普适单体偏振高维量子态幺正操作的光学实现

如何有效地实现高维量子态的制备和操控是量子信息科学研究的一个热点,我们在光学领域,基于线性光学技术和交叉相位调制技术,利用偏振高维量子态与单光子空间高维量子态的纠缠操作,实现这两类高维量子态之间的相互转换.由此,利用单光子空间高维量子态幺正操作的易实现性,可以间接实现偏振高维量子态的幺正操作.这一实现模式超越以往只适用于三维量子态操作的限制,适用于任意维度的单体高维量子态操作,可为高维量子态的深入研究提供一定的便利.     

基于d维六粒子Cluster态的任意qudit态远程联合制备

提出利用d维六粒子Cluster态的任意高维量子态确定远程联合制备方案.首先考虑了基于d维六粒子Cluster态的任意双粒子qudit态远程联合制备方案,然后提出了基于六粒子Cluster态的任意单粒子qudit态双向远程联合制备方案.在方案中,两个发送方共享需制备量子态信息,第一发送方依据已知需制备量子态信息对纠缠粒子执行半正定算子测量(POVM),第二发送方依据第一发送方测量结果以及已知需制备量子态信息对纠缠粒子选择执行适当的正交投影测量,接收方依据所有发送方测量结果对纠缠粒子执行相应的局域幺正操作,就可以在他的粒子上重建需制备量子态.通信方使用高维Cluster态为量子纠缠信道完成高维量子态远程联合制备,与其他方案相比,具有信道容量高的优点.     

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.     

任意单光子两自由度量子态的概率远程制备

量子态远程制备为量子信息的传输提供了一种有效的方法.基于共享的部分超纠缠Bell态,提出了任意单光子极化-时间自由度量子态的远程制备方案.发送方根据待制备的量子态信息对手中光子进行幺正操作,通过单光子投影测量和单向经典通信帮助接收方制备出初始量子态,并给出了该方案的成功概率.     

基于多光子叠加态的交叉相位调制耦合参数测量

继纠缠之后,粒子间相互作用已成为量子增强的重要资源.在不利用纠缠资源的前提下,已有研究工作利用单光子叠加态与相干态的相互作用实现了精度为海森堡极限的参数测量,然而海森堡极限并不是这个相互作用模型的测量精度极限.为了得到更高的测量精度,将单光子叠加态替换为多光子叠加态,在弱测量框架下对信号光量子态进行后选择与量子态筛选.将单光子叠加态替换为多光子叠加态,能为测量结果带来全方位提升.提出基于多光子叠加态的交叉相位调制耦合参数测量的物理方案,该物理方案具有搭建难度低及操作简单的优点.     

基于部分纠缠信道的高维赤道态概率可控远程联合制备

作为一种特殊类型的量子态,赤道态由于在量子信息处理中的独特应用引起了广泛关注.基于不同多粒子纠缠态信道,研究者提出了任意双量子比特、三量子比特以及多量子比特赤道态多方联合制备方案.本文研究了基于高维部分纠缠态的高维多粒子赤道态多方概率可控远程联合制备方案,提出了两个基于高维部分纠缠态的高维多粒子赤道态多方可控远程联合制备方案.一个方案以m个高维类GHZ态为纠缠信道,可实现多个控制方控制下的高维赤道态多方远程联合制备,另一个方案以一个多粒子部分纠缠态为量子纠缠信道完成单个控制方控制下的高维赤道态远程联合制备.多个发送方共享多粒子赤道态信息,依据已知信息对手中纠缠粒子选择执行相应正交投影测量,控制方对他的纠缠粒子执行单粒子测量.接收方与所有发送方和控制方合作,仅需引入一个附加量子比特就可在他的纠缠粒子上重建需制备量子态.方案不需要使用最大纠缠态作为量子纠缠信道实现高维赤道态远程可控联合制备,具有可行性强的优点.     

非简并偏振纠缠源用于光纤量子通信

报道了一个基于自发参量下转换(SPDC)的偏振纠缠实验.采用两块BBO晶体粘合的方法,利用SPDC过程制备810和1 550 nm的偏振纠缠光子对源.用单模光纤耦合来收集参量光子对,在产生的纠缠态中,两光子符合计数为200 cps(每秒钟计数).为了验证产生的纠缠态,利用两个偏振分析器系统测量纠缠相关度,其中,干涉可见度约为90%.     

n-粒子赤道态的受控远程制备

为了解决多量子态的制备问题,首先提出一种构造2n+1-量子纠缠态的方法,并给出其量子线路图。其次,采用2n+1-量子纠缠态为信道,出来远程制备一个任意n-量子赤道纠缠态的方案。该方案在控制者Charlie的协助下,Alice通过多量子投影测量和经典通信,Bob采用简单酉变换就能以100%的概率成功重构任意n-量子赤道态。进一步,通过任意二量子态和任意三量子态的制备的具体实例,说明了上述关于一般多量子赤道纠缠态远程制备协议是可行的。     

用4个量子态实现的多方与多方之间的量子秘密共享

提出了一个用4个量子态实现的多方与多方之间的量子秘密共享方案,并证明此方案对用多光子信号的特洛伊马攻击、用Einstein-Podolsky-Rosen对的伪信号攻击、单光子攻击以及不可见光子攻击是安全的.另外,给出了不忠实者使用任意两粒子纠缠态伪信号攻击方法窃听秘密信息平均成功概率的上界。     

高效实现针对光子丢失问题的容错量子计算

基于两个基本操作——控制路径操作和纠缠子,首先给出非破坏性多光子奇偶校验的实现.随后,将这些操作用于光学量子计算中光子丢失问题的处理.可以实现奇偶量子态的制备,以及单体操作与两体控制非操作,由此可以实现普适容错量子计算.不管是态制备还是门操作都是确定性的,同时方案的实现复杂度也要远低于此前的方案,对于辅助单光子资源的需求也降到很低的程度.并且实现过程中除奇偶校验外,不需要进行任何单光子探测,因此不会造成任何单光子的损失,最大限度地利用了光子资源.比之此前的线性光学方案,我们方案的确定性、高效性、简单性以及资源的高利用率等特点,使得方案更具可行性和更加适用于大规模量子计算.     

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.     

Electromagnetically induced transparency with tunable single-photon pulses

Techniques to facilitate controlled interactions between single photons and atoms are now being actively explored. These techniques are important for the practical realization of quantum networks, in which multiple memory nodes that utilize atoms for generation, storage and processing of quantum states are connected by single-photon transmission in optical fibres. One promising avenue for the realization of quantum networks involves the manipulation of quantum pulses of light in optically dense atomic ensembles using electromagnetically induced transparency (EIT, refs 8, 9). EIT is a coherent control technique that is widely used for controlling the propagation of classical, multi-photon light pulses in applications such as efficient nonlinear optics. Here we demonstrate the use of EIT for the controllable generation, transmission and storage of single photons with tunable frequency, timing and bandwidth. We study the interaction of single photons produced in a 'source' ensemble of 87Rb atoms at room temperature with another 'target' ensemble. This allows us to simultaneously probe the spectral and quantum statistical properties of narrow-bandwidth single-photon pulses, revealing that their quantum nature is preserved under EIT propagation and storage. We measure the time delay associated with the reduced group velocity of the single-photon pulses and report observations of their storage and retrieval.     

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.     

利用四维的二粒子纠缠态传送一个未知的二维二粒子量子态

利用四维的二粒子最大纠缠态作为量子信道可成功地传送一个未知的二维二粒子量子态,其成功几率及态的保真度均为100%。若量子信道为部分纠缠态,也可以几率传送此量子态,其成功几率由部分纠缠态的最小Schm idt系数决定,态的保真度为100%。     

在量子信息分裂中利用局部测量提高量子参数估计

我们利用多体量子纠缠信道在环境噪声中实现了关于量子相位参量的信息分裂.这个量子信息分裂方案是指从一个发送端,人们将含有信息的任意量子态传输到多个接收端,然后利用么正测量在任意一个接收端上还原量子参量信息.我们发现,在退振幅阻尼噪声通道下,合理引入局部测量能提高量子参量估计精度.当环境表现为马尔科夫性时,经过量子信息分裂,量子参量估计精度的动力学演化会呈现单调衰减趋势.当环境具有一定记忆效应时,其演化会呈现振荡起伏行为.     

Entanglement of the orbital angular momentum states of photons.

Entangled quantum states are not separable, regardless of the spatial separation of their components. This is a manifestation of an aspect of quantum mechanics known as quantum non-locality. An important consequence of this is that the measurement of the state of one particle in a two-particle entangled state defines the state of the second particle instantaneously, whereas neither particle possesses its own well-defined state before the measurement. Experimental realizations of entanglement have hitherto been restricted to two-state quantum systems, involving, for example, the two orthogonal polarization states of photons. Here we demonstrate entanglement involving the spatial modes of the electromagnetic field carrying orbital angular momentum. As these modes can be used to define an infinitely dimensional discrete Hilbert space, this approach provides a practical route to entanglement that involves many orthogonal quantum states, rather than just two Multi-dimensional entangled states could be of considerable importance in the field of quantum information, enabling, for example, more efficient use of communication channels in quantum cryptography.     

Single photons on demand from a single molecule at room temperature

The generation of non-classical states of light is of fundamental scientific and technological interest. For example, 'squeezed' states enable measurements to be performed at lower noise levels than possible using classical light. Deterministic (or triggered) single-photon sources exhibit non-classical behaviour in that they emit, with a high degree of certainty, just one photon at a user-specified time. (In contrast, a classical source such as an attenuated pulsed laser emits photons according to Poisson statistics.) A deterministic source of single photons could find applications in quantum information processing, quantum cryptography and certain quantum computation problems. Here we realize a controllable source of single photons using optical pumping of a single molecule in a solid. Triggered single photons are produced at a high rate, whereas the probability of simultaneous emission of two photons is nearly zero--a useful property for secure quantum cryptography. Our approach is characterized by simplicity, room temperature operation and improved performance compared to other triggered sources of single photons.