Broadband source of polarization entangled photons

A broadband source of polarization entangled photons based on type-II spontaneous parametric down conversion from a chirped PPKTP crystal is presented. With numerical simulation and experimental evaluation, we report a source of broadband polarization entangled states with a bandwidth of approximately 125 nm for use in quantum interferometry. The technique has the potential to become a basis for the development of flexible broadband sources with designed spectral properties.     

泵浦光强度对纠缠光子品质的影响

纠缠源亮度和品质直接决定其作为量子信息处理资源的应用价值,研究参量转换过程中纠缠对的产生效率和品质随泵浦激光强度的变化关系对制备高品质纠缠源具有基础意义．利用参量下转换、极化后选择和符合计数技术,制备了Einstein-Podolsky-Rosen (EPR)偏振纠缠态．研究了泵浦激光强度对纠缠光子产生效率、对比度和保真度的影响．结果表明,随泵浦激光强度增大,纠缠对产生效率保持不变,也即纠缠源亮度随激光强度增大而线性增大;纠缠光子的对比度、保真度随激光功率增大而下降．     

Generation of polarization-entangled photon pairs by a single crystal interferometric source pumped by femtosecond laser pulses

Photon pairs highly entangled in polarization have been generated under femtosecond laser pulse excitation by a type-I crystal source operating in a single-arm interferometric scheme. The relevant effects of temporal walkoff existing in these conditions between the ordinary and extraordinary photons were experimentally investigated. By introducing a suitable temporal compensation between the two orthogonal polarization components, highly entangled pulsed states were obtained.     

Generation and characterization of Werner states and maximally entangled mixed states by a universal source of entanglement

We present a novel technique for generating two-photon polarization mixed states of any structure, which is based on the peculiar spatial characteristics of a high brilliance source of entangled pairs. Werner states and maximally entangled mixed states, two well-known families of mixed states important for quantum information, have been created and fully characterized by this technique. We have also investigated and tested the nonlocal properties of these states.     

Quantum packet switching generation using correlated photons in a microring resonator system

We propose a new system of a packet of quantum bits generation using a soliton pulse within a microring resonator. A quantum gate can be formed by using a polarization control unit incorporating into the system. The random signal and idler pairs can be formed within the photon correlation bandwidth, which can be generated, and randomly form the packet quantum bits, i.e. quantum packet switching. Each random code (logic) can be performed by combining the signal and idler of each entangled photon pair via the quantum gate. Results obtained have shown that the packet of quantum logic bits can be generated using the entangled photon pairs generated by the proposed system.     

Switchable dual-wavelength Erbium-doped fiber ring laser with cascaded fiber Bragg gratings

We have experimentally shown wavelength mode switching in a dual-wavelength Erbium-doped single cavity fiber laser where the initial two wavelengths of 1 nm spacing are determined by the cascaded reflection type short-period fiber Bragg gratings having two different centre wavelengths of 1550.5 and 1551.5 nm. The lasing mode depends on the polarization in the ring cavity to migrate from one wavelength to another or operates in both modes in a polarization beam splitter output. To effectively control the polarization in the ring cavity, the polarization controllers were positioned before and after the polarization beam splitter. This method of wavelength switching provides a simple way of mode tuning in dual-wavelength fiber lasers.     

Experimental realization of one-way quantum computing with two-photon four-qubit cluster states

We report an experimental realization of one-way quantum computing on a two-photon four-qubit cluster state. This is accomplished by developing a two-photon cluster state source entangled both in polarization and spatial modes. With this special source, we implemented a highly efficient Grover's search algorithm and high-fidelity two-qubit quantum gates. Our experiment demonstrates that such cluster states could serve as an ideal source and a building block for rapid and precise optical quantum computation.     

基于自发参量下转换源二阶激发过程产生四光子超纠缠态

目前,多光子纠缠态的制备大多通过线性光学器件演化自发参量下转换一阶激发过程产生的纠缠光子对得到.本文考虑由自发参量下转换源二阶激发产生四个不可区分的纠缠光子制备四光子超纠缠态的情况.通过几组分束器、半波片和偏振分束器等线性光学器件设计量子线路演化四光子系统,结合四模符合探测,可得到同时具有偏振纠缠和空间纠缠的四光子超纠缠态.     

Storage and long-distance distribution of telecommunications-band polarization entanglement generated in an optical fiber

We demonstrate storage of polarization-entangled photons for 125 micros, a record storage time to date, in a 25-km-long fiber spool, using a telecommunications-band fiber-based source of entanglement. With this source we also demonstrate distribution of polarization entanglement over 50 km by separating the two photons of an entangled pair and transmitting them individually over separate 25-km fibers. The measured two-photon fringe visibilities were 82% in the storage experiment and 86% in the distribution experiment. Preservation of polarization entanglement over such long-distance transmission demonstrates the viability of all-fiber sources for use in quantum memories and quantum logic gates.     

Polarization controlled multiwavelength switchable erbium-doped fiber laser based on high birefringence few-mode fiber loop mirror

A multiwavelength switchable erbium-doped fiber laser with linear cavity is proposed and experimentally demonstrated. It is based on a fiber loop mirror incorporating a piece of high birefringence few-mode fiber and polarization controller. Due to dependence of its reflection properties on the polarization controllers, the fiber laser can operate in the random combination of three wavelengths by only adjusting the polarization controllers in the linear cavity. The wavelengths involved in the switching operation are determined by the merged Sagnac and intermodal interferences elicited in the fiber loop mirror. The optical signal to noise extinction ratio of every oscillation line is higher than 40 dB and power fluctuation less than 0.5 dB.     

Generation of polarization entangled photons using concurrent type-I and type-0 processes in AlGaAs ridge waveguides

A technique to generate polarization entangled photons using concurrent type-I and type-0 second-order nonlinear processes in monolithic Bragg reflection waveguides is presented and analyzed. Concurrent phase matching is achieved by lithographic tuning of the waveguide ridge width. Nearly perfect entanglement is achievable on-chip through appropriate epistructure design without the need of spectral filtering and group velocity compensation. Theoretical calculations predict that a high quantum interference visibility could be experimentally achieved with the pair generation rate of each process being approximately 3.0×10(6) pairs/s/mW/GHz.     

Optimal quantum cloning on a beam splitter

We demonstrate how a beam splitter in combination with different light sources can be used as an optimal universal 1-->2 quantum cloner and as an optimal universal quantum NOT machine for the polarization qubit of a single photon. For the cloning a source of single photons with maximally mixed polarization is required and for the NOT operation a source of maximally entangled photon pairs. We demonstrate both operations with near optimal fidelity. Our scheme can be generalized in a natural way to clone and NOT the spin state of electrons.     

Exciton polarization, fine-structure splitting, and the asymmetry of quantum dots under uniaxial stress

We derive a general relation between the fine-structure splitting (FSS) and the exciton polarization angle of self-assembled quantum dots under uniaxial stress. We show that the FSS lower bound under external stress can be predicted by the exciton polarization angle and FSS under zero stress. The critical stress can also be determined by monitoring the change in exciton polarization angle. We confirm the theory by performing atomistic pseudopotential calculations for the InAs/GaAs quantum dots. The work provides deep insight into the dot asymmetry and their optical properties and a useful guide in selecting quantum dots with the smallest FSS, which are crucial in entangled photon source applications.     

偶极子位置及偏振对激发光子晶体H1微腔的影响

光子晶体微腔和量子点的集成是实现量子信息处理非常具有潜力的平台之一,利用微腔和量子点的耦合可以制备纠缠光子对,实现对量子态的操控.因为光子晶体微腔具有品质因子高、模场体积小等优点,可以极大地增强光与物质之间的相互作用,从而易于实现量子态在不同物理体系之间的转换.通过单量子点和光子晶体H1微腔的耦合可以产生纠缠光子对,因为H1微腔具有简并的、模式偏振正交的基态模式.通常微腔模式的激发随着量子点在微腔中的位置变化而改变,本文用时域有限差分方法研究了偶极子光源的位置及偏振对激发光子晶体H1微腔模式的影响.结果表明:通过改变偶极子光源位置可以选择性地激发H1微腔简并模式中的一个;具有某一偏振的偶极子光源只能激发相应偏振的微腔模式;模式激发强度的大小也是由偶极子光源在微腔中的位置决定的.鉴于目前量子点在微腔中的位置尚不能精确控制,所以微腔模式受激发光源位置的影响的研究具有重要意义.     

Amplification of entangled light upon reflection from a cavity with nonlinear medium

The kinetic equation for the case of reflection of a broadband entangled light of a nondegenerate parametric source from a cavity is derived with the use of the technique of stochastic differential Ito equations. Statistical properties of the light reflected from a cavity with a nonlinear transparent medium, in which the process of special parametric interaction occurs, are considered on the basis of the obtained equation. Due to the integrals of motion, the entangled light, which is an EPR pair of continuous variables, can be amplified without destruction of the quantum correlation.     

基于周期极化KTiOPO$lt;sub$gt;4$lt;/sub$gt;晶体的高亮度纠缠源研制

量子纠缠是量子信息领域的核心资源,目前利用β型硼酸钡（BBO）晶体参量下转换制备的纠缠光子对的亮度较低,它直接制约了量子通信的最远距离,已无法满足星地实用化量子通信的发展需求.利用周期极化KTiOPO₄晶体,采用准相位匹配技术设计产生了一种后选择的纠缠源,测得的符合计数达到了16×10³s^-1 mW^-1,极化对比度达到27∶1,在亮度上比基于BBO的量子纠缠光源提高了一个数量级以上.这一高亮度的纠缠源可以广泛应用于量子密钥分发、量子隐形传态以及量子计算等新兴量子信息领域,为实现全球化量子通信提供了有力的保障. 关键词： 纠缠态 准相位匹配 4晶体')" href="#">周期极化KTiOPO₄晶体     

Nonlinear magneto-optical Bragg gratings

In magneto-optical gratings the magnetic dipolar coupling superimposed on the electric dipolar one introduces nonreciprocity and spectral discrimination between circular polarization states, measured by a Zeeman-like splitting of the photonic Bragg resonances. In a nonlinear regime the degree of nonreciprocity is modified by the photoinduced interplay of these splittings and their Stark-like shifts. We predict novel magneto-optical modulation schemes for switching between orthogonal circular polarization states of transmission or reflection operated by means of an intense linearly polarized optical pulse train.     

An investigation of the entangled photon walk-off compensation generated by a fiber ring resonator

We firstly present an experimental investigation of the entangled photon states’ recovery using a fiber optic ring resonator incorporating an erbium-doped fiber amplifier (EDFA). The weak light entangled photons can be recovered in both amplitude and states after the optical pumping and polarization control parts are used in the system. A new experimental result of the investigation of thermal effects on the entangled states’ walk-off and compensations has shown that the entangled photon walk-off compensation is achieved by using a pair of polarization control devices. The relationship between thermal effects and the entangled states’ phase shift is investigated and discussed. The walk-off compensation due to temperature changes from 30 to 80 °C is achieved. This shows that the changes in phase of the entangled photons can be negligible when the compensation is employed.     

Deterministic Frequency-Based Polarization Entanglement Concentration with the Multipartite Less-Hyperentangled State

A deterministic entanglement concentration protocol (ECP) is demonstrated in a multipartite less-hyperentangled system, containing the simultaneous entanglements in polarization and frequency degrees of freedom. The motivation is that the entangled system in frequency degree of freedom suffers little from the effects of the channel noise in optical fibers. Consequently, a maximally entangled system can be generated in polarization degree of freedom from the multipartite less-hyperentangled system with cross-Kerr nonlinearity. Compared with the conventional ECPs that have success probabilities less than one while iterating their recursive entanglement concentration processes several times to achieve a maximally entangled system in polarization degree of freedom, the present ECP can generate a maximally entangled system in polarization degree of freedom in two steps with a certainty. It may be useful for enhancing the efficiency of communications in long-distance quantum computation networks.     

转换反射中光自旋霍尔效应的自旋堆积方向

从理论上和实验上研究了转换反射中光自旋霍尔效应的自旋堆积方向的方法,建立了描述光束在空气-棱镜界面反射的自旋堆积模型,揭示了横移与光束入射偏振角的定性关系。研究发现,当入射角小于布儒斯特角时,随着入射偏振角的逐渐增大,自旋堆积的方向发生反转。而当入射角大于布儒斯特角时,自旋堆积的方向不再随入射偏振角的变化而反转。结果表明,在光束入射角为确定值且小于布儒斯特角的情况下,可以通过调控光束的入射偏振角转换自旋堆积的方向。转换自旋堆积方向的研究为有效调控光自旋霍尔效应提供了新的途径。