《自然·光子学》研究亮点专栏报道我校量子物理研究进展

今年9月出版的《Nature Photonics》(《自然·光子学》)杂志,在research highlights专栏中,专题报道了华南师范大学广东省量子调控工程与材料重点实验室在窄线宽纠缠光子对方面的最新研究成果[Nature Photonics 8,674(2014)doi:10.1038/nphoton.2014.202].《自然·光子学》杂志编辑部Noriaki Horiuchi博士撰稿的上述专栏报道,以"Subnatural-linewidth biphotons"为题,评论指出:"为了实现光子与原子间量子态的高效率转换,光子线宽窄于原子自然线宽是必不     

基于单边微腔量子点耦合系统的极化纠缠浓缩

量子纠缠是实现量子信息处理任务的重要资源,一些在经典领域中无法实现的任务,可以借助量子纠缠来完成.然而,纠缠态在噪声的作用下会发生退相干,进而变成非最大纠缠态或混态,不再能完成预定任务.纠缠浓缩是从非最大纠缠态中提取最大纠缠态的过程.我们提出两个非最大纠缠Bell态纠缠浓缩方案,一个是未知系数非最大纠缠Bell态纠缠浓缩方案,另一个是已知系数非最大纠缠Bell态纠缠浓缩方案.方案中利用量子点微腔耦合系统来实现光子与光子之间的非线性相互作用,进而可以通过分级浓缩的方法提高浓缩方案的成功概率.两个方案都可以推广为多光子非最大纠缠GHZ态的纠缠浓缩.     

非对称偏振三维纠缠态的预警制备

在量子信息处理过程中,量子纠缠态扮演着极为重要的角色,其特殊的物理性质,使得量子信息具有经典信息所没有的许多新的特征,为信息传输和信息处理提供了新的物理资源.针对非对称偏振三维纠缠态的制备,基于交叉相位调制技术,以纠缠光子对和两个单光子比特作为初态,通过单光子与相干光的相互作用以及双光子干涉来实现.如果通过三个非计数单光子探测器来预警制备三维最大纠缠态,其概率为3/64.而如果采用特殊的分段式光子探测器,其概率可以提高到3/8,达到理论极限值.该方案在理论上是可行的,效率相对较高,而且预警式的制备为其后续在量子信息过程中的使用提供了很大的灵活性.     

任意多光子Dicke态的制备

量子Dicke态对于多粒子量子纠缠结构和性质的研究,以及量子网络的构建有着重要的意义.基于线性光学和交叉相位调制技术,利用三个基本量子门,给出了四光子Dicke态制备、任意系数W态制备以及任意系数和光子数的Dicke态制备方案.这些方案的确定性、任意性、高效性将为研究量子纠缠结构性质以及量子网络提供一定的便利.     

基于半导体量子点的量子通信

光子源和纠缠光子对的制备是量子信息产生和传输过程的源头,是实现量子通信的重要前提条件.半导体量子点固体系统具有可集成性和可扩展性的优点,并且与现有的半导体光电子学技术密切相关,近年来在单光子源和纠缠光子对制备方面取得了重要的进展,是未来全固态量子通信的重要元器件.从量子通信的基本原理出发,阐述了制备单光子源和纠缠光子对的重要性,介绍如何解析推导出圆形常规半导体量子点中的电子结构,描述了圆形拓扑绝缘体量子点中边缘态具有双重简并的电子结构,能级间隔与量子点的具体形状无关,并且具有自旋轨道锁定的特性,总结了实验和理论上在利用这一独特的电子结构制备单光子源和纠缠光子对方面取得的重要进展.     

多模光场与二能级原子N_j度简并N~∑光子共振相互作用系统中场量子熵与纠缠的演化

利用冯·纽曼量子约化熵理论研究了多模相干态光场与单个二能级原子简并多光子共振相互作用系统中场量子熵与纠缠的时间演化特性,得到了多模光场量子熵的解析表达式,并给出了三模场与原子相互作用时场量子熵的数值计算结果,详细讨论了此时初始平均光子数、原子分布角以及原子偶极相位角对场量子熵与纠缠的影响.数值计算结果表明:初始光场越强,场与原子之间的量子纠缠越弱,当光场足够强时,两子系统几乎始终处于退纠缠状态;场量子熵强烈地依赖于原子分布角,光场与原子总是处于纠缠态,并且在短时间的振荡之后几乎停留在最大纠缠态,而原子分布角越大,场量子熵演化到其最大值的时间越短;原子偶极相位角对场与原子之间的量子纠缠几乎没有影响.根据场量子熵的这些特性,可以制备纠缠态或纯态.     

压缩真空环境下双腔系统的两原子纠缠

原子间纠缠的制备及其演化特性的研究对量子信息和量子网络的实现具有重要的意义.文章研究了压缩真空环境下双JC模型中两原子纠缠的演化,并讨论了不同初态、腔场的平均光子数和压缩角对纠缠的影响.结果表明:两原子初态的纠缠较强则任意时刻的纠缠也会较强;减小腔场的平均光子数可以延长纠缠存在的时间;腔场的压缩角对纠缠演化的影响比较微弱.     

基于量子点-双边光学腔的两自由度并行非定域Toffoli门远程实现

三量子比特Toffoli门非定域远程实现在量子通讯网络、分布式量子计算中有重要应用.已有的Toffoli门非定域远程实现方案大都基于量子系统单一自由度,本文提出超纠缠信道下基于量子点-双边光学腔的两自由度并行非定域Toffoli门远程实现方案.通讯方Alice, Bob使手中光子依次进入量子点-双边光学腔,对光学腔中电子以及纠缠光子执行单粒子测量; Charlie依据纠缠光子单粒子测量结果,执行相应局域幺正操作和极化、路径自由度单粒子测量. Alice和Bob依据单粒子测量结果执行相应局域幺正变换即可完成两自由度并行非定域Toffoli门远程实现.方案以极化、路径两自由度超纠缠光子为量子纠缠信道,具有信道容量高、并行性强的优点.     

同频率纠缠双光子态的量子相干特性

基于量子相干原理,分析了脉冲激光泵浦的第二类型自发参量下转换产生的同频率纠缠双光子态.研究了同频率纠缠双光子的量子相干性质,并与由脉冲激光所产生的不同频率纠缠双光子的量子相干性质做了对比.结果表明,随着激光脉冲宽度的增加,不同频率纠缠双光子态的相位匹配函数的不对称性增加,纠缠光子对的相干性减小,同频率纠缠双光子态的相位匹配函数仍保持对称,纠缠光子对的相干性达到最大.     

用多个光子晶体实现量子纠缠态的稳定性

量子纠缠态的纠缠度随传播距离增加或外界干扰而变小,  在光与环境相互作用的退相干效应下,  可使纠缠度降低或不再纠缠.  基于此, 设计一组一维光子晶体, 当两光子或三光子的纠缠光通过该组光子晶体, 且叠加参数c1=0.03~0.98时, 纠缠度E=0.8~1, 从而使量子纠缠态可远距离传输, 更好地实现量子通信.     

Generation of ultraviolet entangled photons in a semiconductor

Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources.     

Experimental purification of two-atom entanglement

Entanglement is a necessary resource for quantum applications--entanglement established between quantum systems at different locations enables private communication and quantum teleportation, and facilitates quantum information processing. Distributed entanglement is established by preparing an entangled pair of quantum particles in one location, and transporting one member of the pair to another location. However, decoherence during transport reduces the quality (fidelity) of the entanglement. A protocol to achieve entanglement 'purification' has been proposed to improve the fidelity after transport. This protocol uses separate quantum operations at each location and classical communication to distil high-fidelity entangled pairs from lower-fidelity pairs. Proof-of-principle experiments distilling entangled photon pairs have been carried out. However, these experiments obtained distilled pairs with a low probability of success and required destruction of the entangled pairs, rendering them unavailable for further processing. Here we report efficient and non-destructive entanglement purification with atomic quantum bits. Two noisy entangled pairs were created and distilled into one higher-fidelity pair available for further use. Success probabilities were above 35 per cent. The many applications of entanglement purification make it one of the most important techniques in quantum information processing.     

Quantum information: source of triggered entangled photon pairs?

The realization of an entangled photon source will be of great importance in quantum information--for example, for quantum key distribution and quantum computation--and Stevenson et al. have described such a source. However, we show here that first, their source is not entangled; second, they use inappropriate entanglement indicators that rely on assumptions invalidated by their data; and third, their source has insignificant entanglement even after simulating subtraction of the significant quantity of background noise. We therefore find that the standard of proof required for a semiconductor source of triggered entangled photon pairs has not been met by Stevenson et al..     

光量子纠缠态的应用

介绍了目前对光量子纠缠态应用的研究状况，成功的将自发参量下转换产生的纠缠光子对应用于产生真随机数．在数据采集过程中采用了符合测量技术，对采集的多组随机序列采用国际上比较通用的随机性测试程序-ENT进行检验，结果表明数据的随机性很好．     

A semiconductor source of triggered entangled photon pairs

Entangled photon pairs are an important resource in quantum optics, and are essential for quantum information applications such as quantum key distribution and controlled quantum logic operations. The radiative decay of biexcitons-that is, states consisting of two bound electron-hole pairs-in a quantum dot has been proposed as a source of triggered polarization-entangled photon pairs. To date, however, experiments have indicated that a splitting of the intermediate exciton energy yields only classically correlated emission. Here we demonstrate triggered photon pair emission from single quantum dots suggestive of polarization entanglement. We achieve this by tuning the splitting to zero, through either application of an in-plane magnetic field or careful control of growth conditions. Entangled photon pairs generated 'on demand' have significant fundamental advantages over other schemes, which can suffer from multiple pair emission, or require post-selection techniques or the use of photon-number discriminating detectors. Furthermore, control over the pair generation time is essential for scaling many quantum information schemes beyond a few gates. Our results suggest that a triggered entangled photon pair source could be implemented by a simple semiconductor light-emitting diode.     

非线性光学频率变换纠缠光子源实验制备的研究进展

量子纠缠源是量子通信和量子计算的基础,实现隐形传态最重要的关键和难点是制备高纠缠度的、稳定高效的EPR光子纠缠源。系统综述了国内外采用非线性光学频率变换制备纠缠光子源的实验方法以及实验研究的最新进展。     

Experimental extraction of an entangled photon pair from two identically decohered pairs

Entanglement is considered to be one of the most important resources in quantum information processing schemes, including teleportation, dense coding and entanglement-based quantum key distribution. Because entanglement cannot be generated by classical communication between distant parties, distribution of entangled particles between them is necessary. During the distribution process, entanglement between the particles is degraded by the decoherence and dissipation processes that result from unavoidable coupling with the environment. Entanglement distillation and concentration schemes are therefore needed to extract pairs with a higher degree of entanglement from these less-entangled pairs; this is accomplished using local operations and classical communication. Here we report an experimental demonstration of extraction of a polarization-entangled photon pair from two decohered photon pairs. Two polarization-entangled photon pairs are generated by spontaneous parametric down-conversion and then distributed through a channel that induces identical phase fluctuations to both pairs; this ensures that no entanglement is available as long as each pair is manipulated individually. Then, through collective local operations and classical communication we extract from the two decohered pairs a photon pair that is observed to be polarization-entangled.     

Experimental entanglement distillation and 'hidden' non-locality

Entangled states are central to quantum information processing, including quantum teleportation, efficient quantum computation and quantum cryptography. In general, these applications work best with pure, maximally entangled quantum states. However, owing to dissipation and decoherence, practically available states are likely to be non-maximally entangled, partially mixed (that is, not pure), or both. To counter this problem, various schemes of entanglement distillation, state purification and concentration have been proposed. Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarization-entangled photon pairs generated by spontaneous parametric down-conversion. We have also applied our methods to initial states that are partially mixed. After filtering, the distilled states demonstrate certain non-local correlations, as evidenced by their violation of a form of Bell's inequality. Because the initial states do not have this property, they can be said to possess 'hidden' non-locality.     

Quantum teleportation and entanglement distribution over 100-kilometre free-space channels

Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics. Although quantum teleportation and entanglement distribution over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging. Free-space channels, first used for quantum key distribution, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600?metres (ref. 14) and 13?kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel. Most recently, following a modified scheme, free-space quantum teleportation over 16?kilometres was demonstrated with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4?±?0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8?kilometres. Violation of the Clauser-Horne-Shimony-Holt inequality is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.