Nonorthogonal unitaries in two-way quantum key distribution

Two-way Quantum Key Distribution (QKD) schemes commonly make use of a set of unitaries corresponding to binary encodings which can in principle be distinguished perfectly. In this paper, inline with the proposal in Chiribella et al. (2008) [13], we introduce a non-entangled two-way QKD scheme with two sets of unitaries of which the elements in one set can be viewed as ‘nonorthogonal’ to elements in the other with the aim of naturally suppressing an eavesdropper's information to provide for a higher security threshold. Security analysis is done in the context of individual attack strategies for a quick comparison with the conventional two-way QKD scheme. Given the richer structure of the improvement, future direction is also discussed.     

Influence of light source linewidth in differential-phase-shift quantum key distribution systems

Differential-phase-shift (DPS) quantum key distribution (QKD) is one of the QKD protocols, featuring simplicity for practical implementation. It uses a coherent pulse train whose phase should be stable at least within the pulse interval. This paper quantitatively investigates the phase stability required for DPS-QKD systems. The phase stability is characterized by the spectral linewidth of the light source. A theoretical model and experiments are presented, the results of which indicate that the linewidth should be, for example, less than 0.35% of the free-spectral-range of an asymmetric Mach-Zehnder interferometer in a receiver to achieve quantum bit error rate of less than 0.5% due to linewidth broadening of the light source.     

Improved two-way six-state protocol for quantum key distribution

A generalized version for a qubit based two-way quantum key distribution scheme was first proposed in the paper [Phys. Lett. A 358 (2006) 85] capitalizing on the six quantum states derived from three mutually unbiased bases. While boasting of a higher level of security, the protocol was not designed for ease of practical implementation. In this work, we propose modifications to the protocol, resulting not only in improved security but also in a more efficient and practical setup. We provide comparisons for calculated secure key rates for the protocols in noisy and lossy channels.     

Independent attacks in imperfect settings: A case for a two-way quantum key distribution scheme

We review the study on a two-way quantum key distribution protocol given imperfect settings through a simple analysis of a toy model and show that it can outperform a BB84 setup. We provide the sufficient condition for this as a ratio of optimal intensities for the protocols.     

Sagnac quantum key distribution over telecom fiber networks

We present a new concept for compensation of single mode fiber (SMF) birefringence effects in a Sagnac quantum key distribution (QKD) setup, based on a polarization control system and a polarization insensitive phase modulator. Our experimental data show stable (in regards to birefringence drift) QKD over 1550 nm SMF telecom networks in Sagnac configuration, using the BB84-protocol [C.H. Bennett, G. Brassard, in: Proceedings of the IEEE International Conference on Computers, Systems, and Signal Processing, Institute of Electrical and Electronic Engineers, New York, 1984, p. 175] with phase encoding. The achieved total Sagnac transmission loop distances were between 100 km and 150 km with quantum bit error rates (QBER) between 5.84% and 9.79% for the mean-photon-number μ = 0.1. The distances were much longer and rates much higher than in any other published Sagnac QKD experiments. We also show an example of our one-decoy state protocol implementations (for the 45 km distance between Alice and Bob, corresponding to the 130 km total Sagnac loop length), providing an unconditional QKD security. The measurement results have showed feasibility of QKD over telecom fiber networks in Sagnac configuration, using standard fiber telecom components.     

基于相位调制偏振态QKD系统的量子信道校正发送方案

提出了一种基于相位调制偏振态QKD系统的量子信道校正发送的新方案—采用波导型相位调制器研制成电控连续可调光学相移器,它可随相位调制器输入电压的连续改变而产生连续的相移,从而控制输出的偏振态;在其前放置一个半波片,即可校正接收端偏振态量子态在信道中所发生的改变,从而保证信道传送密钥的可用性.通过理论推导和实验研究验证了基于相位调制偏振态QKD系统的量子信道校正发送方案的核心部件的可行性.由于本方案能实现高速调制（GHz）,为解决光纤传输的偏振编码QKD系统中偏振态漂移问题提供了一种新的途径.     

Effect of spontaneous Raman scattering on quantum channel wavelength-multiplexed with classical channel

We quantitatively estimate the effect of spontaneous Raman scattering on a quantum channel wavelength-multiplexed with a classical channel. Based on an experiment that measured the generation power of spontaneous Raman scattering in a fiber, the performance of wavelength-multiplexed quantum key distribution (QKD) systems using the differential-phase-shift protocol is evaluated for various system conditions.     

Error corrections of quantum key distribution of the quantum codes via optical wireless link

Quantum key generated by using a fiber optic Mach Zehnder interferometer incorporating a fiber optic ring resonator is proposed. The generated quantum key is distributed via an optical wireless link system, which is available for the free space link. The random quantum bits can be formed and used as the secret codes in quantum communication system. By adding an ancillary photon after the signal photon within the correlation time of the fiber noise and by performing quantum parity checking, the high fidelity to the noiseless quantum in free space is achieved by the technique known as the quantum error correction.     

各向异性量子点单光子发射的高偏振度特性

研究了线偏振脉冲光场激发下,单个各向异性InGaAs量子点的高偏振光发射. 给出其偏振因子与两个正交本征态之间的交叉弛豫之间的关系式. 分析表明，交叉弛豫随激发场强度增大，并导致偏振因子随激发场入射脉冲面积减小. 关键词： 量子点 单光子发射 偏振度     

单光子量子密钥分发系统中干涉稳定性分析

为了研究位相编码量子密钥分发系统中的干涉稳定性, 运用解析和数值计算方法详细研究了光纤双折射、温度变化、非对称性以及耦合器分光比对量子密钥分发系统干涉条纹可见度的影响, 结果表明光纤双折射、温度变化和非对称性都将影响系统的干涉可见度, 调整接收端的调制位相能消除温度变化的影响, 并减小非对称性对系统的影响, 但不能改善光纤双折射对系统干涉的影响. 另外, 耦合器分光比影响系统的效率.     

Enhanced emission of single quantum dot formed by interface fluctuations in photonic-crystal microcavities

We fabricated photonic-crystal (PhC) microcavities tuned to GaAs quantum dots (QDs) formed by interface fluctuation for the first time and observed the spontaneous emission enhancement in a weak coupling regime. A QD is a very thin GaAs quantum well (QW), and its interface steps exhibit quantum dot-like behavior. The emission intensity from the PhC cavity was stronger than that from the area where no PhC pattern was fabricated and the overall shape of the photoluminescence (PL) agreed with the cavity mode calculated with the three-dimensional (3D) finite-difference time domain (FDTD) method. The spontaneous emission enhancement factor was 10.     

Reasons of emission inhomogeneity in InAs quantum dot structures

The photoluminescence (PL) inhomogeneity has been studied in InAs quantum dots (QDs) embedded in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with QDs grown at different temperatures. It was shown that three reasons are responsible for the emission inhomogeneity in studied QD structures: (i) the high concentration of nonradiative recombination centers in the capping In_0.15Ga_0.85As layer at low QD growth temperatures, (ii) the QD density and size distributions for the structure with QD grown at 510 ^°C, (iii) the high concentration of nonradiative recombination centers in the GaAs barrier at higher QD growth temperatures.     

Thermodynamic properties of two-dimensional few-electrons quantum dot using the static fluctuation approximation (SFA)

The two-dimensional GaAs parabolic quantum dot (QD) is investigated in the static fluctuation approximation (SFA) where both the magnetic field and the electron-electron interaction are fully taken into account. The thermodynamic properties of the system are computed as functions of the temperature and magnetic field for different numbers of electrons (N). It is noted that the mean internal energy increases with increasing number of electrons and magnetic field. This increase is not smooth, and a sudden change in slope is observed at a certain value of magnetic field for N≥4. It is found that the magnetization has sharp jumps for N≥4; for N≤3 a smooth monotonic diamagnetism is exhibited. A sharp peak is observed in the specific heat when the magnetic field is increased. It is found that the lowest-state transitions occur at a critical value of magnetic field. These transitions appear in the mean internal energy and magnetization.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

An Efficient Routing Protocol for Quantum Key Distribution Networks

Quantum key distribution (QKD) can provide point-to-point information-theoretic secure key services for two connected users. In fact, the development of QKD networks needs more focus from the scientific community in order to broaden the service scale of QKD technology to deliver end-to-end secure key services. Of course, some recent efforts have been made to develop secure communication protocols based on QKD. However, due to the limited key generation capability of QKD devices, high quantum secure key utilization is the major concern for QKD networks. Since traditional routing techniques do not account for the state of quantum secure keys on links, applying them in QKD networks directly will result in underutilization of quantum secure keys. Therefore, an efficient routing protocol for QKD networks, especially for large-scale QKD networks, is desperately needed. In this study, an efficient routing protocol based on optimized link-state routing, namely QOLSR, is proposed for QKD networks. QOLSR considerably improves quantum key utilization in QKD networks through link-state awareness and path optimization. Simulation results demonstrate the validity and efficiency of the proposed QOLSR routing protocol. Most importantly, with the growth of communication traffic, the benefit becomes even more apparent.     

CdS量子点荧光光度法测定蛋白质的含量

利用荧光光度法测定食品中蛋白质的含量;实验以六偏磷酸钠为稳定剂,巯基乙酸为修饰剂水相合成了具有优异光学性质的CdS量子点.基于CdS与牛血清白蛋白(BsA)反应后,荧光强度显著增强,建立了CdS荧光光度法测定蛋白质的新方法;体系的荧光强度与BSA浓度在0.001 43～0.250 mg·mL-1范围内呈良好的线性关系,...     

THz imaging of cyclotron emission in quantum Hall conductors

Recent studies of cyclotron emission microscopy on quantum Hall related states are reported. The topics include non-equilibrium between edge and bulk states, current-induced breakdown of the quantum Hall effect, and the emission threshold at hot spots. Experimental method of scanning-type terahertz microscopes developed towards photon-counting level sensitivity is also described.     

Optical near-field mapping of bright and dark quantum dot states

We theoretically investigate scanning near-field optical microscopy (SNOM) of semiconductor quantum dots. A general theoretical framework is developed that accounts for photo excitation and relaxation in complex dielectric environments. We find that in the near-field regime bright and dark excitonic states become mixed, opening new channels for the coupling to the electromagnetic field.     

The classicality and quantumness of a quantum ensemble

In this Letter, we investigate the classicality and quantumness of a quantum ensemble. We define a quantity called ensemble classicality based on classical cloning strategy (ECCC) to characterize how classical a quantum ensemble is. An ensemble of commuting states has a unit ECCC, while a general ensemble can have a ECCC less than 1. We also study how quantum an ensemble is by defining a related quantity called quantumness. We find that the classicality of an ensemble is closely related to how perfectly the ensemble can be cloned, and that the quantumness of the ensemble used in a quantum key distribution (QKD) protocol is exactly the attainable lower bound of the error rate in the sifted key.