期探测实验表明,能观察到-93dBm光信号的波形。     

Temperature control for a long distance cooling channel using a microcomputer

Conventional control systems developed so far for long distance cooling channels, are not yet capable of local-temperature control, local-pressure control or sequential control, since thermodynamic properties of supercritical helium show an order of magnitude change and strong nonlinearity for applied pressure and temperature of cables and magnets.In this paper, a new temperature control method for long distance cooling channels using a microcomputer is proposed. This method is controllable without the necessity to consider the thermodynamic properties of supercritical helium, because a type of learning control method is adopted.     

Fiber designs with significantly reduced nonlinearity for very long distance transmission

A class of low-nonlinearity dispersion-shifted fibers based on depressed-core multistep index profiles is investigated. A systematic approach for designing these fibers in which a reference W-index profile is used to initiate the design ispresented. Transmission properties, including effective area, mode-field diameter, dispersion, dispersion slope, and cutoff wavelength, are evaluated for several design examples. The effects of varying fiber dimensions and indices on effective area and mode-field diameter are assessed. It is shown that there is a trade-off between these two properties and, generally, larger effective areas are associated with larger mode-field diameters. Dispersion-shifted single-mode fiber designs with effective areas of from 78 to 210 mum(2) and the corresponding mode-field diameter of from 8.94 to 14.94 mum, dispersion less than 0.07 ps/nm km, and dispersion slope of approximately 0.05 ps/nm(2) km are presented.     

Quantum mechanics as quantum information,mostly

Abstract

In this paper, I try to cause some good-natured trouble. The issue is, when will we ever stop burdening the taxpayer with conferences devoted to the quantum foundations? The suspicion is expressed that no end will be in sight until a means is found to reduce quantum theory to two or three statements of crisp physical (rather than abstract, axiomatic) significance. In this regard, no tool appears better calibrated for a direct assault than quantum information theory. Far from a strained application of the latest fad to a time-honoured problem, this method holds promise precisely because a large part—but not all—of the structure of quantum theory has always concerned information. It is just that the physics community needs reminding.     

Quantum technology: the second quantum revolution

We are currently in the midst of a second quantum revolution. The first quantum revolution gave us new rules that govern physical reality. The second quantum revolution will take these rules and use them to develop new technologies. In this review we discuss the principles upon which quantum technology is based and the tools required to develop it. We discuss a number of examples of research programs that could deliver quantum technologies in coming decades including: quantum information technology, quantum electromechanical systems, coherent quantum electronics, quantum optics and coherent matter technology.     

Quantum evaporation from quantum liquids and solids

Quantum evaporation is the evaporation of atoms from the surface of a liquid or solid by single excitations, such as phonons. We show that this process can only occur in systems for which the deBoer quantum parameter is above a critical value. For superfluid helium we argue that, because the interface between liquid and gas is not sharp, it may be useful to consider quantum evaporation as a process in which excitations coming from the liquid are adiabatically deformed as they propagate through the interface and then emerge as gas atoms. Based on this picture, we make several predictions which can be tested experimentally. Finally, we discuss quantum evaporation in solid hydrogen.     

Instruments and artisanal practices in long distance oceanic voyages

Scientific instruments are not neutral artefacts; the perception of their value is greatly determined not only by the objects themselves and the function they perform, but also by the context of their use. In the 16th and 17th centuries, scientific instruments – not only nautical ones – acquired a prominent place in European societies that greatly transcended the specific narrow professional circles that used them. This has already been noted as being an important feature in the development of science in early modern Europe. But exactly how did this happen? Following other recent studies, I argue in this paper that in order to understand this phenomenon one has to look carefully at artisanal practices of the period.     

液氢长距离管道输送探讨

讨论了液氢在长距离管道输送中存在着最佳流速,并分析了实际液氢输送过程中的输送状态.对在稳定的大流量转注过程中存在的单相流和两相流两种流动状态分别给出了在不同压力、流速、漏热等情况下温升、蒸气含量的理论计算结果,并在试验的结果上对液氢的流动状态给出了一些看法.     

哈依煤气长输管线GIS数据处理技术研究

文章根据长输管线GIS数据结构特点,提出了管线空间定位的方法,并在此基础上提出一种空间距离量算的算法。最后又提出了一种矢量数据(面域)转换为栅格数据的程序算法。