Entangled States Used in Remote Information Concentration and Their Properties

We analyse a process of remote information concentration achieved by the W state. The result turns out to be neither as good as performed by the GHZ state nor as by the Smolin bound entangled state. Based on this particular phenomenon, the properties of the three entangled states are realized.     

Remote Preparation of Photon Polarization States within a Network

We propose a new linear optical protocol for remote state preparation （RSP） between two parties under control of a number of controllers in terms of optical elements. The realization of this protocol is appealing due to the fact that the quantum state of light is robust against the decoherence, and photons are ideal carriers for transmitting quantum information over long distances.     

Generation of GHZ States via Deterministic Entanglement Concentration

We present the generation of six-particle Greenberger Horne-Zeilinger （GHZ） states via deterministic entanglement concentration and generalize the scheme to the case of 2N particles. We show that arbitrary 2N-particle GHZ states can be obtained with certain probability via entanglement concentration. This may provide a new perspective for the preparation of multi-particle GHZ states. This study is also an exploration on the theory of deterministic entanglement concentration.     

Quantum teleportation of arbitrary two-qubit state via entangled cavity fields

We propose a new protocol for quantum teleportation of an arbitrary two qubit state via continuous variables entangling channel. In our scheme two pairs of entangled light fields are employed. An outstanding characteristic of this scheme is that arbitrary state of two atoms is transmitted deterministically and directly to another pair of atoms without the help of the other atoms.     

Preparation of multi-atom cluster state and teleportation of arbitrary two-atom state via thermal cavity

We propose one cavity QED (CQED) scheme for generating an arbitrary 2-level-atom cluster state. Besides, by using a 4-atom cluster state as quantum channel, we propose another CQED scheme for teleporting any unknown two-atom state. In both schemes, the dynamics processes are essentially quite similar. The Rabi frequency of the classical driving field is much bigger than the detuning between the atoms and the cavity. Hence both schemes are insensitive to the cavity decay. The necessary time for implementation is much shorter than the Rydberg-atom lifespan, therefore atom decays do not need to be considered. Moreover, in the teleportation scheme the discrimination of the 16 mutually orthogonal 4-atom cluster states is transformed into the discrimination of single-atom product states, consequently the discrimination difficulty is degraded and the scheme is more easily implemented.     

Simple scheme for preparing W states and cloning via adiabatic passage in ion-trap systems

We propose a simple protocol for the generation of W states and the implementation of phase-covariant cloning and anticloning machines via adiabatic passage in ion-trap system. In the present scheme, the system state evolves in the dark space during the whole procedure. We only use the two-level ions to act as memory and do not require the transfer quantum information from ions to the vibrational mode, which makes the system simple and robust against decoherence. Moreover, the proposal may be feasible based on current technologies.     

Remote state preparation with classically correlated state

We experimentally demonstrate the first remote state preparation with no shared entanglement, employing classically correlated state (CCS). CCS is verified with quantum state tomography process, and the fidelity is 0.99. The states chosen from a special diameter of the Poincaré sphere can be remotely prepared with unit efficiency at the cost of one cbit, if one classically correlated state is shared. The scheme can remotely prepare the other states on the Poincaré sphere with other CCS using the same experimental setup. The efficiency is 50% in general.     

Optical Realization of Deterministic Entanglement Concentration of Polarized Photons

We propose a scheme for optical realization of deterministic entanglement concentration of polarized photons. To overcome the difficulty due to the lack of sufficiently strong interactions between photons, teleportation is employed to transfer the polarization states of two photons onto the path and polarization states of a third photon, which is made possible by the recent experimental realization of the deterministic and complete Bell state measurement. Then the required positive operator-valued measurement and further operations can be implemented deterministically by using a linear optical setup. All these are within the reach of current technology.     

Remote State Preparation with Genuine Multipartite Entanglement

Enlightened by the work of Yeo and Chua [Phys. Rev. Lett. 96 （2006） 060502] for teleportation and dense coding with genuine multipartite entanglement, we present an explicit protocol for faithful remote state preparation in a real coefficient case by using the same four-particle entangled state which is not reducible to pair of Bell states. It is shown that any complex coefficient case can be changed to a real coefficient case. With this protocol, the state can play an analogous role to Einstein-Podolsky-Rosen pairs in the theory of multipartite entanglement.     

Teleportation for an Ionic Entangled Internal State by Entanglement Swapping

We present an effective scheme to teleport an unknown ionic entangled internal state via trapped ions without joint Bell-state measurement. In the constructed quantum channel process, we adopt entanglement swapping to avoid decrease of entanglement during the distribution of particles. Thus our scheme provides new prospects for quantum teleportation over longer distance. The distinct advantages of our scheme are that our scheme is insensitive to heating of vibrational mode and can be generalized to teleport an N-ion electronic entangled GHZ class state. Furthermore, in our scheme the success probability can reach 1.     

Locally Accessible Information from Multipartite Ensembles

We present a universal Holevo-like upper bound on the locally accessible information for arbitrary multipartite ensembles. This bound allows us to analyze the indistinguishability of a set of orthogonal states under local operations and classical communication. We also derive the upper bound for the capacity of distributed dense coding with multipartite senders and multipartite receivers.     

Forcible-Measurement Attack on Quantum Secure Direct Communication Protocol with Cluster State

The security of the quantum secure direct communication （QSDC） protocol with cluster state is analysed. It is shown that the secret would be partially leaked out when an eavesdropper performs forcible measurements on the transmitted particles. With the help of the result in minimum error discrimination, an upper bound （i.e. 40%） of this leakage is obtained. Moreover, the particular measurements which makes the leakage reach this bound are given.     

Scheme for Concentration of Unknown Greeberger-Horne-Zeilinger Entangled States via Cavity Decay

We present a scheme for entanglement concentration of an unknown atomic non-maximally entangled GHZ state via cavity decay. In the scheme, the atom trapped in a cavity is manipulated by laser field, so the maximally entangled GHZ state can be obtained by performing certain operation, which can be realized by illuminating the atom in a cavity. Our method is robust against spontaneous atomic decay.     

A genuine four-partite entangled state

In a recent paper, a genuine four-partite entangled state is proposed [Y. Yeo, W.K. Chua, Phys. Rev. Lett. 96 (2006) 060502], which has been found to have many interesting entanglement properties. We show this state is locally equivalent to some graph states.     

General and Optimal Scheme for Local Conversion of Pure States

We present general and optimal schemes for local conversion of pure states, via one specific example. First, we give the general solution of the doubly stochastic matrix. Then, we find the general and optimal positive-operator-valued measure （POVM） to realize the local conversion of pure states. Lastly, the physical realization of the POVM is discussed. We show that our scheme has a more general and better effect than other schemes.     

Unconditional continuous variable entanglement cascading

We demonstrate that the Gaussian entanglement cascading can be realized unconditionally for any nonzero squeezing in all of the entangled sources. We obtain the upper bound of the Gaussian cascaded entanglement, which cannot be exceeded by any entanglement cascading protocol based on Gaussian local operations and classical communications. We then propose an unconditional entanglement cascading protocol that can produce the cascaded entanglement reaching the upper bound. The protocol, as a generalization of the standard continuous variable teleportation, includes two steps: (i) each repeater site locally implements the Bell measurement and classically sends the results to Bob; (ii) Bob optimally displaces his own mode based on these results.     

Criterion for Genuine Multipartite Entanglement Quantum Channels

We introduce a character matrix for the N-qubit subsystem of a 2N-qubit state and show the criterion for genuine entanglement channel existing between two N-qubit subsystems in the state. The criterion allows us to check conveniently whether genuine quantum channels exist or not in the 2N-qubit state without calculating its N-qubit reduced density matrices.     

Comment on: “Dense coding and teleportation with one-dimensional cluster states” [Phys. Lett. A 364 (2007) 7]

The schemes on quantum dense coding and teleportation [Phys. Lett. A 364 (2007) 7] are revisited. By reformulizing one-dimensional n-qubit cluster states, we decompose the necessary multi-qubit collective unitary operation into a sequence of single-qubit Hadamard (H) and two-qubit controlled-not (C) operations. Our reduction makes the schemes feasible today.     

Perfect teleportation of arbitrary n-qudit states using different quantum channels

We propose a protocol Dn for faithfully teleporting an arbitrary n-qudit state with the tensor product state (TPS) of n generalized Bell states (GBSs) as the quantum channel. We also put forward explicit protocols and for faithfully teleporting an arbitrary n-qudit state with two classes of 2n-qudit GESs as the quantum channel, where the GESs we constructed are a kind of genuine entangled states which cannot be reducible to the TPS of n GBSs.