基于低Q腔的集体旋转无退相干子空间中的量子信息处理

基于低Q腔的单光子输入输出过程实现量子信息处理任务.将2个光子的极化态编码为1个逻辑量子比特,编码方式对于集体旋转噪声免疫.提出了实现原子和逻辑量子比特之间的混合控制相位翻转门,2个逻辑量子比特之间的CNOT门,逻辑量子比特的纠缠制备,原子到逻辑量子比特的量子态转移等方案.     

应用广义量子线性变换理论求解二维耦合量子谐振子

利用广义量子线性变换理论,给出了耦合量子谐振子■=1/2m(■2/1 ■2/2) 1/2mω2(■12 ■22) k/2(■1-■2)2的能量本征值、本征态、演化算符、演化矩阵以及波函数.     

相空间内光子数增加薛定谔猫态的性质

相空间量子干涉技术将量子态叠加与经典光学的干涉概念相结合,深刻地展示了量子态的非经典特征出现的物理机制。利用相空间量子干涉理论对相位薛定谔猫态在光子数增加过程的特性变化进行了理论分析,通过计算其光子数分布、Q参数和Wigner函数,讨论了光子数增加对相位连续变化的薛定谔猫态的非经典特性的影响,发现随着光子数的增加薛定谔猫态趋于呈现出亚泊松分布特性以及压缩效应。     

中科大实现光子比特与原子比特间的量子态隐形传输

中国科技大学微尺度物质科学国家实验室潘建伟研究小组在国际上首次实验实现了光子比特与原子比特之间的量子态隐形传输。光子是量子通信中最好的信息载体,但却难以存储;而原子态可用来存储量子态。在不破坏其量子特性的情况下,将飞行（光）量子比特所载信息传送到静止（原子）量子比特上．并在需要时成功读取原子量子比特内存储的信息,将是未来量子信息处理中的重要组成部分。研究人员利用极化光子态作为量子信息的载体,利用由大约100万铷原子构成的冷原子系综作为量子存储器．制备了光子与原子系综态之间的纠缠。通过这个光子-原子纠缠源,进行了光量子比特到远程原子比特的量子态隐形传输。传输到原子比特的量子信息在存储了8S后,被成功地转换为光量子态以作进一步的量子信息处理。     

旋涡光学与轨道角动量高维编码量子通信研究

阐述了光学旋涡的物理本征态、产生与调控技术,总结了基于OAM(Orbital Angular Momentum)调制的高维编码QKD以及OAM和自旋角动量(Spin Angular Momentum,SAM)的制备方法.利用I类相位匹配BBO晶体经自发参量下转化获得信号光子和闲置光子OAM纠缠光子对;采用SAM和OAM自由度转换器件,将SAM转化为OAM.另外,运用空间光调制器(Spatial Light Modulator,SLM)作为OAM纠缠态的"后选择"联合调制器,基于该单元技术提出基于OAM改进的BB84 QKD系统、SAM-OAM混合纠缠量子态的QKD系统,并提出三光子纠缠W态的制备方案.结果表明,编码信息量达log2(m+2)比特(m为l的可取值个数),有望实现可扩容量子比特的安全通信,具有高维度、强纠缠特性与抗比特丢失能力,实验中单元技术对进一步增加量子通信信道复用能力和改善网络安全性具有十分重要的意义.     

量子态的相位测量

阐述了一种新的测量量子态相位信息的方法及其原理. 通过测量由时域对称场所制备的量子态的量子干涉效应, 实现了对量子态的相位信息的提取和测量. 给出了该物理模型下的测量结果的解析表示, 揭示了被测量子态的相位信息和周期演化规律. 在理论上提出了实现可见光范围内几何相位测量的有效方法.     

激光冷却原子的新奇量子态与探测

随着原子气体的激光冷却和相干操控技术的飞速发展,冷原子系统的新奇量子态研究以及在量子精密测量的应用均取得了丰硕的成果,例如人造规范势、多体相互作用诱导的拓扑态及拓扑量子相变、冷原子喷泉频标、冷原子干涉仪测量引力常数、魔术波长光晶格频标、冷离子频标、冷原子重力仪、冷原子陀螺仪等.新型冷原子精密测量多次刷新了精密测量的记录,并逐渐成为新的计量标准.基于超冷原子的精密测量为新一代全球卫星导航、深空探测、微重力测量、地震预报、地下油田面积的勘测和油井的定位、工业精密测量与控制等提供新的关键技术.本文重点阐述了激光冷却原子的新奇量子态及探测技术,例如在玻色-凝聚体中产生自旋-轨道角动量耦合;由多体相互作用诱导的拓扑相变;在开通道、闭通道失衡Feshbach共振条件下,体系存在从FuldeFerrell-Larkin-Ovchinnikov超流态到Sarma超流态的量子相变;发展了87Rb原子nP里德堡态的光谱测量、光晶格中超冷原子量子态的探测等手段,并实现了多组分BEC干涉仪.     

量子区分与量子克隆简述

在经典信息理论中,编码状态可以精确复制与区分;而在量子信息中,由于态的叠加性存在,使得非正交态不可区分,量子态不可复制与删除.但是,量子态的区分和克隆在新型的量子信息科学中具有广泛的应用,例如量子密码的接收和窃听等.本文简要介绍量子态的区分和克隆的数学概念及相关研究结果.     

利用两个二粒子部分纠缠态实现两个目标共享的量子隐形传态

提出利用两个二粒子部分纠缠态实现单粒子量子态的隐形传态方案。首先考虑两个目标共享的、概率性的量子隐形传态;然后进一步考虑单个目标确定性的量子隐形传态。在这两个方案中,量子态的发送者的局域正定算符值测量(POVM)起着关键作用,并给出了该测量算符的数学表式。该方法可用于当两部分纠缠态的最小Schm idt系数μ和v满足条件2μ 2v-2μv-1≥0的情形。     

从量子谱到经典轨道:圆形腔中的弹子球

用半经典的方法来研究粒子经典运动已成为处理某些量子问题必不可少的工具.周期轨道理论已经成为人们研究定态体系的量子谱和所对应粒子经典运动的关系的主要工具.本文应用周期轨道理论,运用量子谱函数,这种量子谱函数的傅利叶变换包含了体系内许多经典轨道的信息.以二维无限深圆环势阱为例,利用它们的能量本征值和本征函数,用态密度公式计...     

拓扑绝缘体量子点中的磁交换相互作用

从理论上研究了拓扑绝缘体量子点中的磁交换相互作用.在拓扑绝缘体量子点中,边缘态电子数可以通过量子点的尺寸和外加电场进行调控.当量子点中掺入单个磁离子并且边缘态填充奇数电子时,电子与单个磁离子之间的交换相互作用达到最大值;而边缘态填充偶数电子时,电子与单个磁离子之间的交换相互作用消失.当量子点中掺入2个磁离子时,电子与Mn离子的sp-d相互作用会出现奇偶振荡行为,Mn离子间的相互作用取决于Mn离子间距和量子点壳层中的电子数,表现出典型的Ruderman-Kittel-Kasuya-Yosida型间接交换机制.工作澄清了拓扑绝缘体量子点壳层结构对其磁性的影响,有助于人们设计基于拓扑绝缘体量子点的自旋电子学或量子信息器件.     

Geometric quantum computation using nuclear magnetic resonance

A significant development in computing has been the discovery that the computational power of quantum computers exceeds that of Turing machines. Central to the experimental realization of quantum information processing is the construction of fault-tolerant quantum logic gates. Their operation requires conditional quantum dynamics, in which one sub-system undergoes a coherent evolution that depends on the quantum state of another sub-system; in particular, the evolving sub-system may acquire a conditional phase shift. Although conventionally dynamic in origin, phase shifts can also be geometric. Conditional geometric (or 'Berry') phases depend only on the geometry of the path executed, and are therefore resilient to certain types of errors; this suggests the possibility of an intrinsically fault-tolerant way of performing quantum gate operations. Nuclear magnetic resonance techniques have already been used to demonstrate both simple quantum information processing and geometric phase shifts. Here we combine these ideas by performing a nuclear magnetic resonance experiment in which a conditional Berry phase is implemented, demonstrating a controlled phase shift gate.     

Scaling of entanglement close to a quantum phase transition

Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in Si-MOSFETs (metal oxide silicon field-effect transistors; ref. 4) and the superconductor-insulator transition in two-dimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics-a complete classification of the critical many-body state requires the introduction of concepts from quantum information theory. Here we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of one-dimensional magnetic systems, that entanglement shows scaling behaviour in the vicinity of the transition point.     

Realization of the Cirac-Zoller controlled-NOT quantum gate

Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac-Zoller proposal for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac-Zoller proposal. In our experiment, two 40Ca+ ions are held in a linear Paul trap and are individually addressed using focused laser beams; the qubits are represented by superpositions of two long-lived electronic states. Our work relies on recently developed precise control of atomic phases and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques.     

Kane固体量子计算机中核自旋读出

在2个量子位(qubit)的体系中,对Kane固体量子计算机模型的通过电子状态与核子自旋状态的交换进行测量的方法进行了研究。从2个量子位系统的Hamilton量出发,根据总自旋在外磁场方向的总投影分成5个不变子空间,构造它们的块对角矩阵形式。分析了它们所有的本征值和本征态随电子之间交换相互作用的大小的变化关系。结果表明:利用核自旋和电子自旋交换来测量核自旋的方法是有一定适用范围的,在2个量子位与2个电子的系统中的16个态中,只有4个可以利用这种方法来测量。     

EPR粒子对与量子隐形传态

量子隐形传态,由发送者Alice将准备传送的信息分离成一部分纯粹经典的信息和另外一部分纯粹非经典的信息,通过2条不同的信道传送给接收者Bob。首先传送非经典部分,这需要借助于EPR粒子对,考虑由2个自旋皆为12的费密子构成,其中一个分配给Alice,另一个分配给Bob。Alice选择对她一方的原始粒子和她的EPR粒子一并进行冯.罗曼类型的测量,这个在贝尔算符的4个本征态中的测量,导致系统的波函数的波包坍缩为相互关联的4个贝尔基矢。Bob通过对他的EPR粒子的状态进行适当的幺正变换,能够重新构造出在Alice一方被＂毁灭＂了的原始粒子的状态。此外,这个贝尔测量产生2个比特的经典信息,传送给Bob,从而完成一个量子隐形传态。文章中研究了EPR粒子对与量子隐形传态的内在联系。     

一维两体δ作用玻色费米气体的精确本征值

一维两体δ作用玻色费米混合气体就是用具有超矩阵的量子非线性Schrǒdinger 模型描述。利用量子反散射方法精确地求出了该系统的本征态、本征值和 Betheansatz 方程。另外,还讨论了本征态所具有的守恒关系。     

Microwave quantum logic gates for trapped ions

Control over physical systems at the quantum level is important in fields as diverse as metrology, information processing, simulation and chemistry. For trapped atomic ions, the quantized motional and internal degrees of freedom can be coherently manipulated with laser light. Similar control is difficult to achieve with radio-frequency or microwave radiation: the essential coupling between internal degrees of freedom and motion requires significant field changes over the extent of the atoms' motion, but such changes are negligible at these frequencies for freely propagating fields. An exception is in the near field of microwave currents in structures smaller than the free-space wavelength, where stronger gradients can be generated. Here we first manipulate coherently (on timescales of 20 nanoseconds) the internal quantum states of ions held in a microfabricated trap. The controlling magnetic fields are generated by microwave currents in electrodes that are integrated into the trap structure. We also generate entanglement between the internal degrees of freedom of two atoms with a gate operation suitable for general quantum computation; the entangled state has a fidelity of 0.76(3), where the uncertainty denotes standard error of the mean. Our approach, which involves integrating the quantum control mechanism into the trapping device in a scalable manner, could be applied to quantum information processing, simulation and spectroscopy.     

Coherent control of optical information with matter wave dynamics

In recent years, significant progress has been achieved in manipulating matter with light, and light with matter. Resonant laser fields interacting with cold, dense atom clouds provide a particularly rich system. Such light fields interact strongly with the internal electrons of the atoms, and couple directly to external atomic motion through recoil momenta imparted when photons are absorbed and emitted. Ultraslow light propagation in Bose-Einstein condensates represents an extreme example of resonant light manipulation using cold atoms. Here we demonstrate that a slow light pulse can be stopped and stored in one Bose-Einstein condensate and subsequently revived from a totally different condensate, 160 mum away; information is transferred through conversion of the optical pulse into a travelling matter wave. In the presence of an optical coupling field, a probe laser pulse is first injected into one of the condensates where it is spatially compressed to a length much shorter than the coherent extent of the condensate. The coupling field is then turned off, leaving the atoms in the first condensate in quantum superposition states that comprise a stationary component and a recoiling component in a different internal state. The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this 'messenger' atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser. The probe light is driven back on and the messenger pulse is coherently added to the matter field of the second condensate by way of slow-light-mediated atomic matter-wave amplification. The revived light pulse records the relative amplitude and phase between the recoiling atomic imprint and the revival condensate. Our results provide a dramatic demonstration of coherent optical information processing with matter wave dynamics. Such quantum control may find application in quantum information processing and wavefunction sculpting.     

六角密排格子ISING（S=1）反铁磁体的零温相图

本文利用文献[1]所发展的量子多体方法,变S=1的Ising体系为粒子数不守恒的费米体系,严格地求得了具有近邻和次近邻相互作用的六角密排Ising反铁磁体在外场下的基态能量和完整的零温相图。