数态光场与3个两能级原子相互作用系统的保真度

根据量子信息保真度理论,研究了数态光场与初始处于GHZ态的3个两能级原子相互作用系统中量子态保真度的演变规律.对系统中原子和场取迹后不同量子态保真度的变化规律和场的初始光子数对系统中不同量子态保真度的影响进行了分析与研究.结果表明,初始光子数越大,量子态保真度随时间振荡频率越大.     

薛定谔猫态光场与多光子跃迁运动二能级原子相互作用系统的保真度

利用全量子理论,研究了薛定谔猫态光场与多光子跃迁运动二能级原子相互作用系统的保真度.分别讨论了初始光强、原子运动速度、跃迁光子数以及薛定谔猫态中两宏观相干态的相位角等物理参量对系统保真度的影响.结果表明:随着初始光强的增大,量子态保真效果越来越差;当初始光强增大到一定程度时,偶相干态和奇相干态的保真度与时间演化特性相同;当跃迁光子数为奇数时,原子运动速度的增大使量子态保真效果更好.     

Tavis-Cummings模型中的量子态平均保真度

运用全量子理论,研究了Tavis-Cummings模型中量子态平均保真度的演化特性,对原子和光场的初态以及两原子的关联程度对平均保真度的影响进行了研究.结果表明,原子、光场和系统的平均保真度依赖于初态时2个原子处在基态的几率以及光场处于真空态的几率;初态两原子在基态无关联,光场处于真空态,体系不失真.     

实现量子信息传输的一种新方案

就多粒子(N>3)构成的自旋链传输单态信息进行了研究,提出了实现量子信息传输的一种新方案.采取只考虑近邻相互作用的经过修正的Heisenberg XX模型,当λt较小时,可以把演化算符作一个简单的泰勒展开,然后根据演化算符的具体作用来决定展开项中指数m只能取N-1,就可以得到自旋链上的一端单信息量子态演化到另一端的概率幅的具体形式:|F(t)|=|(-iλt/2)|N-1=1,那么只要选取适当的时间t,就使得单态量子信息在此自旋链上得以完备传输.     

非马尔科夫环境中海森堡XYZ自旋链的隐形传态

利用量子隐形传态方案研究两比特各向异性海森堡XYZ模型隐形传态保真度随时间的演化过程.以最大纠缠态■作为信道，通过量子态扩散方法求解非马尔科夫主方程，从而获得系统的约化密度矩阵；再通过标准的隐形传态方案计算海森堡XYZ自旋链的平均保真度.经过研究发现：当环境关联系数γ较小时，平均保真度明显地呈现出上升的趋势，这表明非马尔科夫环境能够增加平均保真度；同时较大的自旋耦合系数J和余弦磁场强度B也能够增加平均保真度.     

含能流的各向同性XY链中的基态纠缠

分析了含能流的横磁场三比特各向同性XY自旋链的基态纠缠.发现随着外磁场的变化,系统会发生量子相变.以三比特系统为例给出有效哈密顿量的能谱,讨论了体系基态纠缠随λ的变化.当λ＜3/3时,在相变点处基态由W态跳跃到非纠缠态,此时自旋链中没有能量流动.当3/3≤λ≤3时,在能级交错点处发生量子相变,体系由非能流相进入能流相,同时基态由W态ф1跳跃到另一W态ф5;此时,由于能流的影响,使得基态保持在W态,而不再处于非纠缠态;并且随着λ的增大,系统将在更大范围内处于能流相.当λ＞3时,基态波函数表示为ф5,系统完全处于能流相.     

数态光场与2原子相互作用系统保真度的调控

利用数值计算的方法研究了数态光场与初始处于激发态的2个两能级原子相互作用系统的保真度,讨论了光场的失谐量、光场的光子数和对原子操作后对系统保真度的影响.结果表明,不对原子进行操作,初始时刻信息完全不失真,并且可以通过控制场和原子的失谐量和光场的光子数来操控系统保真度的振荡频率、峰值和周期性;对原子进行操作,初始时刻信息就出现失真现象,而且可以通过操作经典场来实现对系统保真度的调控.     

高阶相干光子扣除超叠加改善量子传输保真度

利用光子扣除算符构成的相干光子超叠加操作作用于2个模压缩真空态,引入一类非高斯量子态——高阶相干光子扣除超叠加单模压缩真空态.通过导出该量子态的正规乘积形式,推导了该量子态的归一化系数,与Legendre多项式密切相关.利用相干态的超完备关系和平移算符的正规乘积,导出了态的特征函数的相干态表示,进而解析导出了利用该量子态作为纠缠源实现相干态隐形传输的保真度.研究结果表明:当压缩参数相等时,保真度可获得最大值,对于奇数阶操作,保真度比无叠价操作情况更差,而对于偶数阶情况,在一定的压缩参数范围内,保真度优于无纠缠情况,且保真度可优于经典的最大保真度1/2,同时保真度可以随m的增加而改善.     

LaSrCoO_4 的基态研究

通过运用实空间的Recursion方法和Hartree -Fock近似 ,发现层状钙钛矿LaSr CoO4有三个可能的基态。当晶体场强度比较小 ,系统处于反铁磁高自旋态 ;随着晶体场强度10Dq的变大 ,系统处于铁磁高低自旋混合态 ;当晶体场强度继续增大时 ,系统处于无磁的低自旋态。根据LaSrCoO4在低温下的磁矩大约是 2 .6 μB ,推断LaSrCoO4基态应该是铁磁高低混合自旋态 ,这和光电导谱实验的结果是一致的     

一维量子点阵列自旋链上多比特态的信息传输

利用演化算符作泰勒展开,作用于初态的信息上;使用湮灭算符和其所产生的算符,作用在单比特态上,使一维量子点阵列自旋链上的多比特态量子信息实现完美传输;通过计算初态演化到末态的概率幅,使保真度达到1时实现信息完美传输．结果表明:当相邻量子点之间的交换相互作用和自旋链长度都相同时,保真度传输的比特数越大,传输时间越短．      

Optical pumping of a single hole spin in a quantum dot

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon.     

Quantum annealing with manufactured spins

Many interesting but practically intractable problems can be reduced to that of finding the ground state of a system of interacting spins; however, finding such a ground state remains computationally difficult. It is believed that the ground state of some naturally occurring spin systems can be effectively attained through a process called quantum annealing. If it could be harnessed, quantum annealing might improve on known methods for solving certain types of problem. However, physical investigation of quantum annealing has been largely confined to microscopic spins in condensed-matter systems. Here we use quantum annealing to find the ground state of an artificial Ising spin system comprising an array of eight superconducting flux quantum bits with programmable spin-spin couplings. We observe a clear signature of quantum annealing, distinguishable from classical thermal annealing through the temperature dependence of the time at which the system dynamics freezes. Our implementation can be configured in situ to realize a wide variety of different spin networks, each of which can be monitored as it moves towards a low-energy configuration. This programmable artificial spin network bridges the gap between the theoretical study of ideal isolated spin networks and the experimental investigation of bulk magnetic samples. Moreover, with an increased number of spins, such a system may provide a practical physical means to implement a quantum algorithm, possibly allowing more-effective approaches to solving certain classes of hard combinatorial optimization problems.     

一种成功概率几乎完美的基于相干纠缠态的隐形传态方案

提出以四模纠缠相干态作为量子信道,待传的量子信息为三模相干叠加态,系统组成的系综态经过一个由分束器和移相器组成的线性光学系统,然后通过奇偶态测量,并把测量结果通过经典信道告诉对方,对方选择合适的幺正变换就可以恢复待传的量子信息,通过计算,该方案所得到的成功传态概率几乎完美,同时保真度也趋近于1.     

考虑Rashba效应自组织量子点中极化子性质

在对半导体量子点的研究中考虑自旋一轨道相互作用对极化子基态能量的影响．采用LLP变分的方法研究了电子一声子相互作用．结果表明声子对极化子基态能量起了很重要的作用,而且由于极化子分裂能对极化子基态能量的贡献很大,故在量子点中研究极化子性质时不可忽略极化子分裂能的影响．自旋分裂能随动量增加呈抛物线型增加．随Rashba自旋轨道耦合常数的增加极化子基态能量表现为增加和减少两种截然相反的情况,而两个分裂态中自旋向下的能态更稳定．Rashba效应不可忽略．     

横磁场中各向同性XY链的基态纠缠研究

分析了横磁场中各向同性XY自旋链的基态能量和纠缠问题。研究发现,三量子比特系统中存在一个相变点,此点上,基态能量和纠缠可发生量子相变,基态从W态进入非纠缠;而四量子比特系统存在两个相变点,基态的能量和纠缠均可在相变点处发生量子相变,使纠缠性质发生改变。随着磁场强度的增大,基态纠缠逐渐减小,直到完全消失。四比特系统纠缠的减小要比三比特系统纠缠减小的速度缓慢。     

Storage and retrieval of single photons transmitted between remote quantum memories

An elementary quantum network operation involves storing a qubit state in an atomic quantum memory node, and then retrieving and transporting the information through a single photon excitation to a remote quantum memory node for further storage or analysis. Implementations of quantum network operations are thus conditioned on the ability to realize matter-to-light and/or light-to-matter quantum state mappings. Here we report the generation, transmission, storage and retrieval of single quanta using two remote atomic ensembles. A single photon is generated from a cold atomic ensemble at one site , and is directed to another site through 100 metres of optical fibre. The photon is then converted into a single collective atomic excitation using a dark-state polariton approach. After a programmable storage time, the atomic excitation is converted back into a single photon. This is demonstrated experimentally, for a storage time of 0.5 microseconds, by measurement of an anti-correlation parameter. Storage times exceeding ten microseconds are observed by intensity cross-correlation measurements. This storage period is two orders of magnitude longer than the time required to achieve conversion between photonic and atomic quanta. The controlled transfer of single quanta between remote quantum memories constitutes an important step towards distributed quantum networks.     

不同类型连续变量量子态离物传送的判据

文章利用量子力学的测量理论 ,分析了不同类型连续变量量子态在离物传送过程中的态演化 ,并推导出输出态的保真度表示式。结果表明对不同类型的输入态 ,其保真度和保真度的非经典边界有不同的形式。和相干态比较 ,当输入态为非经典量子态时 ,为达到同样的保真度必须应用具有高关联度的量子纠缠态光场     

3粒子GHZ态的量子远程克隆

提出了一个能实现3粒子GHZ态1→2的量子远程克隆方案:运用2个4粒子纠缠态作为量子信道,通过发送者的2次Bell测量、Hadamard变换、单粒子测量及经典通信;2个接收者进行相应的幺正变换、引入附加粒子和通过Toffoli门,就可以得到原未知态的近似拷贝,此方案的保真度与输入态有关.另外,还推广了一种实现N粒子GHZ态1→2的量子远程克隆方案.     

使用QED技术制备簇态和实现量子信息的传输

提出了一种在腔QED中制备纠缠态并进行量子信息传输的协议,该协议指出量子信息的传输可以在制备纠缠态之后通过纠缠态的分配来实现量子信息的传输．也可以在制备纠缠态的过程中实现量子信息在原子间的直接交换。