Assisted Cloning and Orthogonal Complementing of an Arbitrary Unknown Two-Qubit Entangled State

Based on A.K. Pati＇s original idea [Phys. Rev. A 61 （2000） 022308] on single-qubit-state-assisted clone, very recently Zhan has proposed two assisted quantum cloning protocols of a special class of unknown two-qubit entangled states [Phys. Lett. A 336 （2005） 317]. In this paper we further generalize Zhan＇s protocols such that an arbitrary unknown two-qubit entangled state can be treated.     

Simple experimental scheme of teleporting a two-qubit state via linear optical elements

We propose a simple experimental scheme in which an unknown two-qubit state is faithfully and deterministically teleported from Alice to Bob. The scheme is constructed with four photons from parametric down conversion, linear optical elements, and conventional photon detectors, all of which are available in current technology. It is shown that the probability of successful teleportation ideally reaches 100% based on single-photon two-qubit-assisted Bell-state measurement, which can distinguish all four Bell-states simultaneously via conventional photon detectors. By generalizing the scheme, the teleportation of an unknown multi-qubit system can also be realized.     

Relative entropy of entanglement of two-qubit ’X’ states

Two closest single-qubit states could be diagonalised by the same unitary matrix,which helps to find the relative entropy of entanglement of a two-qubit ’X’ state.We formulate two binary equations for the relative entropy of entanglement and the corresponding closest separable state of a given two-qubit ’X’ state.This approach can be applied to get the relative entropy of entanglement of many widely-discussed two-qubit states,such as pure states,Werner states,and so on.     

Geometric global quantum discord of two-qubit states

We consider the geometric global quantum discord(GGQD) of two-qubit systems. By analyzing the symmetry of geometric global quantum discord we give an approach for deriving analytical formulae of the extremum problem which lies at the core of computing the GGQD for arbitrary two-qubit states. Furthermore, formulae of GGQD of arbitrary two-qubit states and some concrete examples are presented.     

Information under Lorentz Transformation

In the context of quantum information, we investigate extensively some important classes of a general form of a two-qubit system under Lorentz transformation. It is shown Lorentz transformation causes a decay of entanglement and consequently information loses. On the other hand, it generates entangled states between systems prepared initially in a separable states. The partial entangled states are more robust under Lorentz transformation than maximally entangled states. Therefore the rate of information lose is larger for maximum entangled states compared with that for partially entangled states.     

Local Information as an Essential Factor for Quantum Entanglement

Quantum entanglement is not only a fundamental concept in quantum mechanics but also a special resource for many important quantum information processing tasks. An intuitive way to understand quantum entanglement is to analyze its geometric parameters which include local parameters and correlation parameters. The correlation parameters have been extensively studied while the role of local parameters have not been drawn attention. In this paper, we investigate the question how local parameters of a two-qubit system affect quantum entanglement in both quantitative and qualitative perspective. Firstly, we find that the concurrence, a measure of quantum entanglement, of a general two-qubit state is bounded by the norms of local vectors and correlations matrix. Then, we derive a sufficient condition for a two-qubit being separable in perspective of local parameters. Finally, we find that different local parameters could make a state with fixed correlation matrix separable, entangled or even more qualitatively entangled than the one with vanished local parameters.     

Nondestructive discrimination of Greenberger-Horne-Zeilinger-basis states via two-qubit parity detection

We propose new methods to construct universal Greenberger-Horne-Zeilinger (GHZ)-state analyzers without destroying the qubits by using two-qubit parity gates. The idea can be applied to any physical systems where the two-qubit parity gate can be realized. We also investigate the feasibility of nondestructively distinguishing the GHZ-basis states for photonic qubits with such an idea. The nondestructive GHZ-state analyzers can act as generators of GHZ entangled states and are expected to find useful applications for resource-saving quantum information processing.     

Quantum Teleportation Through a Two-Qubit Heisenberg XXZ Chain

We consider a two-qubit Heisenberg XXZ chain as a resource for quantum teleportation via the standard teleportation protocol To. The effects of anisotropic on teleportation fidelity and entanglement are studied in detail. We find anisotropic not only improves the criticM temperature Tc and criticM magnetic field Bc, beyond which quantum teleportation is inferior to classicM communication protocol, but also enhances the fidelity for fixed magnetic field B and temperature T. For entanglement teleportation, the effects of magnetic field on average fidelity and output entanglement are studied.     

用多粒子态进行量子信息分配

我们提出一个用五粒子态将量子信息分裂成几部分的方案.在这个过程中,如果有且只有接受者相互合作才能重建初始比特的态.我们所用的量子通道是Bell态和三比特GHZ态的组合态,这些态在实验上都是很容易制备的.这个方案对某些窃听攻击来说是安全的.而且,它还可以用来实现控制的量子隐形传态.     

Remark on the One-Way Quantum Deficit for General Two-Qubit States

We reinvestigate the one-way quantum deficit,which is a measure of quantum correlation emerging from a thermodynamical approach.We give a tight upper bound of the one-way quantum deficit for general mixed states,and give a sufficient condition for this bound.Finally,we discuss a universal way to evaluate the one-way quantum deficit for general two-qubit states.     

用多粒子态进行量子信息分配

我们提出一个用五粒子态将量子信息分裂成几部分的方案。在这个过程中,如果且只有接受者相互合作才能重建初始比特的态。我们所用的量子通道是Bell太和三比特GHZ态的组合态,这些态在实验上都是很容易制备的。这个方案对某些窃听攻击来说是安全的。而且,它还可以用来实现控制的量子隐形传态。     

Remote preparation of an entangled two-qubit state with three parties

We present a scheme for probabilistic remote preparation of an entangled two-qubit state with three parties from a sender to either of two receivers. The quantum channel is composed of a paxtially entangled two-qubit state and a partially entangled three-qubit state. We calculate the successful total probabilities of the scheme in general and particular cases, respectively. We also calculate total classical communication cost in a general case and two particular cases, respectively.     

Entanglement capacity of two-qubit unitary operator for rank two mixed states

The entanglement capacity of two-qubit unitary operator acting on rank two mixed states in concurrence is discussed. The condition of perfect entangler is the same as that acting on pure states and the entanglement capacity is the mixing parameter v1. For non-perfect entangler, the upper and lower bound of the entanglement capacity are given.     

Remote preparation of an entangled two-qubit state with three parties

We present a scheme for probabilistic remote preparation of an entangled two-qubit state with three parties from a sender to either of two receivers. The quantum channel is composed of a partially entangled two-qubit state and a partially entangled three-qubit state. We calculate the successful total probabilities of the scheme in general and particular cases, respectively. We also calculate total classical communication cost in a general case and two particular cases, respectively.     

Two-Qubit Steerability,Nonlocality, and Average Steered Coherence under Classical Dephasing Channels

When two qubits are prepared in a mixture of two Bell states and exposed to local transmission channels, the dynamics of steerability, Bell nonlocality, and average steered coherence are investigated. Disorders are assumed to influence the channels, resulting in either Markovian Ornstein–Uhlenbeck noise or non-Markovian static noise in two models: a single noisy channel or two local noisy channels. Their findings show that the type and number of classical channels, noise, and initial state must be in an optimal setting in order to preserve quantum correlations.     

Increasing Quantum Correlations Based on Measurement-Induced Disturbance via a Swapping Procedure with Two-Qubit Mixed States

In this paper, quantum correlation (QC) swapping for certain separable two-qubit mixed states is treated. A QC quantifier, measurement-induced disturbance (MID) (Luo in Phys Rev A 77:022301, 2008), is employed to characterize and quantify QCs in the relevant states. Properties of all QCs in the swapping process are revealed. Particularly, it is found that MID can be increased through QC swapping for certain separable two-qubit mixed states.     

Bell States and Two-Qubit Logic Gate with Superconducting Charge Qubits Coupling to a Nanomechanical Resonator

We propose a scheme for generating Bell states involving two SQUID-based charge qubits by coupling them to a nanomechanical resonator. We also show that it is possible to implement a two-qubit logic gate between the two charge qubits by choosing carefully the interaction time.     

Assisted Cloning and Orthogonal Complementing of an Arbitrary Unknown Two-qubit Entangled State via Positive Operator-valued Measure

By using positive operator-valued measure (POVM) instead of the more commonly used collective unitary operation in the quantum teleportation process, Zhan’s scheme (Zhan in Phys. Lett. A 336:317, 2005) is generalized to treat an arbitrary (not a special class of) unknown two-qubit entangled state. Generally speaking, conditioned on Victor’s classical message, Alice can create either a copy or an orthogonal-complement copy of the original state. However, it is shown that Alice’s perfect clone can be realized with higher probability or even in a deterministic manner via an appropriate unitary operation provided that the state to be cloned is chosen from six special ensembles.     

Maximal and total skew information for a two-qubit system using nonlinear interaction models

Maximal and total skew information is studied. For symmetric pure states of two-qubit, they are closely related to the linear entropy, the concurrence, and the spin squeezing parameter. For a two-qubit system implemented in three nonlinear interaction models with an external field, we give the exact state vectors and the expectation value 〈S_z〉 at any time t. Based on 〈S_z〉², we give the maximal and the total skew information and a condition in which the maximal and the total skew information can reach 1 and 2, respectively.