光纤色散与损耗对光量子密钥分发系统的影响

运用量子薛定谔方程，通过计算传输光场干涉强度和分析量子密钥分发系统的误码率，研究了光纤色散和损耗对量子密钥分发系统误码率的影响.研究表明，在目前的基于光纤的量子密钥分发系统中，选择色散位移光纤并以损耗最小波长1.55 μm为工作波长，同时缩短脉冲宽度，可以有效提高量子密钥系统的传输距离. 关键词： 量子保密通信 量子密钥分发 光纤色散 光纤损耗     

磁光效应对量子相位编码系统的影响分析

受光纤双折射的影响,单光子极化态在光纤中传输时容易发生慢变进而影响到量子密钥分发系统的性能。利用概率波叠加理论分析了法拉第旋转镜的时分复用干涉仪相位编码系统。分析结果表明,利用单光子往返通过同一根光纤可以有效抵消光纤双折射的影响,但容易受到非互易性效应如磁光效应的影响。随着磁光旋转角的增加,成功到达干涉仪的单光子形成正常干涉条纹的概率逐渐减小。旋转角大于42°时,形成正常干涉条纹的概率接近于零。     

基于微弱相干脉冲稳定差分相位量子密钥分发

基于差分相位量子密钥分发协议，对微弱相干光脉冲相位差进行编码，在接收端采用Faraday-Michelson系统进行解码.这种量子密钥分发系统具有密钥生成效率高、接收端干涉稳定性好、极限传输距离长等优点，同时还具有光路结构简单、易于在现有的技术条件下实现等特点，特别适用于远程光纤量子密钥分发.在实验系统中利用嵌入式微处理系统来控制量子密钥分发过程，进行了76 km的稳定光纤量子密钥分发实验，其原始密钥的误码率为5.3%. 关键词： 分相位 量子密钥分发 安全性 稳定性     

偏振稳定控制下的量子密钥分发

由于长距离单模光纤传输中存在的双折射效应会引起偏振随机抖动，光纤中利用偏振编码进行量子密钥分发一直难以实现.利用光子计数分析光纤中的偏振变化情况，并通过反馈控制的方式补偿偏振变化，从而实现了基于BB84协议的偏振编码长时间稳定的量子密钥分发实验，传输距离为100km. 关键词： 量子密钥分发 偏振反馈控制 单光子探测 偏振随机抖动     

基于单光子计数反馈的低噪声光纤信道波分复用实时偏振补偿系统

光纤信道由于受环境影响产生的随机双折射等物理效应使得在其中传输的光信号具有敏感的偏振变化,严重影响了偏振编码量子密钥分发系统的性能.本文提出了一种利用单光子计数作为反馈信号的低噪声光纤信道波分复用实时偏振补偿系统,该系统通过探测共轭参考光的光子计数得到光纤信道偏振变化信息,设计补偿算法控制电动偏振控制器实时校准对应偏振基下量子信号光的偏振态,成功实现了稳定的光纤信道偏振补偿.为验证补偿系统的有效性,进行了传输距离为25.2 km的基于BB84协议的量子密钥分发测试,在实验室环境和模拟城域网地埋光纤环境下得到了长达8 h的稳定测试结果,平均量子比特误码率分别为0.52%和1.25%.该实验结果表明本系统可在城域网地埋光纤环境下保障偏振编码量子密钥分发的稳定工作.     

基于双偏振分束器的量子密钥分发系统

提出了利用两个偏振分束器的量子密钥分发系统，有效地解决了相位调制器的偏振依赖性问题. 以1310nm波长在通信距离为25km的光纤中实现了高密钥生成率，干涉对比度99.4%. 有效密钥生成率大于0.6kbit/s, 误码率0.5%. 关键词： 量子密钥分发 偏振分束器 单光子干涉     

单光子探测器性能对量子密钥分发系统的影响

根据量子密钥分发协议的原理和单光子探测器的工作模式系统分析了单光子探测器性能对量子密钥分发系统影响的物理模型,给出了单光子探测器暗计数和量子效率对系统筛选码率和量子误码率等指标产生影响的计算公式,并通过对各类量子密钥分发系统的比较,推导出了适用于一般量子密钥分发系统的普适结果。结果表明,在同等条件下采用较少的探测时间门可以提高量子密钥分发系统的性能。     

双协议量子密钥分发系统实验研究

提出了一种改进的基于时间和相位混合编码的量子密钥分发方案, 并进行了实验研究.在以BB84协议为基础的相位编码量子密钥分发系统上, 利用了系统中原来舍弃的脉冲进行时间编码, 使成码率提高为原方案的二倍. 系统同时获得时间编码密钥和相位编码密钥, 现在可以将两组密钥组合成新密钥, 提高了成码率和监测窃听灵敏度. 同时在系统的接收端用双FM反射式干涉仪代替传统的光纤M-Z干涉仪,提高了系统的稳定性. 实验上已实现90km光纤量子密钥分发, 实验表明本系统具有安全性高,稳定性好,成本低的优点. 关键词： 量子保密通信 量子密钥分发 相位编码 时间编码     

光纤偏振编码量子密钥分发系统荧光边信道攻击与防御

实际安全性是目前量子密钥分发系统中最大的挑战.在实际实现中,接收单元的单光子探测器在雪崩过程的二次光子发射(反向荧光)会导致信息泄露.目前,已有研究表明该反向荧光会泄露时间和偏振信息并且窃听行为不会在通信过程中产生额外误码率,在自由空间量子密钥分发系统中提出了利用反向荧光获取偏振信息的攻击方案,但是在光纤量子密钥分发系统中暂未见报道.本文提出了在光纤偏振编码量子密钥分发系统中利用反向荧光获取信息的窃听方案与减少信息泄露的解决方法,在时分复用偏振补偿的光纤偏振编码量子密钥分发系统的基础上对该方案中窃听者如何获取密钥信息进行了理论分析.实验上测量了光纤偏振编码量子密钥分发系统中反向荧光的概率为0.05,并对本文提出的窃听方案中的信息泄露进行量化,得出窃听者获取密钥信息的下限为2.5×10~(–4).     

基于实时优化单光子干涉可视度的相位编码量子密钥分发

在相位编码量子密钥分发系统中,单光子干涉可视度决定了系统的成码率。本文提出一种优化单光子干涉可视度的方法,即利用遗传算法和四通道偏振控制器实时补偿单光子偏振漂移,同时利用时分复用参考光连续无复位地补偿单光子相位漂移,最终在25km光纤中实时优化的单光子干涉可视度达到98.6%,相位编码量子密钥分发系统成码率为2.2kbit/s。     

Design of defect-core in highly birefringent photonic crystal fibers with anisotropic claddings

The influence of defect-core on the birefringence and confinement losses of rectangular-lattice photonic crystal fibers are investigated numerically by applying the multipole method. Numerical results illustrate that the birefringence in such fibers is determined not only by the arrangement of air holes in the cladding but also the shape of the core. It is found that asymmetry of the core represented by its rectangular shape implies a higher effective index of the mode that is parallel with the longer side of the rectangle, whereas the anisotropic rectangular-lattice cladding gives rise to just the opposite effect and thus the resulting birefringence can be controlled by a proper combinations of both mechanisms. In particular, effect of the asymmetry of the core on the birefringence is dominant for shorter wavelength. Increased birefringence and reduced confinement loss can be achieved, if we form the core by the omission of several air holes in a row to reduce its negative effect on the birefringence. On the other hand, when asymmetry is increased in the other direction, a negative birefringence at shorter wavelength can be achieved. This occurs due to the fact that asymmetry of the core at higher frequencies overcomes the effect of the asymmetric cladding. As a result, its possible to achieve zero birefringence in anisotropic cladding photonic crystal fiber with an asymmetric core.     

Magneto-optic fiber Sagnac modulator based on magnetic fluids

A magneto-optic modulator with a magnetic fluid film inserted into an optical fiber Sagnac interferometer is proposed. The magnetic fluid exhibits variable birefringence and Faraday effect under external magnetic field that will lead to a phase difference and polarization state rotation in the Sagnac interferometer. As a result, the intensity of the output light is modulated under the external magnetic field. Moreover, the modulator has a high extinction ratio and can easily be integrated in a single-mode fiber system. The performance of the modulator is not affected by ambient temperature variation from room temperature to 40 °C.     

光学平衡桥式电流互感器的设计

为了消除传感头随温度变化的线性双折射对光学电流互感器性能的影响,提出了光学平衡桥式电流互感器的概念,并给出了设计原理.推导了与温度有关的线性双折射导致的线偏振光方化角变化量的解析式,并计算了用1/2波片来消除线性双折射影响时波片光轴的方向.给出了光学平衡桥的工作原理和信号处理的方法,推导出计算电流的表达式.进行了实验研究,螺线管电流为2 A,温度在20℃-65℃范围变化时,所设计互感器的输出误差在2.96%以内.提出的光学平衡桥式电流互感器,解决了原有方法存在的线性双折射随温度变化对测量精度影响的难题,有利于促进光学电流互感器的实用化进程.     

Deuterium NMR investigation of the biaxial nematic phase in an organosiloxane tetrapode

Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.     

Temperature dependence of birefringence in olarization-maintaining photonic crystal fibres

In this paper,the temperature dependence of birefringence in polarization maintaining photonic crystal fibres(PMPCFs) is investigated theoretically and experimentally.Utilizing the structural parameters of the PM-PCF samples in the experiment,two effects leading to the birefringence variation under different temperatures are analysed,which are the thermal expansion of silica material and the refractive index variation due to the temperature variation.The actual birefringence variation of the PM-PCF is the combination of the two effects,which is in the order of 10-9 K-1 for both fibre samples.Calculation results also show that the influence of refractive index variation is the dominant contribution,which determines the tendency of the fibre birefringence variation with varying temperature.Then,the birefringence beat lengths of the two fibre samples are measured under the temperature,which varies from -40℃ to 80℃.A traditional PANDA-type polarization maintaining fibre(PMF) is also measured in the same way for comparison.The experimental results indicate that the birefringence variation of the PM-PCF due to temperature variation is far smaller than that of the traditional PMF,which agrees with the theoretical analysis.The ultra-low temperature dependence of the birefringence in the PM-PCF has great potential applications in temperature-insensitive fibre interferometers,fibre sensors,and fibre gyroscopes.     

Asymmetric two-component Fermion systems in strong coupling

We study the phase structure of a dilute two-component Fermi system with attractive interactions as a function of the coupling and a finite number asymmetry or polarization. In weak coupling, a number asymmetry results in phase separation. A mixed phase containing symmetric superfluid matter and an asymmetric normal phase is favored. For strong coupling we show that the stress on the superfluid phase to accommodate a number asymmetry increases. Near the infinite-scattering length, we calculate the single-particle excitation spectrum and the ground-state energy. A picture of weakly interacting quasiparticles emerges for modest polarizations. In this regime a homogeneous phase with a finite population of quasiparticle states characterized by a gapless spectrum is favored over the phase separated state. These states may be realized in cold atom experiments.     

Baryogenesis from the Kobayashi-Maskawa phase

The smallness of quark masses suppresses the CP violation from the Kobayashi-Maskawa phase to a level that is many orders of magnitude below what is required to explain the observed baryon asymmetry. We point out that if, as a result of time variation in the Yukawa couplings, quark masses were large at the time of the electroweak phase transition, then the Kobayashi-Maskawa mechanism could be the source of the asymmetry. The Froggatt-Nielsen mechanism provides a plausible framework where the Yukawa couplings could all be of order 1 at that time, and settle to their present values before nucleo-synthesis. The problems related to a strong first order electroweak phase transition may also be alleviated in this framework. Our scenario reveals a loophole in the commonly held view that the Kobayashi-Maskawa mechanism cannot be the dominant source of CP violation to play a role in baryogenesis.     

Effect of interfacial coupling on rectification in organic spin rectifiers

The effect of interfacial coupling on rectification in an organic co-oligomer spin diode is investigated theoretically by considering spin-independent and spin-resolved couplings respectively. In the case of spin-independent coupling, an optimal interfacial coupling strength with a significant enhanced rectification ratio is found, whose value depends on the structural asymmetry of the molecule. In the case of spin-resolved coupling, we found that only the variation of the interfacial coupling with specific spin is effective to modulate the rectification, which is due to the spin-filtering property of the central asymmetric magnetic molecule. A transition of the spin-current rectification between parallel spin-current rectification and antiparallel spin-current rectification may be observed with the variation of the spin-resolved interfacial coupling. The interfacial effect on rectification is further analyzed from the spin-dependent transmission spectrum at different biases.     

Holographic investigation of induced absorption

A holographic technique for studying induced absorption in transmitting media using a scheme with an additional phase measuring grating is described. A matrix analysis is conducted, and expressions for the induced absorption are derived from the asymmetry of the transmitted beam intensity variation. The dynamics of the induced absorption in a photochromic glass is studied by the standard single-beam diffraction method and a new dual-beam interference method. The influence of a residual absorption grating resulting from the measuring phase grating on the induced absorption dynamics is analyzed. Measuring data obtained by the two methods are shown to be in good agreement     

Leptogenesis via a varying Weinberg operator:a semi-classical approach

In this paper, we introduce leptogenesis via a varying Weinberg operator from a semi-classical perspective.This mechanism is motivated by the breaking of an underlying symmetry which triggers a phase transition that causes the coupling of the Weinberg operator to become dynamical. Consequently, a lepton anti-lepton asymmetry arises from the interference of the Weinberg operator at two different spacetime points. Using the semi-classical approach,we treat the Higgs as a background field and show that a reflection asymmetry between leptons and anti-leptons is generated in the vicinity of the bubble wall. We solve the equations of motion of the lepton and anti-lepton quasiparticles to obtain the final lepton asymmetry.