Single-photon avalanche diode detectors for quantum key distribution

The application of quantum key distribution (QKD) has raised particular demands for single-photon detectors. One of the most promising candidates at the low-loss optical fibre communications windows is the planar geometry InGaAs/InP single-photon avalanche diode. These detectors have been modelled, fabricated and characterised at 1.55 mum wavelength. Their performance in terms of single-photon detection efficiency, dark count rate, timing jitter and afterpulsing behaviour are reported and compared with the best commercially available, linear multiplication avalanche photodiodes operated in Geiger-mode. Their use in the application of QKD is discussed.     

自由空间量子密钥分发中的信号同步解决方案

针对目前自由空间量子密钥分发（QKD）中的信号同步这一难点问题,提出一种采用外置光信号来解决信号同步问题的方案——光同步方案。在发送端利用声光调制器将外置激光器的连续激光分割成周期光脉冲序列,并作为同步光信号发送给接收端。接收端采用光电倍增管接收同步光脉冲信号,生成一个与发送端严格同步的信号,以此作为接收端的时基标准来进行单光子计数。采用高频的内部时钟信号来“监视”接收到的同步信号,从而提高计数准确性。该方案具有长距离性、无线性、低复杂度等特点,已成功应用于一个基于B92协议的自由空间QKD系统中。     

Asynchronous quantum key distribution on a relay network

We show how quantum key distribution on a multi-user, multi-path, network can be used to establish a key between any two end-users in an asynchronous fashion using the technique of bit-transport. By a suitable adaptation of our previous secret-sharing scheme we show that an attacker has to compromise all of the intermediate relays on the network in order to obtain the key. Thus, two end-users can establish a secret key provided they trust at least one of the network relays.     

Fast quantum key distribution with decoy number states

We investigate the use of photon number states to identify eavesdropping attacks on quantum key distribution (QKD) schemes. The technique is based on the fact that different photon numbers traverse a channel with different transmittivity. We then describe two QKD schemes that utilize this method, one of which overcomes the upper limit on the key generation rate imposed by the dead time of detectors when using a heralded source of photons.     

Transmission medium and full fiber-optic setup for quantum key distribution applications

A fiber-optic-based coupled waveguide transmission medium is proposed to distribute secret keys in a single-photon polarization-based quantum cryptography setup. Polarization maintenance properties and coupling phenomena of the transmission medium are exploited to achieve accuracy and security of the transferred key. Elliptic fibers and fiber couplers are used to prepare the transmitted photons at the sender as well as analyze them at the receiver. The uniqueness of the setup stands on the exclusive use of fiber-optic components, enabling its construction on a single fiber line.     

星地量子密钥分配中单光子捕获理论分析

基于TEM10模拉盖尔-高斯光束,推导了自由空间量子密钥分配的单光子捕获概率表达式,分析了低轨卫星-地面站间链路的单光子捕获问题。结果表明,采用TEM10模拉盖尔-高斯型高度衰减激光脉冲作为单光子源,单光子捕获采用前驱波参考脉冲设置时间窗口的方法,可使得卫星上接收机以最大概率捕获光子。单光子捕获概率一般在10-1~10-3量级。与基模高斯光束相比,采用TEM10模拉盖尔-高斯光束的优点是,不会由于卫星运动而产生单光子捕获概率的损耗。通过考虑大气湍流的影响,对单光子捕获概率的表达式进行了修正。     

背景光对星地量子密钥分配量子误码率的影响

量子误码率是量子密钥分配系统的重要参数之一.对于星地量子密钥分配,采用具有泊松分布的高度衰减激光脉冲作为单光子源,建立了由背景光引起的量子误码率理论模型,给出了量子误码率的表达式.针对轨道高度为300km的低轨卫星-地面站间链路,进行了数值仿真分析.研究表明,背景光是限制自由空间量子密钥分配链路距离的主要因素之一,在低轨卫星-地面站间进行量子密钥分配是可行的.在白天,量子误码率的量级为10-3～10-2;在夜晚,满月时为10-4,新月时为10-6～10-5.     

EPR quantum key distribution protocols with potential 100% qubit efficiency

Public discussion is a useful way for quantum key distribution protocols to reveal the presence of eavesdroppers. However, to ensure better security, nearly 50% of the transmitted qubits are spent in public discussions. Consequently, the original EPR quantum key distribution protocol provides only 25% qubit efficiency and Deng et al.'s scheme delivers only 50% qubit efficiency. By bringing classical cryptographic techniques into the quantum arena, this work proposes EPR quantum key distribution protocols with a potential of 100% qubit efficiency     

Galvanostatic pulse deposition of hydroxyapatite for adhesion to titanium for biomedical purposes

Calcium phosphate coatings, in particular synthetic hydroxyapatite, are applied to the surfaces of titanium and its alloys so as to improve the biocompatibility and biological performance. Currently, plasma spraying is the clinically accepted technique for the deposition of calcium phosphate onto titanium. Electrochemical cathodic deposition is emerging as an alternative technique due to it being a nonline-of-sight technique. In this present study, it is demonstrated that increased thickness, crystallinity and adhesion of calcium phosphate coating on titanium is achieved by periodic pulsed low current densities compared to a constant current deposition method. It is believed that the “off” part of the AC deposition cycle gives the calcium and phosphate ions in the bulk solution sufficient time to diffuse to the titanium's surface maintaining more favourable conditions for HA growth. Unfortunately, although pulsed deposition at high current densities is able to produce thick coatings it cannot avoid problems associated with hydrogen bubbles and thus both AC and DC films deposited at high current densities have low crystallinity and poor adhesion.     

The quantum noise of guided wave acoustic Brillouin scattering with applications to continuous-variable quantum key distribution

The spectra and optical quantum state of guided acoustic wave Brillouin scattering in optical fibers are measured. Spectra from 0.95 to 2?GHz are obtained with amplitude resolution as sensitive as 0.01 shot noise unit. Quantum homodyne tomography measurements confirm the thermal quantum statistics of guided acoustic wave Brillouin scattering, which is useful knowledge in the context of experimental continuous-variable quantum key distribution.     

Impact of coarse-grained measurement with finite range on continuous-variable quantum key distribution

In continuous-variable quantum key distribution, detectors are necessarily coarse grained and of finite range. We analyze the impact of both features and demonstrate that while coarse graining adds a fixed error to the estimated excess noise, finite range degrades the estimation accuracy of both transmission and excess noise. Moreover, the inaccurate estimation due to finite range may results in secret key rate underestimation, even misjudgment of security. To compensate these consequences, tuning the modulation variance is a possible way.     

Quantum information technology with Sagnac interferometer: interaction-free measurement,quantum key distribution and quantum secret sharing

The interferometry of single-photon pulses has been used to implement quantum technology systems, like quantum key distribution, interaction-free measurement and some other quantum communication protocols. In most of these implementations, Mach–Zehnder, Michelson and Fabry–Pérot interferometers are the most used. In this work we present optical setups for interaction-free measurement, quantum key distribution, quantum secret sharing and secure classical prisoners' dilemma game using the Sagnac interferometer. The proposed setups are described and as well the quantum protocols using them are explained.     

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.     

The role of squeezing in quantum key distribution based on homodyne detection and post-selection

Abstract

The role of squeezing in quantum key distribution with continuous variables based on homodyne detection and post-selection is investigated for several specific eavesdropping attacks. It is shown that amplitude squeezing creates strong correlations between the signals of the legitimate receiver and a potential eavesdropper. Post-selection of the received pulses can therefore be used to reduce the eavesdropper's knowledge of the raw key, which increases the secret key rate by orders of magnitude over large distances even for modest amounts of squeezing.     

Criterion for quantum turbulence onset after rectangular heat pulse in superfluid helium

On the basis of transient heat transfer measurements of the film boiling onset time in a He II bath, a criterion correlated to the bath temperature and pressure for quantum turbulence onset after a rectangular heat pulse in superfluid helium is shown. Using the criterion parameter α it is possible to determine the time τ needed to initiate quantum turbulence in the He II bath after a rectangular heat pulse. It is found that the criterion value in a He II bath is much smaller than that for He II confined in small channels. A strong pressure effect is also exhibited.     

High-inductance simulator for calibration purposes

A simulator of high inductance (L=10 H) based on a capacitor switching circuit is described. The dependence of the Q factor on the compensation error of the resistive component in various inductance simulation circuits is analyzed. Capacitor switching circuits are shown to have a much higher Q than resistive RC circuits. Translated from Izmeritel'naya Tekhnika, No. 10, pp. 25–28, October, 1997.     

Propagation of single flux quantum pulse of superconducting transmission line

Josephson tunnel junction switching by single flux quantum (SFQ) pulses generated by a two-junction interferometer and propagated on a superconducting transmission line have been investigated experimentally and by numerical simulation. High current density tunnel junctions have been used to enhance voltage SFQ pulse amplitudes and reduce junction damping parameters. The measured effective amplitudes of SFQ pulses allow an evaluation of the applicability of transmission lines to SFQ logic circuits and Josephson samplers.     

Integrated Resonant Self-Pumped Loop and pulse tube cryocooler for cryogenic cooling distribution

Cooling distribution is a vital technology concerning cryogenic thermal management systems for many future space applications, such as in-space, zero boil-off, long-term propellant storage, cooling infrared sensors at multiple locations or at a distance from the cryocooler, and focal-plane arrays in telescopes. These applications require a cooling distribution technology that is able to efficiently and reliably deliver cooling power (generated by a cryocooler) to remote locations and uniformly distribute it over a large-surface area. On-going efforts by others under this technology development area have not shown any promising results.This paper introduces the concept of using a Resonant Self-Pumped Loop (RSPL) integrated with the proven, highly efficient pulse tube cryocooler. The RSPL and pulse tube cryocooler combination generates cooling power and provides a distributive cooling loop that can be extended long distances, has no moving parts, and is driven by a single linear compressor. The RSPL is fully coupled with the oscillating flow of the pulse tube working fluid and utilizes gas diodes to convert the oscillating flow to one-directional (DC) steady flow that circulates through the cooling loop. The proposed RSPL is extremely simple, lightweight, reliable, and flexible for packaging. There are several requirements for the RSPL to operate efficiently. These requirements will be presented in this paper. Compared to other distributive cooling technologies currently under development, the RSPL technology is unique.     

Neutron spectrometry for radiation protection purposes

Determination of the dose equivalent is required for radiation protection purposes, however such a determination is quite difficult for neutron radiation. In order to perform accurate dosimetric determinations, it is advantageous to acquire information about the neutron fluence spectrum in the workplace as well as the reference radiations used to calibrate dosimetric instruments. This information can then be used to select the appropriate dosimetric instrument, the optimum calibration condition or to establish correction factors that account for the differences in calibration and workplace conditions. For quite some time, neutron spectrometry has been used for these purposes. A brief review of the applications of spectrometers in radiation protection and some recommendations for further development are given here.