Optimal teleportation with a mixed state of two qubits

We consider a single copy of a mixed state of two qubits and derive the optimal trace-preserving local operations assisted by classical communication such as to maximize the fidelity of teleportation that can be achieved with this state. These optimal local operations turn out to be implementable by one-way communication and always yield a teleportation fidelity larger than 2/3 if the original state is entangled. This maximal achievable fidelity is an entanglement measure and turns out quantifying the minimal amount of mixing required to destroy the entanglement in a quantum state.     

Generation of high-fidelity four-photon cluster state and quantum-domain demonstration of one-way quantum computing

We experimentally demonstrate a simple scheme for generating a four-photon entangled cluster state with fidelity over 0.860+/-0.015. We show that the fidelity is high enough to guarantee that the produced state is distinguished from Greenberger-Horne-Zeilinger, W, and Dicke types of genuine four-qubit entanglement. We also demonstrate basic operations of one-way quantum computing using the produced state and show that the output state fidelities surpass classical bounds, which indicates that the entanglement in the produced state essentially contributes to the quantum operation.     

Efficient Scheme for One-Way Quantum Computing with Radiofrequency SQUID Qubits byAdiabatic Passage

We proposed an efficient scheme for constructing a quantum controlled phase-shift gate and generating the cluster states with rf superconducting quantum interference devices (SQUIDs) coupled to a microwave cavity through adiabatic evolution of dark eigenstates. During the operation, the spontaneous emission is suppressed since the rf SQUIDs are always in the three lowest flux states. Considering the influence from the cavity decay with achievable
experimental parameters, we numerically analyze the success probability and the fidelity for generating the two-SQUID maximally entangled state and the controlled phase-shift gate by adiabatic passage.     

Remote state preparation of a photonic quantum state via quantum teleportation

We demonstrate an experimental realization of remote state preparation via the quantum teleportation algorithm, using an entangled photon pair in the polarization degree of freedom as the quantum resource. The input state is encoded on the path of one of the photons from the pair. The improved experimental scheme allows us to control the preparation and teleportation of a state over the entire Bloch sphere with a resolution of the degree of mixture given by the coherence length of the photon pair. Both the preparation of the input state and the implementation of the quantum gates are performed in a pair of chained displaced Sagnac interferometers, which contribute to the overall robustness of the setup. An average fidelity above 0.9 is obtained for the remote state preparation process. This scheme allows for a prepared state to be transmitted on every repetition of the experiment, thus giving an intrinsic success probability of 1.     

Tripartite quantum state sharing

We demonstrate a multipartite protocol to securely distribute and reconstruct a quantum state. A secret quantum state is encoded into a tripartite entangled state and distributed to three players. By collaborating, any two of the three players can reconstruct the state, while individual players obtain nothing. We characterize this (2,3) threshold quantum state sharing scheme in terms of fidelity, signal transfer, and reconstruction noise. We demonstrate a fidelity averaged over all reconstruction permutations of 0.73+/-0.04, a level achievable only using quantum resources.     

Four-dimensional entangled state generation in remote cavities

We propose a scheme to generate a maximally four-dimensional entangled state of two six-level atoms in two remote cavities. By choosing suitable intensities and detunings of fields, atomic spontaneous radiation and photon leakage out of cavity and fibre are efficiently suppressed. Thus, the intended state can be generated with high fidelity in the presence of decoherence. We extend the scheme to generate an N-atom four-dimensional entangled state.     

Simply quantum information processing with RF superconducting qubit

Utilizing rf superconducting quantum interference devices coupled with transmission line resonator, we propose a scheme to implementing quantum information processing. In this system, the high fidelity two-qubit maximally entangled states and quantum logic gate are realized. Under the large detuning condition, the excited state of an rf superconducting quantum interference device is adiabatically eliminated. So the excited state spontaneous emission of the superconducting qubit can be effectively avoided in this paper. At last, the experimental feasibility and the challenge of our schemes have been discussed.     

Teleportation of a Bipartite Entangled Coherent State via a Four-Partite Cluster-Type Entangled State

We present an optical scheme to almost completely teleport a bipartite entangled coherent state using a four-partite cluster-type entangled coherent state as quantum channel. The scheme is based on optical elements such as beam splitters, phase shifters, and photon detectors. We also obtain the average fidelity of the teleportation process. It is shown that the average fidelity is quite close to unity if the mean photon number of the coherent stateis not too small.     

Creation of an entangled state between two NV centers in a single-photon interferometric scheme

We study a method for creating an entangled state between two NV centers in diamond that do not directly interact with each other. The method is based on excitation of centers with a single photon via a half divider with subsequent coherent inversion of the spin of the excited center by a microwave pulse and on the detection of a photon that was emitted by the excited center and that passed through the divider. We consider the general case in which the transition frequencies of the centers are shifted with respect to each other and the control pulses are detuned with respect to these transitions. We find the probability of creating an entangled state and its fidelity in relation to the parameters of the method and transition frequencies of the centers. Optimal values of these parameters are determined.     

Two schemes of teleportation one-particle state by a three-particle GHZ state

In accordance with transformation operator, we give two schemes for teleporting an unknown one-particle state via a general GHZ state, Two Von Neumann type measurements are given for teleporting an unknown one-particle state. The first Von Neumann type measurement use four orthogonal states and the second Von Neumann type measurement is eight orthogonal states. For maximally entangled GHZ state, the successful probability and fidelity of two schemes both reach 1.     

Two-qubit entangled state teleportation via optimal POVM and partially entangled GHZ state

Quantum teleportation is of significant meaning in quantum information. In this paper, we study the probabilistic teleportation of a two-qubit entangled state via a partially entangled Greenberger- Horne-Zeilinger (GHZ) state when the quantum channel information is only available to the sender. We formulate it as an unambiguous state discrimination problem and derive exact optimal positive-operator valued measure (POVM) operators for maximizing the probability of unambiguous discrimination. Only one three-qubit POVM for the sender, one two-qubit unitary operation for the receiver, and two cbits for outcome notification are required in this scheme. The unitary operation is given in the form of a concise formula, and the fidelity is calculated. The scheme is further extended to more general case for transmitting a two-qubit entangled state prepared in arbitrary form. We show this scheme is flexible and applicable in the hop-by-hop teleportation situation.     

Generation of maximally entangled state from a single-photon state in fiber-connected cavities

Starting from the physical model consisting of two remotely located atoms trapped in high finesse fiber-connected cavities, generation of maximally entangled state is firstly realized theoretically from a initial single-photon state. Result shows that under the regime of strong atom-cavity coupling strength, the averaged fidelity can remarkably exceed 0.9980. The distant creation of maximally entangled state may provides certain opportunities and be helpful in the accomplishment of scalable quantum networks.     

Approximate Disentanglement

We construct a quantum machine which, by using an asymmetric cloner, disentangles an entangled state and nearly retains it. The attainable maximum value of the scaling parameter for disentangling is identical to that obtained in previous works. The fidelity of the output residual entangled state with respect to the input entangled state is state-dependent. The result shows that it is possible to deal with disentanglement and broadcasting entanglement in a single unitary evolution.     

A generalized two-mode entangled state: Its generation,properties, and applications

Using the technique of integration within an ordered product of operators we construct a generalized two-mode entangled state, which can be generated by an asymmetrical beam splitter （BS）. Some important properties of this state, such as orthogonality and Schmidt decomposition, are also dis- cussed by deriving the expression of BS operator in coordinate representation. As its applications, to conjugate state, obtain operator identities, generate new squeezing operators （squeezed state） are also presented. It is shown that the fidelity of quantum teleportation can be enhanced under certain case by using the asymmetrical new squeezed state as entangled resource.     

用三粒子纠缠态作量子信道远程操纵单比特旋转

提出用三粒子纠缠态作量子信道远程操纵单比特旋转的理论方案。首先，我们利用最大纠缠的GHZ态的性质远程操纵单比特旋转，其保真度和成功几率均为1。 我们还提出了两个用部分纠缠的GHZ态作量子信道实现保真度为1的远程操纵单比特旋转的方案。这些方案的特点是，两地之间还存在一第三者，他作为监控方参与量子远程操纵过程，特别地，当量子信道为部分纠缠态时，他能矫正被非理想量子信道致畸的量子态。除了GHZ型态外，我们还证明了W型态亦可用作量子信道远程操纵单比特旋转，但后者的成功几率总是小于前者。     

利用单个三粒子最大GHZ态或两个EPR态隐形传送任意三粒子GHZ态

提出利用单个三粒子最大Greenberger-Horne-Zeilinger (GHZ)态或两个Einstein-Podolsky-Rosen (EPR)态作为量子信道确定性隐形传送任意三粒子GHZ态的两个方案，并将方案推广至隐形传送任意n(n≥4)粒子GHZ态的情况.讨论了量子信道受噪声影响时隐形传态的保真度.研究发现，当作为量子信道的单个三粒子最大GHZ态受到噪声影响时，隐形传态的保真度仅与量子信道的纠缠度有关，而当作为量子信道的两个EPR态受到噪声影响时，隐形传态的保真度不仅与量子信道的纠缠度有关，还与待传送态的纠缠度有关.所提出的方案具有节省量子信道纠缠资源的特点. 关键词： 隐形传态 三粒子Greenberger-Horne-Zeilinger态 量子逻辑门 保真度     

Thermal entanglement and teleportation of a thermally mixed entangled state of a Heisenberg chain through a Werner state

The thermal entanglement and teleportation of a thermally mixed entangled state of a two-qubit Heisenberg XXX chain under the Dzyaloshinski Moriya （DM） anisotropic antisymmetric interaction through a noisy quantum channel given by a Werner state is investigated. The dependences of the thermal entanglement of the teleported state on the DM coupling constant, the temperature and the entanglement of tbe noisy quantum channel are studied }n detail for both the ferromagnetic and the antiferromagnetic cases. The result shows that a minimum entanglement of the noisy quantum channel must be provided in order to realize the entanglement teleportation. The values of fidelity of the teleported state are also studied for these two cases. It is found that under certain conditions, we can transfer an initial state with a better fidelity than that for any classical communication protocol.     

Teleportation of a two-particle four-component squeezed vacuum state by linear optical elements

We present a linear optical scheme for achieving a unity fidelity teleportation of a two-particle four- component squeezed vacuum state using two entangled squeezed vacuum states as quantum channel. The devices used are beam splitters and ideal photon detectors capable of distinguishing between odd and even photon numbers. Moreover, we also obtain the success probability of the teleportation scheme.     

Scheme for teleportation of an unknown atomic state via a cluster state in cavity QED

This paper proposes an experimentally feasible scheme for teleportation of an unknown two-atom entangled state, where a cluster state is used as the quantum channel. This scheme does not need any joint measurement. In addition, the successful probability and fidelity of teleportation can both reach 1.0. The current scheme can be realized within the current experimental technology.     

Remote preparation of a Greenberger--Horne--Zeilinger state via a two-particle entangled state

We present two schemes for realizing the remote preparation of a Greenberger--Horne--Zeilinger (GHZ) state. The first scheme is to remotely prepare a general N-particle GHZ state with two steps. One is to prepare a qubit state by using finite classical bits from sender to receiver via a two-particle entangled state, and the other is that the receiver introduces N - 1 additional particles and performs N - 1 controlled-not (C-Not) operations. The second scheme is to remotely prepare an N-atom GHZ state via a two-atom entangled state in cavity quantum electrodynamics (QED). The two schemes require only a two-particle entangled state used as a quantum channel, so we reduce the requirement for entanglement.