混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

多光子Tavis-Cummings模型中两纠缠原子的纠缠演化特性

研究了两个纠缠的二能级原子通过多光子跃迁与单模相干光场进行耦合相互作用系统中两原子纠缠的演化特性.计算分析表明,两个原子之间的纠缠呈现出周期性的演化特性,初始两原子的状态、原子间的偶极相互作用、相干光场的参数以及跃迁光子数对两个原子的纠缠有着显著的影响;并发现两原子初始处于某最大纠缠态时,两原子会永远处于该最大纠缠态,因此这一类最大纠缠态可以作为一种量子信息存储器. 关键词： 量子纠缠 部分转置矩阵负本征值 纠缠原子 相干态     

多光子Tavis-Cummings模型中两纠缠原子的纠缠演化特性

研究了两个纠缠的二能级原子通过多光子跃迁与单模相干光场进行耦合相互作用系统中两原子纠缠的演化特性.计算分析表明，两个原子之间的纠缠呈现出周期性的演化特性，初始两原子的状态、原子间的偶极相互作用、相干光场的参数以及跃迁光子数对两个原子的纠缠有着显著的影响；并发现两原子初始处于某最大纠缠态时，两原子会永远处于该最大纠缠态，因此这一类最大纠缠态可以作为一种量子信息存储器.     

原子与双模相干强场依赖强度耦合多光子过程中纠缠量度与制备

给出了依赖强度耦合双模多光子过程Jaynes-Cummings模型的有效哈密顿量.在强场条件下,分别用量子约化熵和量子相对熵研究了上述模型中原子与场之间的纠缠以及双模相干场的模间纠缠演化.研究表明,这两类纠缠演化均与原子跃迁时吸收(或发射)的光子数k密切相关.同时,还揭示了双光子过程(k=1)和多光子过程(k≥2)中不同的纠缠特性.讨论了纠缠态的制备,制备了与时间无关的原子-场的Einstein-Podolsky-Rosen态和双模相干场的模间纠缠态.     

两纠缠原子与光场相互作用系统场熵演化特性

研究了两纠缠原子与相干态光场相互作用系统场熵的时间演化特性,运用全量子力学理论和数值方法,讨论了初始两原子所处的纠缠状态、纠缠度和腔内光场的强弱对场熵的影响.随着光场平均光子数的增加,系统场熵均值和振荡频率增大;光场较弱时,场熵呈现一定的周期性振荡;光场增强后,场熵呈现出周期性的崩塌与回复,且随初始两原子纠缠度的增加,场熵的振幅增大.     

宏观场与环境作用过程中的纠缠突然死亡与突然产生

研究了两个初始处于纠缠相干态上的宏观场各自独立地与一个环境相互作用的系统, 环境对腔场的影响只体现在腔场光子数的泄漏上. 采用共生纠缠(concurrence)度量两个宏观场间的纠缠, 并给出宏观场纠缠的解析解, 以分析这种系统中宏观场纠缠的动力学特性. 研究表明当场的初始平均光子数较大时, 即使很小的光子泄漏率也会导致腔场间出现纠缠突然死亡现象. 同时研究结果也表明光子从腔场泄漏到环境后会导致两环境间的纠缠突然产生, 而这种纠缠产生的时机直接与腔场的初始光子数相关. 本文还进一步发现在大光子数的情况下, 在任何时刻任意一个腔场与任意一个环境间都不会产生纠缠. 关键词： 纠缠相干态 环境 纠缠突然死亡 纠缠突然产生     

与腔场耦合的两个量子点中激子的纠缠动力学

利用共生纠缠度研究了单模腔场内两个耦合量子点中激子的纠缠动力学行为.结果表明:无论腔场初始制备于奇相干态还是偶相干态,两个量子点间直接耦合作用均能减弱激子的纠缠度.在腔场初始为奇相干态时,激子的纠缠度随场模强度的增加而减小;偶相干态时,激子的纠缠度呈现一个转折变化.此外,也研究了单模腔场内平均光子数与激子准最大相干纠缠态的关联.     

Stark位移对热环境下双Jaynes-Cummings模型中原子纠缠的影响

研究双光子跃迁的双Jaynes-Cummings模型中, 两原子初始处于最大纠缠态、光场初始处于单模热态时的原子纠缠动力学, 考虑双光子过程中的Stark位移、热光场的平均光子数对原子纠缠的影响. 结果表明: 不考虑Stark位移时, 两原子在系统演化过程中周期性地出现退纠缠现象, 而考虑Stark位移时, 两原子长时间地纠缠. 特别是当Stark位移参数取值较大且热光场的平均光子数值比较小时, 两原子能保持 稳定地纠缠. 这些结果表明, 可以利用Stark位移对热环境下双Jaynes-Cummings模型中的原子纠缠进行调控.     

Bell态原子与双模纠缠相干光场相互作用的纠缠特性

运用Negativity熵研究了Bell态原子与双模纠缠相干光场相互作用系统中两个全同二能级原子之间的纠缠演化特性.分析了光场强度、光场纠缠度及原子间相互作用强度对纠缠的影响.结果表明:原子处于|β11〉时,两原子始终处于最大纠缠态;原子处于|β00〉时,初始纠缠的两原子始终较长时间处于退纠缠状态;原子处在|β10〉时,增大双模光场的平均光子数可以明显增大两原子之间的纠缠度并保持较大的纠缠状态;原子初态处在|β01〉时,原子间的相互作用强度对双原子间纠缠度有较显著的非线性调制作用.     

Detecting entanglement of quantum channels

Entanglement is the crucial resource for different quantum information processing tasks. While conventional studies focus on the entanglement of bipartite or multipartite quantum states, recent works have extended the scenario to the entanglement of quantum channels, an operational quantification of the channel entanglement manipulation capability. Based on the recently proposed channel entanglement resource framework, here we study a further task of resource detection—witnessing entanglement of quantum channels. We first introduce the general framework and show how channel entanglement detection is related to the Choi state of the channel, enabling channel entanglement detection via conventional state entanglement detection methods. We also consider entanglement of multipartite quantum channels and use the stabilizer formalism to construct entanglement witnesses for circuits consisting of controlled-Z gates. We study the effectiveness of the proposed detection methods and compare their performance for several typical channels. Our work paves the way for systematic theoretical studies of channel entanglement and practical benchmarking of noisy intermediate scaled quantum devices.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper,we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition.From our analysis,we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment.Specially,our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly.Additionally,the discussion of the case of the multipartite states with high dimensions is made.     

Steering in computational science: Mesoscale modelling and simulation

This paper outlines the benefits of computational steering for high performance computing applications. Lattice-Boltzmann mesoscale fluid simulations of binary and ternary amphiphilic fluids in two and three dimensions are used to illustrate the substantial improvements which computational steering offers in terms of resource efficiency and time to discover new physics. We discuss details of our current steering implementations and describe their future outlook with the advent of computational grids.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper, we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition. From our analysis, we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment. Specially, our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly. Additionally, the discussion of the case of the multipartite states with high dimensions is made.     

两原子与数态场相互作用系统中纠缠的调控

通过计算并发度研究了两个处于初始激发态的两能级原子与数态场相互作用系统的纠缠动力学特性,并讨论了场的光子数、原子和场的失谐量以及原子操作对并发度的影响。结果表明当不存在原子操作时,两原子之间的纠缠出现突然产生现象,并且可以通过调节光子数和原子与场的失谐量来控制产生纠缠的阈值时间和纠缠的最大值。当存在原子操作时,两原子之间的纠缠随着时间的演化可以立即产生,而且通过对经典场的操作和控制可以实现两原子之间纠缠的调控。     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol(EPP) with controlled-not(CNOT) gates and linear optics.With the CNOT gates,our EPP can reach a higher fidelity than the conventional one.Moreover,it does not require the fidelity of the initial mixed state to satisfy F > 1/2.If the initial state is not entangled,it still can be purified.With the linear optics,this protocol can get pure maximally entangled pairs with some probabilities.Meanwhile,it can be used to purify the entanglement between the atomic ensembles in distant locations.This protocol may be useful in long-distance quantum communication.     

Efficient entanglement purification in quantum repeaters

We present an efficient entanglement purification protocol (EPP) with controlled-not (CNOT) gates and linear optics. With the CNOT gates, our EPP can reach a higher fidelity than the conventional one. Moreover, it does not require the fidelity of the initial mixed state to satisfy F>1/2. If the initial state is not entangled, it still can be purified. With the linear optics, this protocol can get pure maximally entangled pairs with some probabilities. Meanwhile, it can be used to purify the entanglement between the atomic ensembles in distant locations. This protocol may be useful in long-distance quantum communication.     

Universal Global Cloning of Continuous Variables Entanglement

It is impossible to perfectly duplicate an unknown entangled state while preserving inseparability, which is known as the entanglement no-cloning principle. Nevertheless, approximate cloning of entanglement is allowed by quantum mechanics. A universal entanglement cloning machine (UECM) duplicates an entangled state such that the quality of its entanglement replicas does not depend on the input. To duplicate entanglement shared between two parties, 1-to-N universal local entanglement cloning machine (ULECM) has already been proposed (Weedbrook, et al., Phys. Rev. A, 77, 052313 (2008)), which employs two local UECMs to copy each party of the entangled state. However, the ULECM can never preserve the inseparability in its replicas. Here, a 1-to-N universal global entanglement cloning machine (UGECM) that takes the entire entangled state as the input and then globally clone it to produce replicas is proposed. It is demonstrated that the UGECM outperforms the ULECM both in terms of the fidelity and the inseparability preservation. In addition, the UGECM is of more simple and easy structure, compared with the UGECM. Such a UGECM may find its new applications in quantum entanglement broadcasting.     

Efficient electronic entanglement concentration assisted by single mobile electrons

We present an efficient entanglement concentration protocol （ECP） for mobile electrons with charge detection. This protocol is quite different from other ECPs for one can obtain a maximally entangled pair from a pair of less-entangled state and a single mobile electron with a certain probability. With the help of charge detection, it can be repeated to reach a higher success probability. It also does not need to know the coefficient of the original less-entangled states. All these advantages may make this protocol useful in current distributed quantum information processing.     

Consequent entanglement concentration of a less-entangled electronic cluster state with controlled-not gates

We present a highly efficient entanglement concentration protocol （ECP） for a four-electron system in a less-entangled cluster state. In this ECP, we only require one pair of less-entangled electron cluster states and one ancillary electron to complete the task. With the help of the controlled-not （CNOT） gate, the concentrated maximally entangled state can be retained for further application with some success probability. On the other hand, the discarded items can be reused to obtain a high success probability. All the features make this ECP useful in the current quantum information field.