可控电磁场下量子相干动力学和量子信息流动

针对可控电磁场作用下超导量子比特系统中的量子纠缠和量子相干性的动力学性质进行了探讨,所研究的系统是由两个非相互作用的超导比特与独自的数据总线构成的,同时每个超导比特都受到可控的电磁场的驱动。研究发现可控电磁场能够增加两超导量子比特之间的量子纠缠和量子相干性,特别,增加的幅度会随着可控电磁场作用强度的加大而提高。另外,利用迹距离的方法,探索了可控电磁场对于系统中量子信息流动的影响,发现可控电磁场可以抑制超导量子比特与数据总线之间的相互作用,减缓超导量子比特子系统与其它子系统之间的信息交换,阻碍量子信息在不同子系统中的额流动,并保护超导量子比特子系统初始拥有的量子特性。     

初始量子相干性对玻色-爱因斯坦凝聚系统中量子纠缠的影响

研究拉曼耦合玻色-爱因斯坦凝聚原子系统,在初始态具有量子相干性时纠缠相干态的生成,讨论了初始量子相干性对所生成的纠缠态的影响,指出初始量子相干性只影响所生成的纠缠相干态各组分之间的相位,不影响它们的振幅.     

量子关联研究综述

由EPR佯谬,薛定谔“猫态”等超越直观的纯量子现象产生的量子纠缠理论从其概念提出以来一直被人们认为既是量子理论最为重要的概念之一,也是在量子通讯中实现“稠密编码”和“隐形传态”的关键。然而,最近研究结果显示,量子纠缠并不能够完全解释量子关联所有特性。人们发现,除了纠缠以外,还存在对量子信息和量子计算具有极其重要意义的其它非经典关联,如量子失协是一个纯量子比特确定性量子计算机具有计算效率的原因。这说明,量子失协完全可以成为量子计算新的一种资源。文章介绍了非经典关联（包括量子纠缠）的基本概念及其度量方法,对量子失协在各类模型中表现出的量子关联特性进行分析和与量子纠缠,经典关联比较,从而体现出在各类量子体系中对量子失协进行研究的意义,同时引导理论和实验研究者去研究量子失协的潜在研究价值。     

相互作用的量子比特在不同环境下的共生纠缠度和量子失协分析

研究了初始纠缠的量子比特在不同的环境下共生纠缠度和量子失协的动力学演化.在所研究的几种模型中,发现共生纠缠度强烈地依赖于初始环境,并在演化过程中发生纠缠死亡现象.共生纠缠度和量子失协在演化过程中均出现坍塌和复苏效应,但当量子比特间的相互作用为零时,坍塌和复苏效应消失.可无论在何种情况下,量子失协均不会减小到零,即仍能反映量子比特和系统间的量子关联,显示量子失协比量子纠缠具有更强的抗退相干能力.因此量子失协可作为获得有效量子信息的一种更有效的量子资源.     

有噪量子信道中的量子相干性

量子态的相干叠加是量子力学的重要特征,同时也是量子计算能够实施的重要资源.由于噪声的影响,量子比特系统的相干性不能长时间的保持.有噪量子信道能够多大程度的保持量子相干性?本文利用相对熵相干性度量,首先讨论两体量子系统相干性、纠缠以及量子关联之间的关系.然后讨论噪声对相干性的影响.通过定义最大相干性的衰减量,即相干性衰减,进而讨论相干性衰减与系统和环境之间纠缠的不确定性关系.最后,具体讨论几类常见量子信道保持最大相干性的能力.     

一类两体非X-型量子态的量子失谐

量子失谐是近年来被发现的除量子纠缠外很重要的一种量子关联,是量子信息中两种表达式的差异.针对两体X-型量子态的量子失谐的研究有很多,但对两体非X-型量子态的研究较少.讨论一类含4个参数的两体非X-型量子态的量子失谐.通过对量子测量进行酉变换,得到经典量子关联.最后运用拉格朗日乘数法来求其量子失谐的极值,给出这类两体非X-型量子态量子失谐的解析解.     

两体量子模型的代数动力学方法求解

引入广义坐标和广义动量,将非线性自洽两体量子模型表述为经典不含时哈密顿系统并实现了去约束经典哈密顿量的正则化。量子系统的整体规范不变性,体现在去约束经典哈密顿量和哈密顿动力学关系的不变性中。利用代数动力学方法求解经典哈密顿方程,得到了两体量子系统的六阶近似分析解。     

基于关联方向测量的量子关联

通过讨论量子失协的上界,给出可达熵量子关联态的一个必要条件．对于双量子比特系统,利用可获得经典关联的最佳测量定义了关联方向上最小的量子失协,分析得到该度量是量子失协很好的逼近结果．最后,讨论量子关联的动力学过程,发现量子关联在衰减的过程中出现了突然的改变,利用关联方向上最小的量子失协给出了该现象的合理解释．     

锂原子系统的量子相变与量子纠缠

运用矩阵对角化的方法研究了锂6原子系统中的量子相变和量子纠缠．通过计算该量子系统的冯诺依曼熵发现纠缠度与量子相变存在关联．计算结果表明,量子纠缠只存在于量子相变之前,当磁场超过相变临界点,纠缠现象随即消失．     

2个二能级原子与最小热环境作用系统中的量子特性

利用全量子理论研究了2个二能级原子与单模热场相互作用的简单模型(其中一个原子是孤立的,另一个原子与单模热场相耦合,该热场为一个小微外环境),通过对该模型进行精确求解,探究小微外环境对2个相互作用二能级原子系统量子特性的影响.通过研究量子纠缠和相干特性的时间演化,讨论了相关耦合常数及环境有效温度对有关物理量演化的影响.结果表明：这一系统中,可观察到2原子间量子纠缠的突然死亡和再生的现象,分别对应弱耦合及强耦合情况,当平均光子数不同时,相干性及原子的线性熵随时间变化呈现不同的演化规律.     

Atomic homodyne detection of continuous-variable entangled twin-atom states

Historically, the completeness of quantum theory has been questioned using the concept of bipartite continuous-variable entanglement. The non-classical correlations (entanglement) between the two subsystems imply that the observables of one subsystem are determined by the measurement choice on the other, regardless of the distance between the subsystems. Nowadays, continuous-variable entanglement is regarded as an essential resource, allowing for quantum enhanced measurement resolution, the realization of quantum teleportation and quantum memories, or the demonstration of the Einstein-Podolsky-Rosen paradox. These applications rely on techniques to manipulate and detect coherences of quantum fields, the quadratures. Whereas in optics coherent homodyne detection of quadratures is a standard technique, for massive particles a corresponding method was missing. Here we report the realization of an atomic analogue to homodyne detection for the measurement of matter-wave quadratures. The application of this technique to a quantum state produced by spin-changing collisions in a Bose-Einstein condensate reveals continuous-variable entanglement, as well as the twin-atom character of the state. Our results provide a rare example of continuous-variable entanglement of massive particles. The direct detection of atomic quadratures has applications not only in experimental quantum atom optics, but also for the measurement of fields in many-body systems of massive particles.     

Experimental long-lived entanglement of two macroscopic objects

Entanglement is considered to be one of the most profound features of quantum mechanics. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 1012 atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation of quantum states of matter and quantum memory.     

在纠缠原子与热光场相互作用系统中光场的特性

采用时间演化算符和数值计算方法,研究了两全同二能级纠缠原子与热态光场相互作用过程中光场的量子特性,结果表明:两原子初始纠缠度和光场初始强度对光场的量子特性均有影响.对于适当的初始热光场强度,光场显现出了非经典特性.这种非经典特性随着初始热光场强度的增加而增强,但这种非经典特性并不总是随着原子初始纠缠度的增大而增强,而是呈现出一种复杂的关系.     

Controlled exchange interaction between pairs of neutral atoms in an optical lattice

Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials to isolate and manipulate arrays of paired (87)Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling in a system of neutral atoms. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its 'half-implementation', the root SWAP gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation.     

Stimulated emission of polarization-entangled photons

Entangled photon pairs-discrete light quanta that exhibit non-classical correlations-play a crucial role in quantum information science (for example, in demonstrations of quantum non-locality, quantum teleportation and quantum cryptography). At the macroscopic optical-field level non-classical correlations can also be important, as in the case of squeezed light, entangled light beams and teleportation of continuous quantum variables. Here we use stimulated parametric down-conversion to study entangled states of light that bridge the gap between discrete and macroscopic optical quantum correlations. We demonstrate experimentally the onset of laser-like action for entangled photons, through the creation and amplification of the spin-1/2 and spin-1 singlet states consisting of two and four photons, respectively. This entanglement structure holds great promise in quantum information science where there is a strong demand for entangled states of increasing complexity.     

Dynamical decoupling of electron spins in phosphorus-doped silicon

Quantum coherence is an important enabling feature underpinning quantum computation. However, because of couplings with its noisy surrounding environment, qubits suffer from the decoherence effects. The dynamical decoupling (DD) technique uses pulse-induced qubit flips to effectively mitigate couplings between qubits and environment. Optimal DD eliminates dephasing up to a given order with the minimum number of pulses. In this paper, we first introduce our recent work on prolonging electron spin coherence in γ-irradiated malonic acid crystals and analyze different decoherence mechanisms in this solid system. Then we focus on electron spin relaxation properties in another system, phosphorous-doped silicon (Si:P) crystals. These properties have been investigated by pulse electron paramagnetic resonance (EPR). We also investigate the performance of the dynamical decoupling technique on this system. Using 8-pulse periodic DD, the coherence time can be extended to 296 μs compared with 112 μs with one-pulse control.     

Entanglement of single-atom quantum bits at a distance

Quantum information science involves the storage, manipulation and communication of information encoded in quantum systems, where the phenomena of superposition and entanglement can provide enhancements over what is possible classically. Large-scale quantum information processors require stable and addressable quantum memories, usually in the form of fixed quantum bits (qubits), and a means of transferring and entangling the quantum information between memories that may be separated by macroscopic or even geographic distances. Atomic systems are excellent quantum memories, because appropriate internal electronic states can coherently store qubits over very long timescales. Photons, on the other hand, are the natural platform for the distribution of quantum information between remote qubits, given their ability to traverse large distances with little perturbation. Recently, there has been considerable progress in coupling small samples of atomic gases through photonic channels, including the entanglement between light and atoms and the observation of entanglement signatures between remotely located atomic ensembles. In contrast to atomic ensembles, single-atom quantum memories allow the implementation of conditional quantum gates through photonic channels, a key requirement for quantum computing. Along these lines, individual atoms have been coupled to photons in cavities, and trapped atoms have been linked to emitted photons in free space. Here we demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of these photons signals the entanglement of the atomic qubits. We characterize the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. Although this entanglement method is probabilistic, it is still in principle useful for subsequent quantum operations and scalable quantum information applications.     

Amplifying stationary quantum discord and entanglement between a superconducting qubit and a data bus by time-dependent electromagnetic field

We study the dynamics of quantum discord and entanglement between a superconducting qubit and a data bus,which is driven by a controllable time-dependent electromagnetic field,in the presence of phase decoherence and find that the quantum discord and entanglement remain at a stationary non-zero value for long time evolution.It is shown that the amount of stationary quantum discord and entanglement can be enhanced by applying the time-dependent electromagnetic field.     

Decoherence-protected quantum gates for a hybrid solid-state spin register

Protecting the dynamics of coupled quantum systems from decoherence by the environment is a key challenge for solid-state quantum information processing. An idle quantum bit (qubit) can be efficiently insulated from the outside world by dynamical decoupling, as has recently been demonstrated for individual solid-state qubits. However, protecting qubit coherence during a multi-qubit gate is a non-trivial problem: in general, the decoupling disrupts the interqubit dynamics and hence conflicts with gate operation. This problem is particularly salient for hybrid systems, in which different types of qubit evolve and decohere at very different rates. Here we present the integration of dynamical decoupling into quantum gates for a standard hybrid system, the electron-nuclear spin register. Our design harnesses the internal resonance in the coupled-spin system to resolve the conflict between gate operation and decoupling. We experimentally demonstrate these gates using a two-qubit register in diamond operating at room temperature. Quantum tomography reveals that the qubits involved in the gate operation are protected as accurately as idle qubits. We also perform Grover's quantum search algorithm, and achieve fidelities of more than 90% even though the algorithm run-time exceeds the electron spin dephasing time by two orders of magnitude. Our results directly allow decoherence-protected interface gates between different types of solid-state qubit. Ultimately, quantum gates with integrated decoupling may reach the accuracy threshold for fault-tolerant quantum information processing with solid-state devices.