三能级原子系统中量子态相干保持的控制策略

研究光与物质的相互作用并利用其性质设计新型的量子器件,以实现光信息存储及其消相干抑制.分析了以单色激光场为控制场的A型三能级原子系统的主方程模型,借助于无消相干子空间的构造方法,通过改变耦合激光场Rabi频率的方式,设计了实现系统中量子态相干保持的控制策略.     

三能级原子系统中量子态相干保持的控制策略

研究光与物质的相互作用并利用其性质设计新型的量子器件,以实现光信息存储及其消相干抑制.分析了以单色激光场为控制场的∧型三能级原子系统的主方程模型,借助于无消相干子空间的构造方法,通过改变耦合激光场Rabi频率的方式,设计了实现系统中量子态相干保持的控制策略.

     

基于相干控制的二能级量子系统退相干抑制

对于二能级开放量子系统,研究了利用相干控制抑制退相干效应的问题.首先讨论了二能级开放量子系统在相干控制下的建模问题,将退相干抑制归结为与环境噪声解耦的控制问题.然后,引入开环控制抑制退相干,并证明该控制可使系统状态中的部分分量与环境噪声渐近解耦.最后引入反馈控制,使得系统状态的相应分量可以与环境精确解耦,同时能够避免测量引入的量子噪声的影响.     

两硬币量子游走模型中的相干动力学

量子游走是量子计算的重要模型,而多硬币量子游走模型由于在量子通讯协议中表现突出也越来越受到人们的关注.量子相干不仅可以刻画量子态的特点,也可以反映量子演化过程的性质.主要对一维圆上两硬币量子游走模型的量子相干性进行了分析.一方面,讨论了初始量子态和硬币算子的选取对量子相干的影响.当硬币算子为Hadamard算子且初态只要在位置子空间上是均衡叠加态,整个量子游走演化过程是具有周期性的,且量子相干仅依赖于步数和圆上顶点的个数;当初始态是均衡叠加态而对硬币算子没有任何限制时,量子相干的演化也极具规律性.另一方面,发现在利用量子游走实现完美状态转移(perfect state transfer)的过程中,硬币算子的选取直接影响量子相干的值.最后,探讨了2种量子游走模型之间的等价性,并基于此指出了其在量子隐形传输(quantum teleportation)中的应用和改进的可能性.     

量子力学系统的输出反馈控制

研究量子系统输出反馈控制问题建模以及控制律设计问题.首先讨论了量子力学VonNeumann测量原理与连续测量模型的一致性;然后在连续测量模型的基础上总结了已有量子反馈模型的结果,归纳出量子输出反馈控制系统模型;最后针对单比特振幅退相干抑制问题,利用线性直接输出反馈控制设计反馈控制律,指出利用最优控制的方法设计线性输出反馈控制的比例系数,可以得到较好的结果.     

基于奇相干光源和量子存储的量子密钥分配协议

针对传统量子密钥分配协议使用弱相干光源带来的密钥生成率较低的问题,对光源进行优化,用奇相干光源代替弱相干光源,提出了基于奇相干光源和量子存储的测量设备无关量子密钥分配协议。对比了具有奇相干光源和量子存储的测量设备无关量子密钥分配协议与基于弱相干光源测量设备无关量子密钥分配协议的性能优劣。分析了基于奇相干光源和量子存储的测量设备无关量子密钥分配协议中,密钥生成率、最小退相干时间与安全传输距离之间的关系。仿真结果表明,引入奇相干光源大大减少了传统弱相干光源的多光子数,弥补了其在光源上的不足之处。随着安全传输距离的增加,密钥生成率随之降低,但基于奇相干光源和量子存储的量子密钥分配协议性能仍然较高。     

基于非线性消振器的空间结构被动振动抑制

研究了空间结构振动抑制的被动非线性消振方法.提出了适用于空间环境的非线性消振器结构及动力学模型,该结构通过引入磁力实现空间环境下航天器结构的振动抑制.然后,从理论上建立了含有非线性消振器的空间悬臂梁结构动力学模型,并通过Galerkin截断及数值分析方法分析了瞬态激励下非线性消振器对空间悬臂梁结构的被动振动抑制效果.仿真结果表明,该被动非线性消振器对系统的能量耗散率可以达到92%,可以实现非常好的振动抑制效果,能够适应空间环境,并提高航天系统的可靠性.     

融合空时相干和特征空间波束形成的超声成像

为了进一步提高超声成像的质量,提出了融合特征空间最小波束形成和空时相干系数的成像方法。首先利用最小方差法计算回波数据的协方差矩阵和加权向量;然后对协方差矩阵进行特征分解得到信号子空间,并将加权向量投影到信号子空间,得到特征空间方法的加权向量;同时采用空时平滑方法计算相干系数,最后用空时相干系数作为加权系数对特征空间最小方差波束形成的结果进行优化。为了验证算法的有效性,对医学成像上常用的点目标和斑目标进行了成像,仿真实验结果表明：与特征空间最小方差算法和融合特征空间与相干系数的算法相比,提出的方法提高了对比度和稳健性,其代价是略微降低了成像分辨率。     

改进Geese算法对信号的DOA估计

针对传统的空间谱估计算法对相干源的波达方向（DOA）估计会失效的问题,该文在利用信号子空间特征向量生成广义特征值（Generalized Eigenvalues utilizing Signal subspace Eigenveetors,Geese）算法基础上,结合空间平滑技术对接受数据进行预处理,提出一种改进的Geese算法。该算法由于不需方向搜索且只利用信号子空间,大大降低了计算复杂性。计算机仿真结果表明,与Geese算法相比,该改进算法既能够有效地估计出独立信号源的DOA,也能有效地估计出相干信号源和相隔比较近的低信噪比信号源的DOA。对于独立源的估计,在相同条件下,估计性能也要优于经典的空间谱估计算法（ESPRIT算法和MUSIC算法）,从而证明该算法的合理性。     

一种基于阵列虚拟平移的互耦自校正算法

阵元间的互耦和相干信源对阵列的DOA(Direction-Of-Arrival)估计性能产生了严重影响.针对这一问题,在应用子空间原理的基础上,利用均匀线阵互耦矩阵的带状循环特性及时称Toeplitz性,并结合阵列虚拟平移,提出了一种阵元互耦条件下的相干信源的波达方向估计新算法.该算法无需阵元的互耦参数信息,就可准确地估计出相干信源的DOA,估计性能优良,而且只需一维搜索,计算量较小.计算机仿真结果验证了该算法的有效性.     

Pole placement approach to coherent passive reservoir engineering for storing quantum information

Reservoir engineering is the term used in quantum control and information technologies to describe manipulating the environment within which an open quantum system operates. Reservoir engineering is essential in applications where storing quantum information is required. From the control theory perspective, a quantum system is capable of storing quantum information if it possesses a so-called decoherence free subsystem (DFS). This paper explores pole placement techniques to facilitate synthesis of decoherence free subsystems via coherent quantum feedback control. We discuss limitations of the conventional `open loop'' approach and propose a constructive feedback design methodology for decoherence free subsystem engineering. It captures a quite general dynamic coherent feedback structure which allows systems with decoherence free modes to be synthesized from components which do not have such modes.     

Enhancing the fidelity of remote state preparation by partial measurements

Enhancing the fidelity of quantum state transmission in noisy environments is a significant subject in the field of quantum communication. In this paper, improving the fidelity of a deterministic remote state preparation (RSP) protocol under decoherence is investigated with the technique of weak measurement (WM) and weak measurement reversal (WMR). We first construct the quantum circuit of the deterministic remote preparation of a single-qubit state through an EPR state with the assistance of an auxiliary qubit. Then, we analytically derive the average fidelity of the deterministic RSP protocol under the influence of generalized amplitude damping noises acting on the EPR state. Our results show that when only qubit 2 undergoes the decoherence channel, the average fidelity of the RSP protocol subject to generalized amplitude damping noise is the same as that subject to amplitude damping noise. Moreover, we analyze the optimal average fidelity of the above RSP process by introducing WM and WMR. It is found that the application of WM and a subsequent reversal operation could lead to the remarkable improvement of the average fidelity for most values of the decoherence parameters.     

Analysis of optical parity gates of generating Bell state for quantum information and secure quantum communication via weak cross-Kerr nonlinearity under decoherence effect

We demonstrate the advantages of an optical parity gate using weak cross-Kerr nonlinearities (XKNLs), quantum bus (qubus) beams, and photon number resolving (PNR) measurement through our analysis, utilizing a master equation under the decoherence effect (occurred the dephasing and photon loss). To generate Bell states, parity gates based on quantum non-demolition measurement using XKNL are extensively employed in quantum information processing. When designing a parity gate via XKNL, the parity gate can be diversely constructed according to the measurement strategies. In practice, the interactions of XKNLs in optical fiber are inevitable under the decoherence effect. Thus, by our analysis of the decoherence effect, we show that the designed parity gate employing homodyne measurement would not be expected to provide reliable quantum operation. Furthermore, compared with a parity gate using a displacement operator and PNR measurement, we conclude there is experimental benefit from implementation of a parity gate via qubus beams and PNR measurement under the decoherence effect.     

Quantum discord protection of a two-qutrit V-type atomic system from decoherence by partially collapsing measurements

In this paper, by exploiting the weak measurement and quantum measurement reversal procedure, we propose a scheme to show how one can protect the geometric quantum discord (GQD) of a two-qutrit V-type atomic system each of which interacts with a dissipative reservoir independently. We examine the scheme for the GQD of the initial two-qutrit Werner and Horodecki states for different classes of weak measurement strengths. It is found out that the presented protocol enables us to suppress decoherence due to the amplitude damping channel and preserve the quantum discord of the two-qutrit system successfully.     

弱测量在提高量子参数估计精度中的应用

量子参数估计理论主要研究如何利用量子资源来提高经典参数的估计精度,其应用非常广泛.对于封闭的量子系统,采用纠缠的量子探针测量未知的经典参数,其估计精度可以大幅地超越经典探针所能达到的精度.然而,当退相干效应存在时,量子探针的优越性就会受到限制.本文引入弱测量的方法提高噪声环境下参数估计的精度,并证明了当退振幅噪声存在时,这一方法可以使得参数估计的精度恢复到海森堡极限,尽管实现的概率会随着探针个数的增加呈指数地减少.本文进一步考察了当辅助系统存在时,弱测量的方法对提高参数估计精度的有效性.     

Discrete quantum Fourier transform using weak cross-Kerr nonlinearity and displacement operator and photon-number-resolving measurement under the decoherence effect

We present a scheme for implementing discrete quantum Fourier transform (DQFT) with robustness against the decoherence effect using weak cross-Kerr nonlinearities (XKNLs). The multi-photon DQFT scheme can be achieved by operating the controlled path and merging path gates that are formed with weak XKNLs and linear optical devices. To enhance feasibility under the decoherence effect, in practice, we utilize a displacement operator and photon-number-resolving measurement in the optical gate using XKNLs. Consequently, when there is a strong amplitude of the coherent state, we demonstrate that it is possible to experimentally implement the DQFT scheme, utilizing current technology, with a certain probability of success under the decoherence effect.     

Exact master equation and non-markovian decoherence for quantum dot quantum computing

In this article, we report the recent progress on decoherence dynamics of electrons in quantum dot quantum computing systems using the exact master equation we derived recently based on the Feynman–Vernon influence functional approach. The exact master equation is valid for general nanostructure systems coupled to multi-reservoirs with arbitrary spectral densities, temperatures and biases. We take the double quantum dot charge qubit system as a specific example, and discuss in details the decoherence dynamics of the charge qubit under coherence controls. The decoherence dynamics risen from the entanglement between the system and the environment is mainly non-Markovian. We further discuss the decoherence of the double-dot charge qubit induced by quantum point contact (QPC) measurement where the master equation is re-derived using the Keldysh non-equilibrium Green function technique due to the non-linear coupling between the charge qubit and the QPC. The non-Markovian decoherence dynamics in the measurement processes is extensively discussed as well.     

Protection of quantum correlations against decoherence

The protection of different quantum correlations, such as Bell nonlocality, quantum discord, and geometric quantum discord as trace distance against noise, is explored. By weak measurement and quantum measurement reversal, we show that the mentioned quantum correlations can be effectively preserved probabilistically from the decoherence due to amplitude damping. The results will play an important role in the experiments using the quantum correlations as resource.