Quantum information processing in optical images

Optical images can be used to transport, store and process information in a parallel way. We discuss different results obtained in the domain of ‘quantum imaging’, aiming at exploiting at the same time the quantum properties of optical images and their intrinsic parallelism. We define the notion of standard quantum limit (SQL) in optical resolution, set by the quantum noise of usual coherent light, and show that it can be much lower than the diffraction limit. We also prove that this limit can be circumvented by especially designed nonclassical and multimode light. We present an experiment showing that OPOs oscillating inside an exactly confocal cavity actually produce such transverse multimode nonclassical light. We finally describe another experiment which has surpassed the SQL in the case of beam positioning, both in the 1D and 2D cases.     

From computation to black holes and space-time foam

We show that quantum mechanics and general relativity limit the speed nu of a simple computer (such as a black hole) and its memory space I to I(nu2) less, similar(t(-2))P, where t(P) is the Planck time. We also show that the lifetime of a simple clock and its precision are similarly limited. These bounds and the holographic bound originate from the same physics that governs the quantum fluctuations of space-time. We further show that these physical bounds are realized for black holes, yielding the correct Hawking black hole lifetime, and that space-time undergoes much larger quantum fluctuations than conventional wisdom claims-almost within range of detection with modern gravitational-wave interferometers.     

基于压缩光的量子精密测量

对任何物理量的测量都有一定的噪声, 经典测量所能达到的最小噪声一般称为散粒噪声, 对应着测量的标准量子极限. 利用压缩光可以突破标准量子极限, 从而提高测量精度. 本文介绍了压缩态光场用于突破标准量子极限的基本原理, 以及压缩态光场在相位测量、光学横向小位移及倾斜测量、磁场测量以及时钟同步等精密测量领域的应用和最新进展.     

Meta-Entanglement

We give a meta-logical interpretation of the entanglement mechanism of quantum space-time in terms of the sequent calculus of a quantum sub-structural logic. This meta-logical picture is based mainly on the two meta-rules cut and EPR, and on the new meta-theorem “teleportation” (TEL), built by the use of the above meta-rules, both performed in parallel. The proof of (TEL)-theorem fairly reproduces the protocol of quantum teleportation. In the framework of space-time entanglement, the conclusion of the (TEL)-theorem is that the entangled space-time can convey the quantum teleportation of an unknown quantum state. We also introduce two new structural rules: the Hadamard (H)-rule and the CNOT-rule, the latter being used, together with the cut, in the proof of the new theorem “Entanglement” (ENT).

     

Pulsed homodyne measurements of femtosecond squeezed pulses generated by single-pass parametric deamplification

A new scheme is described for the generation of pulsed squeezed light by use of femtosecond pulses that have been parametrically deamplified through a single pass in a thin (100-microm) potassium niobate crystal with a significant deamplification of approximately -3 dB. The quantum noise of each pulse is registered in the time domain by single-shot homodyne detection operated with femtosecond pulses; the best squeezed quadrature variance was 1.87 dB below the shot-noise level. Such a scheme provides a basic resource for time-resolved quantum communication protocols.     

Quantum fluctuations in holographic teleportation of optical images

We investigate in detail the quantum fluctuations in the quantum holographic teleportation protocol that we recently proposed [11]. This protocol implements a continuous variable teleportation scheme that enables the transfer of the quantum state of spatially multimode electromagnetic fields, preserving their quantum correlations in space-time, and can be used to perform teleportation of 2D optical images. We derive a characteristic functional, which provides any arbitrary spatio-temporal correlation function of the teleported field, and calculate the fidelity of the teleportation scheme for multimode Gaussian input states. We show that for multimode light fields one has to distinguish between a global and a reduced fidelity. While the global fidelity tends to vanish for teleportation of fields with many degrees of freedom, the reduced fidelity can be made close to unity by choosing properly the number of essential degrees of freedom and the spatial bandwidth of the EPR beams used in the teleportation scheme.Received: 16 March 2004, Published online: 11 May 2004PACS: 03.67.-a Quantum information - 03.65.Bz Foundations, theory of measurement, miscellaneous theories (including Aharonov-Bohm effect, Bell inequalities, Berrys phase) - 42.50.Dv Nonclassical states of the electromagnetic field, including entangled photon states; quantum state engineering and measurements     

Generation and application of twin beams from an optical parametric oscillator including an alpha-cut KTP crystal

Measurement slight amounts of absorption of light with accuracy beyond the standard quantum limit has been experimentally demonstrated. The quantum-correlated twin beams used in the measurement were generated from a nondegenerate optical parametric oscillator including an alpha-cut KTiOPO(4) crystal pumped by an intracavity frequency-doubled Nd:YAG laser. The noise in the intensity difference between the twin beams was reduced by 88% below the standard quantum limit (SQL). The signal-to-noise ratio was improved by 7 dB with respect to the SQL of the total light employed in the experiment and by 4 dB with respect to that of the signal light.     

基于啁啾脉冲干涉仪的高精度定位方案设计分析

提出利用啁啾脉冲干涉仪进行高精度空间定位的新方案,该方案利用2个具有强的经典频率关联的正负啁啾脉冲,入射到分束器后经和频晶体产生和频干涉信号,通过探测所得干涉信号精确确定经过不同路径的2个光信号的相对延迟时间,从而精确确定目标位置,实现高精度定位,理论上可以达到微米量级的定位精度。研究正负啁啾脉冲不完全对称即正负啁啾系数大小的差异对定位结果的影响。结果表明:正负啁啾系数大小的差异对定位结果影响很小,定位结果对这种差异有很强的容忍度。该方案除具有量子定位中HOM(Hong-Ou-Mandel)干涉仪的所有优点外,还具有装置简单,抑制损耗等优点。     

Decoherence-free subspaces in quantum key distribution

We demonstrate that two recent innovations in the field of practical quantum key distribution (one-way autocompensation and passive detection) are closely related to the methods developed to protect quantum computations from decoherence. We present a new scheme that combines these advantages, and propose a practical implementation of this scheme that is feasible using existing technology.     

基于量子增强型光纤马赫-曾德尔干涉仪的低频信号测量

利用低频光通信波段真空压缩态光场可实现基于光纤的量子精密测量.本文利用简并光学参量振荡器实验制备出1550 nm低频真空压缩态光场.在分析频段10—500 kHz范围内压缩态光场的压缩度均达3 dB.用实验制备的1550 nm真空压缩态光场填补光纤马赫-曾德尔干涉仪的真空通道,实现了量子增强型光纤马赫-曾德尔干涉仪,完成了突破标准量子极限的相位调制频率为500 kHz的低频信号测量.与光纤马赫-曾德尔干涉仪相比,测量信噪比提高了2 dB.     

Robust entanglement through macroscopic quantum jumps

We propose an entanglement generation scheme that requires neither the coherent evolution of a quantum system nor the detection of single photons. Instead, the desired state is heralded by a macroscopic quantum jump. Macroscopic quantum jumps manifest themselves as a random telegraph signal with long intervals of intense fluorescence (light periods) interrupted by the complete absence of photons (dark periods). Here we show that a system of two atoms trapped inside an optical cavity can be designed such that a dark period prepares the atoms in a maximally entangled ground state. Achieving fidelities above 0.9 is possible even when the single-atom cooperativity parameter is as low as 10 and when using a photon detector with an efficiency as low as eta=0.2.     

Quantum control and manipulation of multi-color light fields

Quantum control of stationary multi-color (MC) light fields in resonant medium with multi-Λ scheme of atomic transition is proposed. We have found the general analytical solution in the adiabatic limit of quantum evolution resulting from the interaction of the slow probe light with the new fields generated in the nondegenerate multi-wave mixing scheme. We have found a critical condition for the stopping and quantum manipulation of the MC-light fields where the united light group velocity can be reduced down to zero with optimal spectral parameters while preserving the delicate quantum correlations of the initial probe light pulse. The manipulations, which provide the effective transference of quantum probe light to the new multifrequency light fields have been analyzed. The text was submitted by the authors in English.     

基于零拍探测和压缩真空光输入增强Sagnac效应

利用量子技术增强Sagnac效应提高陀螺输出精度具有重要的研究意义, 是实现全自主导航的重要途径. 以相干态激光作为输入光源的光学陀螺因真空零点波动使其输出精度限制于散粒噪声极限而难以提高. 为减小真空波动的影响, 提出在激光输入的分束器的另一输入端输入压缩真空光并结合平衡零拍探测技术的方法增强Sagnac效应. 理论分析表明Sagnac效应性能得到有效提升: 干涉输出的灵敏度检测极限和动态范围均随着压缩程度的增加而呈指数级增长. 该方法只需对经典光学陀螺做少量改动就可实现, 是提高光学陀螺输出精度的一种新方法.     

基于相干叠加态的非正交编码诱骗态量子密钥分发

非正交编码协议和诱骗态方法可以有效地抵御光子数分离攻击. 由于相干叠加态中单光子成分高达90%, 常作为单光子量子比特的替代出现, 用于量子信息过程处理和计算. 本文结合非正交编码协议和诱骗态方法提出一种新的量子密钥分发方案, 光源采用相干叠加态, 推导了单光子的密钥生成速率、计数率下限和误码率的上限, 利用Matlab 模拟了无限多诱骗态情况下和有限多诱骗态情况下密钥生成速率和传输距离的关系, 得出该方案可以提升密钥生成速率并且提高安全传输距离, 验证了该方案可以进一步提高量子密钥分发系统的性能.     

Gravitational Effects in the Propagation and in the Emission of Light

Following Chou's viewpoint that the Cartesian system of space coordinates and timedefines a Minkowskian space-time in which Einstein's field equations and the harmonic conditionare the laws of gravitation (Ref.[1]), we study the effects of an external gravitational fieldin the propagation and in the emission of light respectively from the generalized Maxrwell andDirac equations of the general theory of relativity by geometric optics and quantum mechanics,and get the usual results for the red shift. For the naively homogeneous and isotropic model ofcosmology (the case with k = 0 in the Robertson-Walker scheme) closed form for the relationbetween distance and red shift is obtained, which indicates that a new way of analyzing theobservational data with not too small red shifts is needed.     

Experimentally feasible quantum erasure-correcting code for continuous variables

We devise a scheme that protects quantum coherent states of light from probabilistic losses, thus achieving the first continuous-variable quantum erasure-correcting code. If the occurrence of erasures can be probed, then the decoder enables, in principle, a perfect recovery of the original light states. Otherwise, if supplemented with postselection based on homodyne detection, this code can be turned into an efficient erasure-filtration scheme. The experimental feasibility of the proposed protocol is carefully addressed.     

PbSe量子点近红外光源的CH_4气体检测

研制了一种新型的PbSe量子点近红外光源,其光致发光谱较窄,能有效匹配气体的红外吸收峰。采用配位溶剂法合成出5.1 nm的PbSe量子点,并将其沉积到Ga N芯片上(沉积厚度为165.5μm),经过紫外光照处理和固化后制成了光致发光的近红外光源。该光源第一激子吸收峰位于1 592 nm,光致发光峰位于1 665 nm,其发射光谱包含了1 625~1 840 nm之间的CH_4气体的全部的吸收谱。利用其进行CH_4气体浓度检测实验,结果表明,系统的检测下限可以达到100×10~(-6),检测误差为2%。这种由PbSe量子点近红外光源构成的新型检测系统具有低功耗、低成本和高效能等优点,将其应用在气体检测中时可以略去滤光片,其在红外气体检测领域中有着较广阔的应用前景。     

混沌光场光子统计分布及二阶相干度的分析与测量

利用通信波段双通道单光子探测器,采用Hanbury Brown-Twiss关联测量方案,理论分析并实验测量了光反馈半导体激光器产生的混沌光场的光子统计分布及不同混沌状态光场的二阶相干度.通过对混沌光场二阶相干度g~((2))(τ)的理论分析,得出随着延迟时间和相干时间的变化,其与相干光、热光及单光子态的二阶相干度可明显区分并呈现出不同分布.同时实验上产生了频谱宽度6.7 GHz的混沌光场,测量了不同光子数分布的结果,并用高斯随机分布、泊松分布、玻色-爱因斯坦分布对光子数分布进行理论拟合,发现随着入射平均光子数的增加,光子数分布从玻色-爱因斯坦分布过渡到泊松分布,但整个过程都与高斯随机分布符合较好,且光场的二阶相干度g~((2))(0)由2降至1.通过改变偏置电流(I=1.0Ith-2.0Ith)和反馈强度(0—10%),实验上研究了混沌光场由低频起伏到相干塌陷的过程中不同状态宏观动力学特性与二阶相干度的对应关系.结果表明:混沌光场在此过程中始终呈现出明显的聚束效应,并在频谱宽度最大时达到最强;同时给出了光子计数测量中聚束效应减弱的物理原因.实验表明该系统及方法能很好地揭示不同状态混沌光场的光子统计特性.     

A cooperative positioning with Kalman filters and handover mechanism for indoor microcellular visible light communication network

We propose a novel handover scheme for indoor microcellular visible light communication (VLC) network. With such a scheme, the room, which is fully coverage by light, is divided into several microcells according to the layout of light-emitting diodes (LEDs). However, the directionality of light arises new challenges in keeping the connectivity between the mobile devices and light source under the mobile circumstances. The simplest solution is that all LEDs broadcast data of every user simultaneously, but it wastes too much bandwidth resource, especially when the amount of users increases. To solve this key problem, we utilize the optical positioning assisting handover procedure in this paper. In the positioning stage, the network manager obtains the location information of user device via downlink and uplink signal strength information, which is white light and infrared, respectively. After that, a Kalman filter is utilized for improving the tracking performance of a mobile device. Then, the network manager decides how to initiate the handover process by the previous information. Results show that the proposed scheme can achieve low-cost, seamless data communication, and a high probability of successful handover.