A proposal for preparation of cluster states with linear optics

Measurement-based quantum computation in an optical setup shows great promise towards the implementation oflarge-scale quantum computation. The difficulty of measurement-based quantum computation lies in the preparation ofcluster state. In this paper, we propose the method of generating the large-scale cluster state, which is a platform formeasurement-based quantum computation. In order to achieve more complex quantum circuits, the preparation protocolof N-photon cluster state will be proposed as a generalization of the preparation of four- and five-photon cluster states.Furthermore, our proposal is experimentally feasible.     

基于偏振无关分束器的两光子部分纠缠态的简化浓缩方案

经由偏振无关的光学分束器实现对未知两光子部分纠缠态的纠缠浓缩,包括利用单光子探测器的线性光学浓缩方案,和利用弱交叉Kerr非线性媒质、零拍探测的浓缩方案。两对部分纠缠的光子对中各一个信号光子同时通过偏振无关分束器后,当探测到分束器输出模中各有一个光子时,剩余的两光子将处于最大纠缠态。与以前的浓缩方案相比较,我们的浓缩方案不需要额外的Hadmard门对信号光子施加作用,所需要的光学器件较少,这增加了方案在实验实现上的可行性。     

YBCO薄膜中自旋极化准粒子的注入效应

采用Nd0 .7Sr0 .3MnO3/SrTiO3/YBa2 Cu3O7-δ的异质结构 ,研究了自旋极化准粒子的注入效应 .在 5 6 μm宽的YBCO膜条上成功地制备了与超导膜条同样宽度但不同长度的六个注入结区 ,长度L分别为 80 μm ,4 0 μm ,2 0 μm ,10 μm ,5 μm和 2 μm .80nm厚的YBCO薄膜在 16K温度下Jc 为 2× 10 5A/cm2 .Iin=0 .5mA的自旋电流注入下 ,随L从 80 μm逐渐顺次减小时 ,注入效率 η =ΔJc/ΔJin逐渐增大 .而当L≤ 2 0 μm后 ,η不再增加 ,达到几乎相同的值 (～ 6 ) .初步分析认为这与自旋极化准粒子在超导膜内的有效自旋扩散长度有关 .异质结构中YBCO薄膜的超导电性以及注入窗口的尺寸对获得大的自旋注入效率十分重要 .     

Unconventional Geometric Quantum Gate in Circuit QED

We propose a scheme to implement an unconventional geometric phase gate in circuit QED, i.e. two superconducting charge qubits inside a superconducting transmission line resonator. The quantum operation depends only on global geometric features, and thus is insensitive to the state of the cavity mode.     

噪声对一种三粒子量子探针态的影响

量子度量学是研究量子测量与统计推断的一门学科,主要利用量子手段来提高参数估计的精度,在量子信息处理与测量中起到关键作用.量子参数估计的一般过程包含四个步骤:探针态的制备、参数化过程、对参数化后的输出态进行测量以及根据测量结果估计待测参数.其中探针态的选取对测量精度起着至关重要的作用.然而在实际的量子探针态的制备过程中,初始探针态会受到环境噪声的影响.目前人们已经研究了W态与Greenberger-Horne-Zeilinger(GHZ)态的量子Fisher信息(QFI)在典型噪声通道下的变化行为.由于W态与GHZ态有着不同的纠缠性质,对于W态与GHZ态的叠加态的QFI动力学研究具有重要的实际意义.故此,本文主要研究典型噪声通道对这两种状态的叠加态的QFI动力学行为的影响,得出了QFI随噪声参数的变化行为.结果表明,叠加态中W态组分可明显对抗相位阻尼噪声对探针态的QFI的影响,而其中的GHZ态组分可明显对抗振幅阻尼噪声的影响,从而为在实际环境中选取高精度的参数估计过程提供参考.     

Implementation Scheme of Two-Photon Post-Quantum Correlations

The pre-and post-selection processes of the "two-state vector formalism" lead to a fair sampling loophole in Bell test, so it can be used to simulate post-quantum correlations. In this paper, we propose a physical implementation of such a correlation with the help of quantum non-demolition measurement, which is realized via the cross-Kerr nonlinear interaction between the signal photon and a probe coherent beam. The indirect measurement on the polarization state of photon is realized by the direct measurement on the phase shift of the probe coherent beam, which enhances the detection efficiency greatly and leaves the signal photon unabsorbed. The maximal violation of the CHSH inequality 4 can be achieved by pre-and post-selecting maximally entangled states. The reason why we can get the post-quantum correlation is that the selection of the results after measurement opens fair-sampling loophole. The fair-sampling loophole opened here is different from the one usually used in the currently existing simulation schemes for post-quantum correlations,which are simulated by selecting the states to be measured or enlarging the Hilbert space. So, our results present an alternative way to mimic post-quantum correlations.     

Nearly Deterministic Teleportation of a Photonic Qubit with Weak Cross-Kerr Nonlinearities

Scheme for teleporting an unknown single-qubit photonic state is revisited. With the help of quantum nondemolition measurements, the scheme now can be achieved in an almost deterministic way. The weak-cross-Kerr-nonlinearities-based quantum nondemolition measurement acts as an entangler as well as the Bell-state analyzer, and thus plays a similar role as the CNOT gate in the teleportation process. This improvement makes the present scheme more efficient than the schemes using nonunitary projective measurements and feasible with the current experimental technology.     

Preparation of cluster state in large detuned cavity

An experimentally realizable physical scheme for preparing multiqubit cluster states from a large detuned atomcavity interaction is proposed. The scheme is free of any type of measurement and insensitive to the cavity decay, and the cavity field is only virtually excited so that there is no information exchanging between atom and cavity. The time required for the gate operations is very short, which is important for decoherence. We also discuss the experimental feasibility of our scheme.     

频率变化光场与二能级原子相互作用系统的光场的振幅平方压缩效应

在旋波近似下,利用推广的J-C模型,考虑光场的频率随时间按照正弦函数的形式作小量变化,采用数值计算的方法,研究分析二项式光场与二能级原子相互作用时,光场的振幅平方压缩效应。最终的研究结果表明：场频率的变化对光场的振幅平方压缩的深度和周期性都存在影响