Resource-Efficient Direct Entanglement Measurement of Werner State with Hybrid Spin-Photon Interaction System

We propose two resource-efficient schemes of direct entanglement measurement of two-qubit Werner states via hybrid interaction system with nitrogen-vacancy (NV) center coupled to micro-cavity. Based on the unconventional encoding mode on auxiliary qubits, our physical unit can realize the hybrid controlled phase gate and controlled-NOT gate between spin and polarization qubits. Utilizing only one copy of initial entangled state, we implement direct concurrence measurement of spin Werner states in NV centers and polarization Werner states of single photons. Both schemes can be transformed into remote ones with the initial entangled states possessed by spatially separated participants. Experimental feasibility analyses indicate that the presented schemes have reliable performance in the current available experimental conditions.

     

Direct measurement of the concurrence for two-qubit electron spin entangled pure state based on charge detection

We propose a protocol for directly measuring the concurrence of a two-qubit electronic pure entangled state. To complete this task, we first design a parity-check measurement(PCM) which is constructed by two polarization beam splitters(PBSs) and a charge detector. By using the PCM for three rounds, we can achieve the concurrence by calculating the total probability of picking up the odd parity states from the initial states. Since the conduction electron may be a good candidate for the realization of quantum computation, this protocol may be useful in future solid quantum computation.     

Remote Preparation of an Arbitrary Two-Qubit State with Three-Party

In this paper, we propose a scheme to remotely prepare an arbitrary two-qubit state from one sender to either of two receivers. Two cases of the prepared quantum state, an arbitrary two-qubit state with real coefficients and complex coefficients, are considered. Firstly, one single EPR pair and a GHZ state are used as the quantum channel. Then the present scheme is extended to some partially entangled states as the quantum channel. To design these schemes, some useful and general measurement bases have been constructed. The successful probability and classical communication cost of these schemes are also calculated to weigh the efficiency and cost.     

Entanglement dynamics of two interacting qubits under the influence of local dissipation

We investigate the dynamics of entanglement given by the concurrence of a two-qubit system in the non-Markovian setting. A quantum master equation is derived, which is solved in the eigenbasis of the system Hamiltonian for X-type initial states. A closed formula for time evolution of concurrence is presented for a pure state. It is shown that under the influence of dissipation non-zero entanglement is created in unentangled two-qubit states which decay in the same way as pure entangled states. We also show that under real circumstances, the decay rate of concurrence is strongly modified by the non-Markovianity of the evolution.     

Quantum state sharing with a genuinely entangled five-qubit state and Bell-state measurements

We construct several distinct schemes for tripartite Quantum state sharing (QSTS) of arbitrary single- and two-qubit states. Our schemes use genuinely entangled five-qubit state that has recently been introduced by Brown et al. [J. Phys. A 38 1119 (2005)] as the quantum channel. The Bell-state measurements and the single-qubit measurement are needed in our schemes. In comparison with the QSTS scheme using the same quantum channel [Phys. Rev. A 77 (2008) 032321], not joint measurement, which makes this scheme simpler than the latter.     

单光子纠缠态并发度的直接测量

通过分析光学分束器对单光子态的作用关系,提出了一个利用分束器和光子数探测器的单光子纠缠的直接测量方案.方案中用到单光子与空间模纠缠及其两个备份,并让它们通过一个50:50的分束器.选用并发度为纠缠度量,其可由单光子探测器的探测概率直接获得.此方案不需复杂的量子态层析方法,同时只用到在量子信息处理中常用的光学器件,增强了方案在实验上实现的可行性.     

Schemes for Splitting Quantum Information via Tripartite Entangled States

In this paper we propose two schemes for quantum information splitting via tripartite entangled states. Explicit protocols for the quantum information splitting of a single qubit state and an arbitrary two-qubit entangled state are illustrated. We also consider the security against certain eavesdropping attacks. Moreover, a generalization of the scheme to multi-particle case is also outlined.     

Quantum Information Splitting by Using Asymmetric Multi-particle State

We propose schemes for quantum information splitting by using asymmetric multi-particle entangled state. Explicit protocols for the quantum information splitting of a single-qubit state and a two-qubit entangled state via asymmetric three-particle entangled state are illustrated. The four-particle asymmetric entangled state are also used as quantum channel to split the unknown two-qubit entangled state. We also consider the security against certain eavesdropping attacks.     

Quantum logic networks for probabilistic teleportation

By eans of the primitive operations consisting of single-qubit gates.two-qubit controlled-not gates,Von Neuman measurement and classically controlled operations.,we construct efficient quantum logic networks for implementing probabilistic teleportation of a single qubit,a two-particle entangled state,and an N-particle entanglement.Based on the quantum networks,we show that after the partially entangled states are concentrated into maximal entanglement,the above three kinds of probabilistic teleportation are the same as the standard teleportation using the corresponding maximally entangled states as the quantum channels.     

Simple all-microwave entangling gate for fixed-frequency superconducting qubits

We demonstrate an all-microwave two-qubit gate on superconducting qubits which are fixed in frequency at optimal bias points. The gate requires no additional subcircuitry and is tunable via the amplitude of microwave irradiation on one qubit at the transition frequency of the other. We use the gate to generate entangled states with a maximal extracted concurrence of 0.88, and quantum process tomography reveals a gate fidelity of 81%.     

Concurrence, tangle and fully entangled fraction

We show that although we cannot distil a singlet from many pairs of bound entangled states, the concurrence and the tangle of two entangled quantum states are always strictly larger than those of one of them, even both entangled quantum states are bound entangled. We present a relation between the concurrence and the fidelity of optimal teleportation. We also give new upper and lower bounds for concurrence and tangle.     

Deterministic Joint Assisted Cloning of Unknown Two-Qubit Entangled States

We present two schemes for perfect cloning unknown two-qubit and general two-qubit entangled states with assistance from two state preparers, respectively. In the schemes, the sender wish to teleport an unknown two-qubit (or general two-qubit) entangled state which from two state preparers to a remote receiver, and then create a perfect copy of the unknown state at her place. The schemes include two stages. The first stage of the schemes requires usual teleportation. In the second stage, to help the sender realize the quantum cloning, two state preparers perform two-qubit projective measurements on their own qubits which from the sender, then the sender can acquire a perfect copy of the unknown state. To complete the assisted cloning schemes, several novel sets of mutually orthogonal basis vectors are introduced. It is shown that, only if two state preparers collaborate with each other, and perform projective measurements under suitable measuring basis on their own qubit respectively, the sender can create a copy of the unknown state by means of some appropriate unitary operations. The advantage of the present schemes is that the total success probability for assisted cloning a perfect copy of the unknown state can reach 1.     

Standard Teleportation of One-Qubit State and Partial Teleportation of Two-Qubit State via X-States

In this article,we derive the explicit entanglement and fidelity expressions for a larger class of two-qubit states,namely,a seven-parameter family of so called X-states.The analytical expressions of the entanglement,the output entanglement and the average fidelity are obtained for this general model by using the concept of concurrence and average fidelity.We study the relations between quantum entanglement,the output entanglement,and the average fidelity for standard teleportation of one-qubit and partial teleportation of two-qubit state.We discover that the average fidelity and entanglement is not a simple dependence relation.The higher entangled system is helpful for teleportation.     

Splitting Unknown Qutrit or Ququart States via Two-Qubit Partially Entangled States Channel

In this paper we propose two quantum information splitting (QIS) schemes respectively for splitting an unknown single-qutrit and single-ququart state via three pairs of two-qubit partially entangled state. The necessary measurements and operations are given detailedly and the success probabilities are worked out. The two schemes can be directly generalized to QIS of multi-qutrit and multi-ququart states by using Bell-states channel.     

Robustness of Entanglement During Decoherence

Finding the most robust entangled states during the whole process of decoherence is a particularly fundamental problem for quantum physics and quantum information processing. In this paper, the decoherence process of two-qubit system under two individual identical decoherence channels is investigated systematically. We find that although the robustness of two-qubit states with same initial entanglement is usually different, the Bell-like states are always the most robust entangled states during decoherence. That is to say, affected by the same amount of noise, the remain entanglement of an arbitrary two-qubit state is not more than that of a Bell-like state with the same initial entanglement.     

Quantum entanglement and quantum operation

It is a simple introduction to quantum entanglement and quantum operations. The authors focus on some applications of quantum entanglement and relations between two-qubit entangled states and unitary operations. It includes remote state preparation by using any pure entangled states, nonlocal operation implementation using entangled states, entanglement capacity of two-qubit gates and two-qubit gates construction. Supported by the National Fundamental Research Program of China (Grant No. 2001CB309306), the National Natural Science Foundation of China (Grant Nos. 60621064 and 10674127) and the Innovation Funds from Chinese Academy of Sciences     

Mutual Restriction between Concurrence and Intrinsic Concurrence for Arbitrary Two-Qubit States

Concurrence is viewed as the most commonly approach for quantifying entanglement of two-qubit states, while intrinsic concurrence contains concurrence of four pure states consisting of a special pure state ensemble concerning an arbitrary two-qubit state. Thus, a natural question arises: Whether there is a specified relation between them.We firstly examine the relation between concurrence and intrinsic concurrence for the maximally nonlocal mixed state under a special unitary operation, which is not yet rigorously proved. In order to obtain a general result, we investigate the relation between concurrence and intrinsic concurrence using randomly generated two-qubit states,and derive an inequality relation between them. Finally, we take into account the relation between concurrence and intrinsic concurrence in open systems, and reveal the ratio of the two quantum resources, which is only correlated with the experiencing channels.     

两量子比特非线性相互作用模型中的量子关联

运用量子态变换的方法论证了两个qubit纯态中量子关联与纠缠的等价性.并利用三种带有横场的非线性相互作用模型研究了两个qubit体系中的量子关联.发现合适的横场对于最大量子关联态的获得、平均量子关联的提高都有着积极的作用.两个qubit体系获得最大量子关联时,不同模型,不同的横场,对应的量子态却各不相同.     

Quantum Teleportation and Quantum Information Splitting by?Using?a?Genuinely?Entangled Six-Qubit State

A new application of the genuinely entangled six-qubit state introduced recently by Tapiador et al. (J. Phys. A 42:415301, 2009) is investigated for the quantum teleportation of an arbitrary three-qubit state and for quantum information splitting (QIS) of an arbitrary two-qubit state. For QIS, we have shown that it can be completed perfectly with two distinct measurement methods. In our scheme, the joint Bell-state measurement and the joint multi-qubit measurement are needed. This quantum teleportation and QIS schemes are deterministic.     

多粒子纠缠的保护方案

量子纠缠是量子信息的重要物理资源. 然而当量子系统与环境相互作用时, 会不可避免地产生消相干导致纠缠下降, 因此保护纠缠不受环境的影响具有重要意义. 振幅衰减是一种典型的衰减机制. 如果探测环境保证没有激发从系统中流出, 即视为对系统的一种弱测量. 本文基于局域脉冲序列和弱测量, 提出了一种可以保护多粒子纠缠不受振幅衰减影响的有效物理方案, 保护的对象是在量子通信和量子计算中发挥重要作用的Cluster态和Maximal slice态.