光极限探测技术在空间通信中的应用

光是人类最早的科学研究对象之一,光子是光的最小能量单元,具备量子的基本特征.随着科学技术发展,人类已经能够实现对单个光子的极限探测.光的常规探测已经普遍应用于地面光纤通信中,而光的极限探测则在空间量子通信及深空超远距离光通信中具备重要的应用价值.文章介绍了光极限探测技术在空间量子科学实验、空间光子通信中的典型应用及涉及...     

远距离量子通信实验中的高维纠缠源

设计了适用于远距离量子通信实验的高维纠缠源.利用连续激光器抽运产生了极化-时间两体四维纠缠光子对,在抽运功率20 mW下测到每秒700对符合,保真度为89%±3%.相比已有的高维纠缠源,在本文中发展的源具有传输便利、相位稳定性好等优点,适用于未来远距离高维量子通信实验和量子力学基本问题实验检验,如远距离高维量子密码实验、两粒子Greenberger-Horne-Zeilinger定理检验、两粒子量子赝"心灵感应"(quantum pseudo telepathy)实验演示等. 关键词： 高维纠缠 极化纠缠 时间箱纠缠 量子通信     

Experimental realization of quantum cryptography communication in free space

Quantum cryptography is a new secure communication protocol with the combina- tion of quantum mechanics and information theory[1]. Its security depends on the laws of physics and has been proved strictly[2,3]. Quantum communication is the art of generat- ing and transmitting the keys through a quantum channel between two parties, usually called Alice and Bob. Unlike the classical key distribution, the quantum keys are gener- ated in the process of transmission instantaneously. The keys can be…     

安全传送明文的量子直传实验方案设计

乒乓直传协议是一种新颖的量子直传通信协议。基于量子纠缠特性,乒乓协议允许绝对安全地进行明文直接通信和渐进地进行密钥安全分发。是一种绝对安全的即时通信协议。该量子直传协议将来有望成为安全稳定的光量子通信的主流方式。然而,目前还没有该协议的物理实现方案。分析了乒乓直传协议的工作原理．给出了该协议的流程图,进而分析了协议的物理实现机制,结合纠缠光子源、自由空间单光子路由控制和单光子检测技术,提出了在自由空间中实现乒乓直传协议的实验技术方案,并设计出了乒乓直传协议的实验装置。此实验技术方案为从实验角度进一步研究乒乓协议,以及未来此协议的商业化应用提供了参考。     

Long distance quantum teleportation in a quantum relay configuration

A long distance quantum teleportation experiment with a fiber-delayed Bell state measurement (BSM) is reported. The source creating the qubits to be teleported and the source creating the necessary entangled state are connected to the beam splitter realizing the BSM by two 2 km long optical fibers. In addition, the teleported qubits are analyzed after 2.2 km of optical fiber, in another laboratory separated by 55 m. Time-bin qubits carried by photons at 1310 nm are teleported onto photons at 1550 nm. The fidelity is of 77%, above the maximal value obtainable without entanglement. This is the first realization of an elementary quantum relay over significant distances, which will allow an increase in the range of quantum communication and quantum key distribution.     

Photon subtraction-based continuous-variable measurement-device-independent quantum key distribution with discrete modulation over a fiber-to-water channel

We propose a discrete-modulated continuous-variable measurement-device-independent quantum key distribution protocol over a fiber-to-water channel. Different from optical fibers, the underwater channel has more severe optical attenuation because of optical absorption and scattering, which reduces the maximum communication distance. To enhance the performance of the protocol, the photon subtraction operation is implemented at the modulator side. We carry out a performance simulation in two different kinds of seawater channel, and the result shows that the scheme with photon subtraction has a longer secure communication distance under certain conditions.     

A Possible Origin of Quantum Correlations

We intend to eliminate the known conflict between relativity and quantum mechanics. We believe the “instant” correlation between entangled distant quantum particles can be explained by the fact that in a laboratory reference frame the photon traveling duration is positive and finite while its proper (in vacuum) traveling duration is equal to zero. In the latter case, any two events that are separated (in a laboratory reference frame) by an arbitrary finite distance can be considered as simultaneous ones. So, the photon nonlocal correlation turns out to be a relative property and may be explained like known twins paradox in relativity. In such a situation, any standard causal interaction between the correlated particles is absent in a laboratory reference frame; however, some specific mutual couple appears between them; this couple is strictly oscillating without some oriented energy or/and information transferring. We also motivate the basic hypothesis extension on quantum particles having nonzero masses.     

An improved quantum repeater protocol using hyperentangled state purification

This article indicates an improved protocol of quantum repeaters through the scheme of nest entanglement purification [Phys. Rev. Lett. 81, 5932 (1998)]. The hyperentangled photon pairs are utilized to overcome the limitations of long distance quantum communication via noisy channels. In addition, it is mentioned that if there are imperfect quantum operations and noises in realistic communication, the fidelities of hyperentangled quantum channels will be improved compared with the traditional entangled states.     

Radiation-induced nonlinear optical response of quartz fibers

The intensity of radiation-induced luminescence and transient optical losses in KU-1 (Russia) and K-3 (Japan) quartz glass optical tibers irradiated in a fast pulsed fission reactor (a pulse duration of 80 μs and a neutron flux up to 7 × 10¹⁶ cm^–2 s^–2) has been measured in the visible range. The intensity of the fast luminescence component nonlinearly depends on the neutron flux. The luminescence intensity and the transient optical losses depend on the probe light intensity. Suppression of radiation-induced luminescence is observed at wavelengths that are longer or shorter than the probe light wavelength. Light probing leads to an increase in transient optical losses and a more rapid recovery of transparency. A model of two photon fluxes is proposed to analyze the relationship of the effects of suppression of radiation-induced luminescence and the increase in optical losses upon light probing. The effect of suppression of radiation-induced luminescence can be used to control the optical properties of fibers in radiation fields.     

门控下InGaAs/InP单光子探测器用于符合测量的时域滤波特性研究

基于砷化镓/磷化铟雪崩光电二极管(InGaAs/InP APD)的半导体单光子探测器因工作在通信波段,且具有体积小、成本低、操作方便等优势,在实用化量子通信技术中发挥了重要作用.为尽可能避免暗计数和后脉冲对单光子探测的影响,InGaAs/InP单光子探测器广泛采用门控技术来快速触发和淬灭雪崩效应,有效门宽通常在纳秒量级.本文研究揭示了门控下单光子探测器可测量的最大符合时间宽度受限于门控脉冲的宽度,理论分析与实验结果良好拟合.该研究表明,门控下InGaAs/InP单光子探测器用于双光子符合测量具有显著的时域滤波特性,限制了其在基于双光子时间关联测量的量子信息技术中的应用.     

Four-State Continuous-Variable Quantum Key Distribution with Photon Subtraction

Four-state continuous-variable quantum key distribution (CVQKD) is one of the discretely modulated CVQKD which generates four nonorthogonal coherent states and exploits the sign of the measured quadrature of each state to encode information rather than uses the quadrature \(\hat {x}\) or \(\hat {p}\) itself. It has been proven that four-state CVQKD is more suitable than Gaussian modulated CVQKD in terms of transmission distance. In this paper, we propose an improved four-state CVQKD using an non-Gaussian operation, photon subtraction. A suitable photon-subtraction operation can be exploited to improve the maximal transmission of CVQKD in point-to-point quantum communication since it provides a method to enhance the performance of entanglement-based (EB) CVQKD. Photon subtraction not only can lengthen the maximal transmission distance by increasing the signal-to-noise rate but also can be easily implemented with existing technologies. Security analysis shows that the proposed scheme can lengthen the maximum transmission distance. Furthermore, by taking finite-size effect into account we obtain a tighter bound of the secure distance, which is more practical than that obtained in the asymptotic limit.     

一种基于双偏振分束器的量子秘密共享方案

提出一个基于双偏振分束器的单量子比特全光纤量子秘密共享方案, 该方案具有自动补偿光纤及光学器件的双折射效应和相位抖动的功能, 在干涉对比度测试和稳定性测试时, 该方案在5 km通信距离中, 获得的干涉对比度优于993%, 且可长时间保持稳定. 关键词： 量子秘密共享 偏振分束器 单光子干涉     

低轨道量子卫星通信星上交换算法及仿真

量子卫星通信能够解决量子移动通信在航海、航空领域中对于远距离和大范围的需求,而星上交换是量子卫星通信的关键技术之一.本文以低轨道量子卫星通信星上交换为研究对象,提出了一种新的星上交换算法—终端测距法.利用该算法测得终端到相邻小区中心的距离,并将测得的数据上传给当前服务卫星系统,再由卫星系统通过比较距离大小决定终端是否切换.数学分析和仿真结果表明,该算法可靠性高、操作方便,能够在各卫星之间实现平稳切换.     

基于弱相干光源测量设备无关量子密钥分发系统的误码率分析

测量设备无关量子密钥分发系统可以免疫任何针对探测器边信道的攻击, 并进一步结合诱惑态方法规避了准单光子源引入的实际安全性问题. 目前实验中一般采用弱相干光源, 但是该光源含有一定比例的空脉冲和多光子脉冲. 本文针对弱相干光源的具体特性, 采用量子力学的描述, 将各个器件进行量子化处理, 并同时考虑探测器的具体性能参数的影响, 分别给出了通信双方各自发送的脉冲含有特定光子数时产生的成功贝尔态和错误贝尔态的概率公式, 从理论上对相位编码和偏振编码测量设备无关量子密钥分发系统的误码率进行了定量分析, 分别推导并模拟了通信双方采用的平均光子数对称和不对称时误码率随传输距离的变化情况, 结果表明在偏振编码Z基中, 多光子脉冲不会引起误码; 在偏振编码X基和相位编码中, 受多光子影响, 产生的误码率较大. 对于不同的编码方式, 误码率均随传输距离的增加有不同程度的升高, 长距离传输时, 平均光子数越小, 产生的误码率越大; 在偏振编码X基和相位编码的短距离传输中, 相对于对称, 通信双方采用的平均光子数不对称时产生的误码率较大.     

Ionization of deep quantum wells: Optical trampoline effect

A new mechanism of transitions of an electronic system from the ground state to states with excitation energies exceeding many times the energy of a light photon initiating the transitions has been considered. This mechanism is based on the so-called optical “trampoline” effect: one of the interacting electrons receives energy from another electron and, simultaneously absorbing a photon ?ω, overcomes the energy gap significantly exceeding ?ω. Ionization of deep quantum wells by low-frequency light of moderate intensity due to the optical trampoline effect was calculated.     

Remote preparation of <Emphasis Type="Italic">N</Emphasis> photon GHZ polarization entangled states within a network

We propose a new linear optical protocol for remote state preparation (RSP) between two parties under control of a number of controllers in terms of optical elements. The proposed setup involves simple linear optical elements, a N-photon polarization entangled state, and photon de tectors, witch have been widely used in experiment. The realization of this protocol is appealing due to the fact that quantum state of light is robust against the decoherence and photons are ideal carriers for transmitting quantum information over long distances.     

All linear optical quantum memory based on quantum error correction

When photons are sent through a fiber as part of a quantum communication protocol, the error that is most difficult to correct is photon loss. Here we propose and analyze a two-to-four qubit encoding scheme, which can recover the loss of one qubit in the transmission. This device acts as a repeater, when it is placed in series to cover a distance larger than the attenuation length of the fiber, and it acts as an optical quantum memory, when it is inserted in a fiber loop. We call this dual-purpose device a "quantum transponder."     

Mesoscopic interferometers for electron waves

Mesoscopic interferometers are electronic analogues of optical interferometers, with “quantum point contacts” playing the role of optical beam splitters. Mesoscopic analogues of two-slit, Mach-Zehnder and Fabry-Perot interferometers have been built. A fundamental difference between electron and photon interferometry is that electron interferometry is nonlocal.     

From quantum trajectories to classical orbits

The evolution of open quantum systems can be “unraveled” into individual “trajectories” in a variety of ways. In the mesoscopic regime, quantum jump (QJ) trajectories approach a diffusive limit similar to quantum state diffusion (QSD). In the classical limit, both unravelings show the rise of classical orbits for both regular and chaotic systems.