基于偏振编码的副载波复用量子密钥分发研究

提出了一种采用相位合成偏振态编码的副载波复用量子密钥分发方案,该方案在不增加系统设备的前提下允许多路量子密钥通过并行的边带信道同时进行分发．每一个边带的偏振态可通过控制副载波的相位随机且独立地合成．该方案允许量子密钥系统的成钥率成倍的增加．仿真实验证明,通过调整每个副载波相位能够理想地调整和控制边带的偏振态．     

Soliton wavelength-division multiplexing system with channel-isolating notch filters

We propose a wavelength-division multiplexing system in which transmission of solitons is stabilized by fixed- or sliding-frequency notch filters (a soliton rail), providing channel isolation. We demonstrate analytically and numerically that a soliton trapped in a channel between two notches is very robust. We also predict an optimum ratio between the channel separation and the soliton's spectral width. The effects of interchannel collisions are considered, and it is demonstrated that these effects can be largely eliminated by notch filters, which require a compensatory gain that is comparable with the basic gain balancing the fiber loss.     

A new synchronization scheme based on time division multiplexing and wavelength division multiplexing technology for practical quantum key distribution system

Three clock synchronization schemes for a quantum key distribution system are compared experimentally through the outdoor fibre and the interaction physical model of the the clock signal and the the quantum signal in the quantum key distribution system is analysed to propose a new synchronization scheme based on time division multiplexing and wavelength division multiplexing technology to reduce quantum bits error rates under some transmission rate conditions.The proposed synchronization scheme can not only completely eliminate noise photons from the bright background light of the the clock signal,but also suppress the fibre nonlinear crosstalk.     

Incompatibility of polarization-division multiplexing with wavelength-division multiplexing in soliton-transmission systems

In a recent experiment it was demonstrated that polarization-division multiplexing was incompatible with wavelength-division multiplexing. We discuss a theoretical model that explains this result.     

All-fiber zero-insertion-loss add-drop filter for wavelength-division multiplexing

We developed and fabricated an all-fiber add-drop filter by recording a Bragg grating in the waist of an asymmetric mode converter-coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-mode optical fibers. The insertion loss of the device was ~0.1 dB .A narrow spectral bandwidth (<1 nm) and a large add-drop efficiency (>90%) were also demonstrated. In addition, the filter was polarization independent.     

FPGA based digital phase-coding quantum key distribution system

Quantum key distribution(QKD) is a technology with the potential capability to achieve information-theoretic security. Phasecoding is an important approach to develop practical QKD systems in fiber channel. In order to improve the phase-coding modulation rate, we proposed a new digital-modulation method in this paper and constructed a compact and robust prototype of QKD system using currently available components in our lab to demonstrate the effectiveness of the method. The system was deployed in laboratory environment over a 50 km fiber and continuously operated during 87 h without manual interaction. The quantum bit error rate(QBER) of the system was stable with an average value of 3.22% and the secure key generation rate is 8.91 kbps. Although the modulation rate of the photon in the demo system was only 200 MHz, which was limited by the FaradayMichelson interferometer(FMI) structure, the proposed method and the field programmable gate array(FPGA) based electronics scheme have a great potential for high speed QKD systems with Giga-bits/second modulation rate.     

Substrate-strain-induced tunability of dense wavelength-division multiplexing thin-film filters

Classic dense wavelength-division multiplexing thin-film filters can be spectrally tuned through the substrate's strain. We analyze the theoretical shift of the design wavelength of a narrow-bandpass filter when uniform, uniaxial compressive stress is applied to the substrate, and we compare calculated sensitivity with experimental data. We measure the transmittance shape of a 200-GHz standard filter for several loading cases to quantify the increase of insertion losses.     

光波分复用实验的改进和扩展

利用自聚焦透镜代替集成的波分复用器件,对2路波分复用实验结构进行了改进,并在实验中增加光谱仪用于观察光纤中的光谱.通过上述改进,使学生进一步了解波分复用器内部结构,加深对复用和解复用的理解.     

Measurement-device-independent quantum key distribution

How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem--measurement-device-independent quantum key distribution (QKD). It not only removes all detector side channels, but also doubles the secure distance with conventional lasers. Our proposal can be implemented with standard optical components with low detection efficiency and highly lossy channels. In contrast to the previous solution of full device independent QKD, the realization of our idea does not require detectors of near unity detection efficiency in combination with a qubit amplifier (based on teleportation) or a quantum nondemolition measurement of the number of photons in a pulse. Furthermore, its key generation rate is many orders of magnitude higher than that based on full device independent QKD. The results show that long-distance quantum cryptography over say 200 km will remain secure even with seriously flawed detectors.     

Side-channel-free quantum key distribution

Quantum key distribution (QKD) offers the promise of absolutely secure communications. However, proofs of absolute security often assume perfect implementation from theory to experiment. Thus, existing systems may be prone to insidious side-channel attacks that rely on flaws in experimental implementation. Here we replace all real channels with virtual channels in a QKD protocol, making the relevant detectors and settings inside private spaces inaccessible while simultaneously acting as a Hilbert space filter to eliminate side-channel attacks. By using a quantum memory we find that we are able to bound the secret-key rate below by the entanglement-distillation rate computed over the distributed states.     

Frequency-coded quantum key distribution

We report an intrinsically stable quantum key distribution scheme based on genuine frequency-coded quantum states. The qubits are efficiently processed without fiber interferometers by fully exploiting the nonlinear interaction occurring in electro-optic phase modulators. The system requires only integrated off-the-shelf devices and could be used with a true single-photon source. Preliminary experiments have been performed with weak laser pulses and have demonstrated the feasibility of this new setup.     

Tunable wavelength-division multiplexing based on metallic nanoparticle arrays

We report a tunable wavelength-division multiplexing (WDM) structure based on two-dimensional silver nanoparticle arrays. The linewidth of the multiple geometric resonances of the arrays is of the order of several nanometers generally, which guarantees high wavelength selectivity. Optical channels can be selectively activated by setting the polarization of the incident wave. The operation wavelength can be tuned from the visible to the near infrared, and the free spectral range can be adjusted from hundreds to tens of nanometers by varying the size of the constituent particles and the interparticle distances. The proposed structure can provide an extinction ratio of ~10 and a quality factor of ~700. This tunable, easy-to-produce, and subwavelength WDM structure is desirable for plasmonic integrated circuits.     

Experimental demonstration of subcarrier multiplexed quantum key distribution system

We provide, to our knowledge, the first experimental demonstration of the feasibility of sending several parallel keys by exploiting the technique of subcarrier multiplexing (SCM) widely employed in microwave photonics. This approach brings several advantages such as high spectral efficiency compatible with the actual secure key rates, the sharing of the optical fainted pulse by all the quantum multiplexed channels reducing the system complexity, and the possibility of upgrading with wavelength division multiplexing in a two-tier scheme, to increase the number of parallel keys. Two independent quantum SCM channels featuring a sifted key rate of 10 Kb/s/channel over a link with quantum bit error rate <2% is reported.     

Integrated quantum key distribution system using single sideband detection

We report a new quantum cryptographic system involving single sideband detection and allowing an implementation of the BB84 protocol. The transmitted bits are reliably coded by the phase of a high frequency modulating signal. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally at 1 550 nm using compact and conventional device technology. Single photon interference has been obtained with a fringe visibility greater than 98%, indicating that the system can be used in view of quantum key distribution potentially beyond 50-km-long standard single-mode fiber. Received 13 July 2001 and Received in final form 30 November 2001     

连续变量量子密码术

文章综述了连续变量量子密码术的基本原理，突出了其在光源制备、光子探测以及量子密钥生成的码率等办面相对于单光子量子密码术的优越性，给出了连续变量量子密码术的安全性以及对线路噪声的具体要求，提出了连续变量量子密码术目前所面临的主要困难和今后的发展前景。     

Decoy state quantum key distribution

There has been much interest in quantum key distribution. Experimentally, quantum key distribution over 150 km of commercial Telecom fibers has been successfully performed. The crucial issue in quantum key distribution is its security. Unfortunately, all recent experiments are, in principle, insecure due to real-life imperfections. Here, we propose a method that can for the first time make most of those experiments secure by using essentially the same hardware. Our method is to use decoy states to detect eavesdropping attacks. As a consequence, we have the best of both worlds--enjoying unconditional security guaranteed by the fundamental laws of physics and yet dramatically surpassing even some of the best experimental performances reported in the literature.     

Intrinsically stable phase-modulated polarization encoding system for quantum key distribution

We demonstrate experimentally an intrinsically stable polarization coding and decoding system composed of optical-fiber Sagnac interferometers with integrated phase modulators for quantum key distribution. An interference visibility of 98.35% can be kept longtime during the experiment without any efforts of active compensation for coding all four desired polarization states.     

Traveling-wave polymer devices as wavelength converters for wavelength-division multiplexing applications

Wavelength conversion of optical signals as a result of refraction through a moving interface in traveling-wave electro-optic phase modulators has been analyzed. The connection between wavelength conversion and phase modulation with velocity mismatch has been investigated both analytically and by use of computer simulation. The configuration of a device performing the conversion is proposed, and the operating requirements are determined. Devices based on the described technique are especially promising for wavelength conversion in wavelength-division multiplexing applications and possess several advantages over competing all-optical methods.