Bound entanglement and continuous variables

We introduce the definition of generic bound entanglement for the case of continuous variables. We provide some examples of bound entangled states for that case, and discuss their physical sense in the context of quantum optics. We raise the question of whether the entanglement of these states is generic. As a by-product we obtain a new many parameter family of bound entangled states with positive partial transpose. We also point out that the "entanglement witnesses" and positive maps revealing the corresponding bound entanglement can easily be constructed.     

Generation of remote W-type entangled state via tripartite entanglement swapping of continuous variables

We propose a scheme for generating remote W-type entangled state via tripartite entanglement swapping of continuous variables, where two EPR pairs and a local W-type entangled state are required. Because of the co-existence of both bipartite and tripartite entanglement in a W-type entangled state, the three involved remote regions, without direct interaction, will become entangled after the prescribed entanglement swapping.     

Experimental demonstration of unconditional entanglement swapping for continuous variables

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.     

Multipartite entanglement for continuous variables: A quantum teleportation network

We show that one single-mode squeezed state distributed among N parties using linear optics suffices to produce a truly N-partite entangled state for any nonzero squeezing and arbitrarily many parties. From this N-partite entangled state, via quadrature measurements of N-2 modes, bipartite entanglement between any two of the N parties can be "distilled," which enables quantum teleportation with an experimentally determinable fidelity better than could be achieved in any classical scheme.     

连续变量无条件纠缠交换 --纠缠态的量子离物传送

利用自行设计的两台非简并光学参量放大器，获得了一对具有经典相干性且量子起伏相互独立的连续变量纠缠态光场，并用它完成了连续变量的无条件纠缠交换，即纠缠态的量子离物传送．通过联合贝尔态探测与纠缠塌缩，使两个初始不纠缠而又从未发生过直接相互作用的光场产生了量子纠缠，其正交振幅和位相分量的量子起伏关联方差被直接测量，其测量值分别低于散离噪声极限1．23dB和1．12dB．理论计算与实验结果基本符合．     

Bidirectional quantum teleportation of unknown photons using path-polarization intra-particle hybrid entanglement and controlled-unitary gates via cross-Kerr nonlinearity

We propose an arbitrary controlled-unitary(CU) gate and a bidirectional quantum teleportation(BQTP) scheme. The proposed CU gate utilizes photonic qubits(photons) with cross-Kerr nonlinearities(XKNLs), X-homodyne detectors, and linear optical elements, and consists of the consecutive operation of a controlled-path(C-path) gate and a gathering-path(Gpath) gate. It is almost deterministic and feasible with current technology when a strong coherent state and weak XKNLs are employed. Based on the CU gate, we present a BQTP scheme that simultaneously teleports two unknown photons between distant users by transmitting only one photon in a path-polarization intra-particle hybrid entangled state. Consequently, it is possible to experimentally implement BQTP with a certain success probability using the proposed CU gate.     

利用交叉克尔非线性效应实现纠缠转移

提出了一种基于交叉克尔非线性效应的纠缠态转移方案.利用该方案可以将离散变量光场态之间的纠缠关系转移到连续变量光场态(相干态)上.通过适当设置初始相干态的振幅,该方案可以使转移后的纠缠相干态处于最大纠缠态. 关键词： 交叉克尔效应 纠缠转移 纠缠相干态     

Generation of entanglement molecules via weak coherent field in cavity QED

This paper presents a method for generating entanglement molecules, which is introduced by Dur (2001 Phys. Rev. A). In this scheme, N ladder-type three-level atoms are sent through a resonant weak coherent cavity field, then the system states are measured. And the system field may collapse onto some possible types of entanglement molecules. Meanwhile it discusses about the interaction time from the experimental point of view, and compare the result with the previous scheme proposed by Huang (2004 J. Phys. B: At. Mol. Opt. Phys.).     

Experimental demonstration of tripartite entanglement and controlled dense coding for continuous variables

A tripartite entangled state of bright optical field is experimentally produced using an Einstein-Podolsky-Rosen entangled state for continuous variables and linear optics. The controlled dense coding among a sender, a receiver, and a controller is demonstrated by exploiting the tripartite entanglement. The obtained three-mode "position" correlation and relative "momentum" correlation between the sender and the receiver, and thus the improvements of the measured signal to noise ratios of amplitude and phase signals with respect to the shot noise limit are 3.28 and 3.18 dB, respectively. If the mean photon number n equals 11 the channel capacity can be controllably inverted between 2.91 and 3.14. When n is larger than 1.0 and 10.52, the channel capacity of the controlled dense coding is predicted to exceed the ideal single channel capacity of coherent and squeezed state light communication, respectively.     

Strong causality from weak via continuous monitoring

Repeated unbiased measurements cause a continual application of the weak causality principle, leading to an apparent arrow of time for continuously-monitored quantum systems.     

Generation of pair coherent state using weak cross-Kerr media

We firstly give a nonlocal method for generating pair coherent state with two traveling wave fields in distinct districts. The experimental scheme proposed is based on a two-mode photon number matching process, which employs weak cross-Kerr media and on/off detection. Then we discuss the details for implementing this scheme, showing that it is robust against the low quantum efficiency of photon detectors and offers nearly perfect pair coherent states. Finally, we show how a two-mode Schrödinger cat state and a generalized two-mode correlated photon number state can be prepared via this matching process.     

利用弱交叉科尔效应实现多光子任意高维空间纠缠态的确定性制备

提出两种实现多光子高维空间纠缠态的制备方案.首先给出一种基于后验选择技术的实现方案,此方案以一定的概率实现.然后以基于弱交叉科尔效应的控制路径(C-path)门为基础,给出任意双光子任意维度的空间纠缠态的制备方案,并将此方案推广到任意多光子任意维度的空间纠缠态的制备.这一制备方案的最大优点在于可以确定性的得到纠缠态,并且不需要复杂的后验选择技术,可以很方便的应用于量子信息过程,同时该方案在目前的实验条件下是可行的.     

High-fidelity teleportation beyond the no-cloning limit and entanglement swapping for continuous variables

We experimentally demonstrate continuous-variable quantum teleportation beyond the no-cloning limit. We teleport a coherent state and achieve the fidelity of 0.70 +/- 0.02 that surpasses the no-cloning limit of 2/3. Surpassing the limit is necessary to transfer the nonclassicality of an input quantum state. By using our high-fidelity teleporter, we demonstrate entanglement swapping, namely, teleportation of quantum entanglement, as an example of transfer of nonclassicality.     

用弱交叉Kerr非线性实现光子极化纠缠Bell态的完全区分

基于相干态和信号位的光子数态与弱交叉Kerr非线性相互作用后,会在相干态上产生相位变化,并结合极化分束器构造了一个奇偶校验测量装置. 用零差探测器对相干态的相位变化进行测量,实现对Bell态的非破坏区分. 再利用控制非门和斜置的极化分束器对两信号位光子进行控制非操作和单光子测量,完成对四个Bell态的完全区分. 用到的弱交叉Kerr非线性增加了区分方案在实验上实现的可行性.     

用弱交叉Kerr非线性实现光子极化纠缠Bell态的完全区分

基于相干态和信号位的光子数态与弱交叉Kerr非线性相互作用后,会在相干态上产生相位变化,并结合极化分束器构造了一个奇偶校验测量装置.用零差探测器对相干态的相位变化进行测量,实现对Bell态的非破坏区分.再利用控制非门和斜置的极化分束器对两信号位光子进行控制非操作和单光子测量,完成对四个Bell态的完全区分.用到的弱交叉Kerr非线性增加了区分方案在实验上实现的可行性.     

Robust schemes for the generation of multipartite entanglement for remote atoms with cross-Kerr nonlinearity

Based on the interference effect of indistinguishable polarized photons leaking out of separated cavities with each atom trapped in separate cavity, using quantum nondemolition detection, we propose the robust schemes for the generation of N-atom GHZ state, three-atom W state and four-atom cluster state with a certain success probability. In Lamb-Dicke limit, the schemes do not require the simultaneous click of the photon-detectors. These made the schemes more realizable in experiments. Meanwhile, the advantage of the scheme is that the fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photodetectors. The schemes would be useful steps towards long-distance quantum communication.     

Detection of two-mode compression and degree of entanglement in continuous variables in parametric scattering of light

Generation of “twin beams” (of light with two-mode compression) in single-pass optical parametric amplifier (a crystal with a nonzero quadratic susceptibility) is considered. Radiation at the output of the nonlinear crystal is essentially multimode, which raises the question about the effect of the detection volume on the extent of suppression of noise from the difference photocurrent of the detectors. In addition, the longitudinal as well as transverse size of the region in which parametric transformation takes place is of fundamental importance. It is shown that maximal suppression of noise from difference photocurrent requires a high degree of entanglement of two-photon light at the outlet of the parametric amplifier, which is defined by Federov et al. [Phys. Rev. A 77, 032336 (2008)] as the ratio of the intensity distribution width to the correlation function width. The detection volume should be chosen taking into account both these quantities. Various modes of single-pass generation of twin beams (noncollinear frequency-degenerate and collinear frequency-nondegenerate synchronism of type I, as well as collinear frequency-degenerate synchronism of type II) are considered in connection with the degree of entanglement.     

利用交叉克尔非线性产生四光子偏振态簇态

提出了一种基于交叉克尔非线性效应产生四光子偏振态簇态的方案,与其他产生四光子簇态的方案比较,该方案引入宇称门的思想和用到基于零差探测的非破坏测量方法,使得该方案在实验上更易于操作及实现.     

Generation of a four-particle entangled state via cross-Kerr nonlinearity

We propose a scheme for generating a genuine four-particle polarisation entangled state ︱χ00 that has many interesting entanglement properties and potential applications in quantum information processing. In our scheme, we use the weak cross-Kerr nonlinear interaction between field-modes and the non-demolition measurement method based on highly efficient homodyne detection, which is feasible under the current experiment conditions.     

Generation of hyperentangled four-photon cluster state via cross-Kerr nonlinearity

We propose a scheme for generating a hyperentangled four-photon cluster state that is simultaneously entangled in polarization modes and spatial modes. This scheme is based on linear optical elements, weak cross-Kerr nonlinearity, and homodyne detection. Therefore, it is feasible with current experimental technology.