隐形传送单光子数态保真度的优化

基于Braunstein与Kimble的连续变量量子隐形传态方案,利用双模压缩真空态作为纠缠通道,通过调节输出端位移因子可以优化单光子数态量子隐形传态的保真度。     

无噪线性放大的连续变量量子隐形传态

连续变量量子隐形传态在实际的量子通信中起着至关重要的作用.然而,实际环境的噪声与损耗会导致量子纠缠的退化,极大地影响系统的传输性能,尤其是降低系统的传输距离,阻碍其实际应用.本文提出了无噪线性放大的连续变量量子隐形传态方案,利用无噪线性放大器对退化的纠缠源进行放大,从而补偿光纤损耗信道对纠缠源的衰减影响,提升系统的性能.本文详细分析了不同损耗信道和纠缠源情况下,无噪线性放大器的增益系数选取范围以及不同增益对方案性能的影响.仿真结果表明,本文提出的方案相比于原方案,在保真度和传输距离方面都有较大的提升.该研究结果为长距离连续变量量子隐形传态的实际应用提供了一种切实有效的方法.     

四组份不同频率连续变量纠缠态光场的产生

量子纠缠态是开展量子信息工作的核心资源。提出在一块光学超晶格中通过有注入信号的非简并光学参量振荡级联一个和频过程,可以产生不同频率的四组份连续变量纠缠态光场的可行实验方案。首先泵浦光和注入信号光通过差频过程产生闲置光。然后泵浦光和闲置光在同一块光学超晶格中通过级联和频过程产生和频光。根据多组份连续变量纠缠的判定方法,从理论上证明泵浦光、信号光、闲置光与和频光场之间的量子纠缠特性。四组份纠缠特性随泵浦功率的增大而减弱,另外选取较大的注入信号功率、级联非线性过程的耦合参数和泵浦光衰减常数可以得到较好的四组份纠缠光场。该实验方案只用到一块光学超晶格就可以产生四色连续变量纠缠态光场,实验装置简单。     

电子自旋辅助实现光子偏振态的量子纠缠浓缩

量子纠缠浓缩可以将非最大的纠缠态转变为最大纠缠态,提高量子通信的安全性.本文基于圆偏振光和量子点-腔系统的相互作用,用一个单光子作为连接远距离纠缠光子对的桥梁,在理想条件下实现了光子偏振纠缠态的浓缩.计算结果显示,这个纠缠浓缩方案在考虑耦合强度和腔泄漏的情况下也可以保持较高的保真度,而且不需要知道部分纠缠态的初始信息,也不必重复执行纠缠浓缩过程.这不仅提高了量子纠缠浓缩的安全性,也有助于通过消耗最少的量子资源来实现高效的量子信息处理.     

连续变量无条件纠缠交换 --纠缠态的量子离物传送

利用自行设计的两台非简并光学参量放大器，获得了一对具有经典相干性且量子起伏相互独立的连续变量纠缠态光场，并用它完成了连续变量的无条件纠缠交换，即纠缠态的量子离物传送．通过联合贝尔态探测与纠缠塌缩，使两个初始不纠缠而又从未发生过直接相互作用的光场产生了量子纠缠，其正交振幅和位相分量的量子起伏关联方差被直接测量，其测量值分别低于散离噪声极限1．23dB和1．12dB．理论计算与实验结果基本符合．     

连续变量六组份和八组份星型Cluster纠缠态光场产生系统

Cluster纠缠态是执行单向量子计算的基本资源。星型Cluster态可用于构建量子信息网络。以非简并光学参变放大器产生的双模压缩态光场为基础,设计了产生连续变量六组份和八组份星型Cluster纠缠态光场的实验方案,并推导了相应的量子不可分判据。计算结果指出,当量子不可分判据中的增益因子取为1时,需要一定的初始压缩度才能制备星型Cluster纠缠态;但当选取最佳增益因子时,极低的压缩也能产生星型纠缠。计算为实验系统设计提供了直接参考。     

连续变量纠缠态光场在光纤中传输特性的实验研究

连续变量纠缠态由于其确定性产生、高效率的特点而被广泛应用于连续变量量子信息处理.在量子信息处理过程中纠缠态与量子信道发生相互作用而退相干,这是限制长距离量子信息发展的重要因素之一.光纤信道作为理想的量子信道,是目前连续变量量子信息研究关注的热点.本文利用Ⅱ类匹配的楔角极化磷酸氧钛钾晶体构成了三共振的非简并光学参量放大器,获得了8.3 dB的光通信波段1.5μm连续变量纠缠态光场.将产生的纠缠态光场注入单模光纤,其量子特性在传输距离达50 km后仍得到保持,纠缠度为0.21dB.该研究可为基于光纤的长距离连续变量量子信息研究提供有效的依据.     

连续变量量子离物传态的实验研究

利用单个Ⅱ类相位匹配OPA产生的双模压缩态实现连续变量压缩纠缠态，完成了连续变量量 子离物传态. Ⅱ类相位匹配OPA参量反放大过程可以实现正交振幅反关联、正交位相关联的 压缩纠缠态，这不同于一般运转于阈值以下的OPO产生的正交振幅关联、正交位相反关联的 压缩纠缠态，可以采用直接平衡测量的方法完成Bell态测量. 利用双KTP补偿非线性过程的 离散效应，获得了最大压缩大于2dB的双模压缩纠缠态，实现保真度06，考虑到探测效率 实际保真度为057. 这种方案简化了测量方法与纠缠光源产生装置，有利于进行量子通信 的实验研究与应用. 同时，讨论了探测过程中的一些非理想因素. 关键词： 量子离物传态 压缩纠缠态 Bell态测量     

基于EPR导引的连续变量安全量子克隆

利用量子信道和经典信道相结合的方法,采用部分脱体传输设计了连续变量1→2量子克隆方案,在此基础上研究了克隆保真度与EPR导引之间的关系。研究结果表明：双向量子导引态是实现相干态安全量子克隆的必要资源。对于克隆输出模Clone 1,取最优增益时克隆保真度超过不可克隆阈值的实现需要共享纠缠源的双向导引,但并不是所有双向导引的资源都能使克隆的保真度大于■。在输出模Clone 1的最优增益下,观察了输出模Clone 1和输出模Clone 2的保真度随分束器反射率和压缩参数的变化,发现使用纠缠度较小但可导引的资源可以实现较高的克隆保真度,且克隆过程中也不需要较高的反射率。此研究结果对安全量子通信网络的构建具有一定的参考意义。     

基于四波混频过程的纠缠光放大

双模纠缠态是量子信息领域一种重要的量子资源,本文基于四波混频过程从理论上提出了对双模纠缠态的单个模式(单模放大方案)和对双模纠缠态的两个模式(双模放大方案)的放大.利用光学分束器模型来模拟在光学传输过程中损耗引入的真空场噪声,利用部分转置正定判据分析了两种不同的放大方案中四波混频过程的增益对初始双模纠缠态的纠缠程度的影响.结果表明,在特定的损耗情况下,两个方案中初始双模纠缠态的纠缠度都随增益的增大而减小,直至消失,且双模放大方案中初始双模纠缠态纠缠消失得比单模放大方案中更快.本文的理论结果为实验上实现基于四波混频过程的双模纠缠态的放大奠定了理论基础.     

Classical and quantum communication without a shared reference frame

We show that communication without a shared reference frame is possible using entangled states. Both classical and quantum information can be communicated with perfect fidelity without a shared reference frame at a rate that asymptotically approaches one classical bit or one encoded qubit per transmitted qubit. We present an optical scheme to communicate classical bits without a shared reference frame using entangled photon pairs and linear optical Bell state measurements.     

Optical hybrid approaches to quantum information

This article reviews recent hybrid approaches to optical quantum information processing, in which both discrete and continuous degrees of freedom are exploited. There are well‐known limitations to optical single‐photon‐based qubit and multi‐photon‐based qumode implementations of quantum communication and quantum computation, when the toolbox is restricted to the most practical set of linear operations and resources such as linear optics and Gaussian operations and states. The recent hybrid approaches aim at pushing the feasibility, the efficiencies, and the fidelities of the linear schemes to the limits, potentially adding weak or measurement‐induced nonlinearities to the toolbox.     

Remote entanglement between a single atom and a Bose-Einstein condensate

Entanglement between stationary systems at remote locations is a key resource for quantum networks. We report on the experimental generation of remote entanglement between a single atom inside an optical cavity and a Bose-Einstein condensate (BEC). To produce this, a single photon is created in the atom-cavity system, thereby generating atom-photon entanglement. The photon is transported to the BEC and converted into a collective excitation in the BEC, thus establishing matter-matter entanglement. After a variable delay, this entanglement is converted into photon-photon entanglement. The matter-matter entanglement lifetime of 100 μs exceeds the photon duration by 2 orders of magnitude. The total fidelity of all concatenated operations is 95%. This hybrid system opens up promising perspectives in the field of quantum information.     

Generation of a displaced qubit and entangled displaced photon state via conditional measurement and their properties

We study optical schemes for generating both a displaced photon and a displaced qubit via conditional measurement. Combining one mode prepared in different microscopic states (one-mode qubit, single photon, vacuum state) and another mode in macroscopic states (coherent state, single photon added coherent state), a conditional state in the other output mode exhibits properties of a superposition of the displaced vacuum and a single photon. We propose to use the displaced qubit and entangled states composed of the displaced photon as components for quantum information processing. Basic states of such a qubit are distinguishable from each other with high fidelity. We show that the qubit reveals both microscopic and macroscopic properties. Entangled displaced states with a coherent phase as an additional degree of freedom are introduced. We show that additional degree of freedom enables to implement complete Bell state measurement of the entangled displaced photon states.     

基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

Quantum Fisher information flow and entanglement in pair coherent states

Realistic quantum systems are the main part of quantum information technology, which causes the rapid destruction of crucial quantum properties. Therefore, the unavoidable interaction between quantum systems, understanding the dynamics of entanglement indicators and finding the correlation between different phenomena may stimulate great interest. In this article the quantum Fisher information and nonlocal correlation between a single qubit and two-mode optical field initially in pair coherent states have investigated. We examine the impact of the qubit motion and intensity dependent function on the flow of quantum Fisher information and quantum entanglement during the time evolution. We discuss the relationship between quantum Fisher information flow and quantum entanglement in the case of moving and unmoving single qubit.     

Entanglement of remote atomic qubits

We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at site , transmitting the photon to site in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality [EQUATION: SEE TEXT]. We experimentally determine [EQUATION: SEE TEXT]. Entanglement of two remote atomic qubits, each qubit consisting of two independent spin wave excitations, and reversible, coherent transfer of entanglement between matter and light represent important advances in quantum information science.     

Simultaneous Preparation of Two Optical Cat States Based on a Nondegenerate Optical Parametric Amplifier

The cat state, known as the superposition of coherent states, has broad applications in quantum computation and quantum metrology. Increasing the number of optical cat states is crucial to implement complex quantum information tasks based on them. Here, two optical cat states are prepared simultaneously based on a nondegenerate optical parametric amplifier. By subtracting one photon from each of two squeezed vacuum states, two odd cat states with orthogonal superposition direction in phase space are prepared simultaneously, which have similar fidelity of 60% and amplitude of 1.2. Compared with the traditional method to generate two odd optical cat states based on two degenerate optical parametric amplifiers, only one nondegenerate optical parametric amplifier is applied in this experiment, which saves half of the quantum resources of nonlinear cavities. The presented results make a step toward preparing the four-component cat state, which has potential applications in fault-tolerant quantum computation.     

Hybrid quantum processors: molecular ensembles as quantum memory for solid state circuits

We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.     

Improved Superconducting Qubit State Readout by Path Interference

High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. We demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined with other qubit readout methods based on the discrimination of cavity photon states to further improve the qubit state readout.