The generation of Entangled Qudits and their Application in Probabilistic Superdense Coding

A scheme of the generation of entangled qutrits is presented, and then is generalized to entangled ququads and entangled qudits. With the entangled qutrits, an experimental scheme of probability superdense coding with only linear optical elements is proposed. It is shown that this scheme will be suitable for the entangled ququads, even for the entangled qudits if some nonlinearity is used. This scheme is feasible in the laboratory with the current experimental technology.     

Controlled teleportation of an arbitrary n-qudit state using nonmaximally entangled GHZ states

In this paper, we explicitly present a general scheme for controlled quantum teleportation of an arbitrary multi-qudit state with unit fidelity and non-unit successful probability using d-dimensional nonmaximally entangled GHZ states as the quantum channel and generalized d-dimensional Bell states as the measurement basis. The expression of successful probability for controlled teleportation is present depending on the degree of entanglement matching between the quantum channel and the generalized Bell states. And the formulae for the selection of operations performed by the receiver are given according to the results measured by the sender and the controller.      

Multi-particle and high-dimension controlled order rearrangement encryption protocols

Based on controlled order rearrange encryption (CORE) for quantum key distribution using EPR pairs [Fu.G. Deng, G.L. Long, Phys. Rev. A 68, 042315 (2003)], we propose generalized controlled order rearrangement encryption (GCORE) protocols of N qubits and N qutrits, and concretely display them in cases using 3-qubit, 2-qutrit maximally entangled basis states. We further show that our protocols will become safer with an increase in dimensions and number of particles. Moreover, we carry out the security analysis using quantum covariant cloning machine. Although the applications of the generalized scheme need to be further studied, GCORE has many distinct features such as large capacity and high efficiency.     

Quantum teleportation via a two-qubit Heisenberg <Emphasis Type="Italic">XXZ</Emphasis> chain – effects of anisotropy and magnetic field

We study quantum teleportation via a two-qubit Heisenberg XXZ chain under an inhomogeneous magnetic field. We first consider entanglement teleportation, and then focus on the teleportation fidelity under different conditions. The effects of anisotropy and the magnetic field, both uniform and inhomogeneous, are discussed. We also find that, though entanglement teleportation does require an entangled quantum channel, a nonzero critical value of minimum entanglement is not always necessary.     

A singlet state prepared with resonant interaction via trapped ions

A simple scheme for the generation of the three-qutrit Λ-type singlet state with resonant interaction via trapped ions is proposed. In the present proposal, the required interaction time is very short due to resonant interaction. The present proposal may be realized based on the current technologies. We also note that the three-ion three-level maximally entangled state can be generated via resonant interaction in one step.     

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.     

Cluster-type entangled coherent states

We present the cluster-type entangled coherent states (CTECS) and discuss their properties. A cavity QED generation scheme using suitable choices of atom-cavity interactions, obtained via detunings adjustments and the application of classical external fields, is also presented. After the realization of simple atomic measurements, CTECS representing nonlocal electromagnetic fields in separate cavities can be generated.     

Teleportation of Mixture of Diagonal Bell States via Bound Entangled States

We propose a protocol for teleportation of arbitrary mixture of diagonal Bell states, it is shown that the channel can be constructed with either pure maximally entangled states or mixed bound entangled states. We also present protocols to realize the controlled teleportation of mixture of diagonal Bell states via multi-particle mixed states. Our results show that bound entangled states are also important and useful resources in quantum communication tasks.     

Scheme for Generating Cluster States with Charge Qubits in a Cavity

Based on superconducting charge qubits （SCCQs） coupled to a single-mode microwave cavity, we propose a scheme for generating charge cluster states. For all SCCQs, the controlled gate voltages are all in their degeneracy points, the quantum information is encoded in two logic states of charge basis. The generation of the multi-qubit cluster state can be achieved step by step on a pair of nearest-neighbor qubits. Considering effective long-rang coupling, we provide an efficient way to one-step generating of a highly entangled cluster state, in which the qubit-qubit coupling is mediated by the cavity mode. Our quantum operations are insensitive to the initial state of the cavity mode by removing the influence of the cavity mode via the periodical evolution of the system. Thus, our operation may be against the decoherence from the cavity.     

Quantum cryptography using partially entangled states

We show that non-maximally entangled states can be used to build a quantum key distribution (QKD) scheme where the key is probabilistically teleported from Alice to Bob. This probabilistic aspect of the protocol ensures the security of the key without the need of non-orthogonal states to encode it, in contrast to other QKD schemes. Also, the security and key transmission rate of the present protocol is nearly equivalent to those of standard QKD schemes and these aspects can be controlled by properly harnessing the new free parameter in the present proposal, namely, the degree of partial entanglement. Furthermore, we discuss how to build a controlled QKD scheme, also based on partially entangled states, where a third party can decide whether or not Alice and Bob are allowed to share a key.     

Teleportation of the three-level three-particle entangled state and classical communication cost

We propose a scheme to probabilistically teleport an unknown three-level three-particle entangled state. The quantum channel is composed of a partial entangled three-level two-particle state and a partial entangled three-level three-particle state. We calculate the successful total probability and the classical communication cost required in the ideal probabilistic teleportation process, respectively. It is shown that an unknown three-level three-particle entangled state can be teleported using fewer entangled particles and lesser classical communication cost than Bennett et al.’s original protocol.     

Splitting a qudit state via Greenberger-Horne-Zeilinger states of qubits

We present a tripartite quantum information splitting scheme which splits a qutrit state via two GHZ states. The scheme is then generalized to splitting a qudit state among any number of receivers. We show that this scheme is also applicable to splitting any multi-qudit entangled states.     

Preparation of Arbitrary Four-Qubit W State with Atomic Ensembles via Rydberg Blockade

The generation of various entangled states is an essential task in quantum information processing. Recently, a scheme （PRA 79, 022304） has been suggested for generating Greenberger-Horne-Zeilinger state and cluster state with atomic ensembles based on the Rydberg blockade. Using similar resources as the earlier scheme, here we propose an experimentally feasible scheme of preparing arbitrary four-qubit W class of maximally and non- maximally entangled states with atomic ensembles in a single step. Moreover, we carefully analyze the realistic noises and predict that four-qubit W states can be produced with high fidelity （F - 0.994） via our scheme.     

Entanglement generation in a two-mode single-atom laser

We present a method of generating two-mode single atom laser based on the nonresonant interaction of a three-level Λ type atom in a two-mode cavity with three strong classical driving fields. An analytical solution for this effective dynamics under the presence of the cavity losses is obtained, which allow us to analyze the entanglement properties and the photon statistics of the two cavity modes exactly. It is also shown that the possible generation of the two-mode entangled coherent states in the transient regime after the atomic measurement.     

Multiparty controlled quantum secure direct communication using Greenberger-Horne-Zeilinger state

Based on the idea of dense coding of three-photon entangled state and qubit transmission in blocks, we present a multiparty controlled quantum secret direct communication scheme by using Greenberger-Horne-Zeilinger state. In the present scheme, the sender transmits three bits of secret message to the receiver directly and the secret message can only be recovered by the receiver under the permission of all the controllers. All three-photon entangled states are used to transmit the secret message except those chosen for eavesdropping check and the present scheme has a high source capacity because Greenberger-Horne-Zeilinger state forms a large Hilbert space.     

Long-distance quantum communication in a decoherence-free subspace

We show how to realize long-distance quantum communication using a long-lived quantum memory, which is embedded in a decoherence-free subspace (DFS). Neutral atoms were used in the present scheme, whose interactions are catalyzed by single photons or weak coherent light. The generation, purification and swapping of logical entangled states are performed with help of cavity-assisted photon scattering which is robust to random variation in the atom-photon coupling rate.     

Multiparty quantum secret sharing based on Bell measurement

A multiparty quantum secret sharing scheme based on Bell measurement is proposed and analyzed. In this scheme, all agents are not required to prepare entangled states or perform any local unitary operation. The security of the protocol is also analyzed. It is shown that any eavesdropper will introduce errors invariably and be detected if he tries to steal information about Trent’s secret. Moreover, because no classical bit needs to be transmitted except those for detection, the total efficiency of the scheme approaches to 100%.     

Continuous-Variable Quantum Teleportation of Entangled Coherent States

We propose a quantum teleportation scheme for tripartite entangled coherent state （ECS） with continuous variable. Our scheme is feasible and economical in the sense that we need only linear optical devices such as beam splitters, phase shifters and photon detectors and employ three bipartite maximally ECSs as quantum channels. We also generalize the tripartite scheme into multipartite ease and calculate the minimum average fidelity for the schemes in tripartite and multipartite cases.     

Partial teleportation of entangled atomic states

In this Letter we propose a scheme for partially teleporting entangled atomic states. Our scheme can be implemented using only four two-level atoms interacting either resonantly or off-resonantly with a single cavity-QED. The estimative of losses occurring during this partial teleportation process is accomplished through the phenomenological operator approach technique.     

Quantum switch for coupling highly detuned superconducting qubits

We propose to implement a quantum switch scheme for coupling highly detuned superconducting qubits connected by a gap-tunable bridge qubit. By modulating the frequency of the bridge qubit, it can be used as a coupler to switch on/off and adjust the coupling strength between the initially non-interaction qubits. It is shown that the proposals of quantum information transfer and quantum entangled gate between two highly detuned qubits can be implemented with high fidelity. Moreover, we extend the case of coupling the switch to multiple qubits for the generation of W states. The advantages of our scheme are that it eliminates the need for tuning the gaps of the qubits and the cross-talk interaction is greatly suppressed. The influence of decoherence and parameter variation is also investigated by numerical simulation, which suggests that the present scheme is feasible in current experiment.