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.     

Deterministic generation of entangled photons by cavity-assisted interactions through adiabatic passage

We propose a deterministic scheme for generation of highly entangled photon states using a high-Q two-mode optical cavity and the dark state evolution. Because of the adiabatic operation, our proposal is robust to ambient noise, and the relevant dynamics is insensitive to the randomness of moderate fluctuations regarding experimental parameters. Our scheme not only works deterministically, but also has the advantage of achieving highly entangled photons by adiabatically increasing or decreasing the Rabi frequencies regarding the classical driving pulses, which would be practical in real implementation. Our scheme can also be extended to generation of multiphoton entanglement.     

Quantum Secret Sharing Protocol between Multiparty and Multiparty with Single Photons and Unitary Transformations

We propose a scheme of quantum secret sharing between Alice＇s group and Bob＇s group with single photons and unitary transformations. In the protocol, one member in Alice＇s group prepares a sequence of single photons in one of four different states, while other members directly encode their information on the sequence of single photons via unitary operations; after that, the last member sends the sequence of single photons to Bob＇s group. Then Bob＇s, except for the last one, do work similarly. Finally the last member in Bob＇s group measures the qubits. If the security of the quantum channel is guaranteed by some tests, then the qubit states sent by the last member of Alice＇s group can be used as key bits for secret sharing. It is shown that this scheme is safe.     

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.     

Realization of Three-Qubit Controlled-Phase Gate Operation with Atoms in Cavity QED System

We propose a scheme for realization of three-qubit controlled-phase gate via passing two three-level atoms through a high-Q optical cavity in a cavity QED system. In the presented protocol, the two stable ground states of the atoms act as the two controlling qubits and the zero- and one-photon Fock states of the cavity-field form the target qubit, and no auxiliary state or any measurement is required. The numerical simulation shows that the gate fidelities remain at a high level under the influence of the atomic spontaneous emission, the decay of the cavity mode and deviation of the coupling strength. The experimental feasibility of our proposal is also discussed.     

Scheme for linear optical preparation of a type of four-photon entangled state with conventional photon detectors

We propose a simple experimental scheme to prepare a type of four-photon entangled state |χ〉 that has many interesting entanglement properties and possible applications in quantum information processing with a certain success probability. The proposed setup involves only simple linear optical devices, a single-photon polarization state, three pairs of two-photon polarization entangled states, and the conventional photon detectors that cannot distinguish a single photon from two or more photons, which greatly simplify the experimental realization of the scheme.     

Comment on “Controlled DSQC using five-qubit entangled states and two-step security test”

A controlled deterministic secure quantum communication protocol [X.M. Xiu, L. Dong, Y.J. Cao, F. Chi, Opt. Commun. 282 (2009) 333] with five-qubit entangled states was proposed recently. The aim of Xiu et al. was that the successful realization of communication between Alice and Bob needed the cooperation of a controller, Charlie. However, we show that the controller Charlie’s role could be excluded unknowingly. Moreover, an eavesdropper can entangle an ancilla without introducing any error in the first security test and then she can distill a quarter of the secret messages from her entangled ancilla.     

Simple Algorithm for Deterministic Entanglement Concentration

We first present the general solution and the simplest special solution of the doubly stochastic matrix used in deterministic entanglement concentration. Then, we give a better scheme to realize deterministic entanglement concentration. Under this scheme, the concentration is realized, and more importantly, two partially entangled pairs concentrate to four-particle Greenberger-Horne-Zeilinger （GHZ） states with a certain probability.     

Generation of Multipartite Ionic Entangled States only by Single-Qubit Measurements via Linear Optical Elements

We propose a scheme for generation of multipartite ionic Greenberger-Horne-Zeilinger （GHZ） states only by single-qubit measurements. Our scheme not only does not need joint measurements but also avoids the difficulty of synchronizing the arrival time of the two scattered photons, which is faced by the previous schemes. Therefore our entanglement generation scheme can be implemented more easily than the schemes based on atomic interference in experiment.     

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.     

Symmetric Telecloning and Entanglement Distribution of Spin Quantum States

We propose a physical realization of symmetric telecloning machine for spin quantum states. The concept of area average fidelity is introduced to describe the telecloning quality. It is indicated that for certain input states this quantity may come to an enough high level to satisfy the need of quantum information processing. We also study the properties of entanglement distribution via the spin chain for arbitrary two-qubit entangled pure states as inputs and find that the decay ratio of entanglement for the output states is only determined by the parameters of spin chain and waiting time, independent of the initial input states.     

A revised controlled deterministic secure quantum communication with five-photon entangled state

A revised controlled deterministic secure quantum communication protocol using five-photon entangled state is proposed. It amends the security loopholes pointed by Qin et al. in [S.J. Qin, Q.Y. Wen, L.M. Meng, F.C. Zhu, Opt. Commun. 282 (2009) 2656] in the original protocol proposed by Xiu et al. in [X.M. Xiu, L. Dong, Y.J. Gao, F. Chi, Opt. Commun. 282 (2009) 333]. The security loopholes are solved by using order rearrangement of transmission photons and two-step security test.     

Quantum processing photonic states in optical lattices

The mapping of photonic states to collective excitations of atomic ensembles is a powerful tool which finds a useful application in the realization of quantum memories and quantum repeaters. In this work we show that cold atoms in optical lattices can be used to perform an entangling unitary operation on the transferred atomic excitations. After the release of the quantum atomic state, our protocol results in a deterministic two qubit gate for photons. The proposed scheme is feasible with current experimental techniques and robust against the dominant sources of noise.     

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.     

Deterministic geometric quantum phase gates for two atoms in decoherence-free subspace

We propose a scheme for realizing conventional geometric quantum phase gates in the context of cavity QED. During the operation neither the atomic system nor the cavity mode is excited, which is important in view of decoherence. The scheme does not require detection of photons, so the gate operation is deterministic and the influence of photodetection imperfection is eliminated. Taking advantage of the geometric manipulation, the phase gate is resilient to fluctuations of experimental parameters.     

Deterministic secure quantum communication against collective-dephasing noise by using EPR pairs and auxiliary photons

A deterministic secure quantum communication against collective-dephasing noise is proposed. Alice constructs two sets of three-photon bases with EPR (Einstein-Podolsky-Rosen) pairs in the state |Ψ⁺〉 or |Ψ^-〉 and auxiliary single photons in the state |H〉. And then she sends them to Bob. Bob can get the secret message by his single-photon measurement outcomes and two public message strings from Alice if the quantum channel is secure. The scheme does not need photon storing technique and only single-photon measurement is necessary.     

A note on approximate teleportation of an unknown atomic state in the two-photon Jaynes-Cummings model

We consider recent schemes [J.M. Liu, B. Weng, Physica A 367 (2006) 215] to teleport unknown atomic states and superposition of zero- and two-photon states using the two-photon Jaynes-Cummings model. Here we do the same using the “full two-photon Jaynes-Cumming”, valid for arbitrary average number of photons. The success probability and fidelity of this teleportation are also considered.     

Creation of atomic W state in a cavity by adiabatic passage

We propose two relatively robust schemes to generate controllable (deterministic) atomic W states of three Λ-like atoms interacting with an optical cavity and a laser beam. Losses due to atomic spontaneous emissions and to cavity decay are efficiently suppressed by employing adiabatic passage technique and appropriately designed atom-field couplings. In these schemes the three atoms traverse the cavity-mode and the laser beam and become entangled in the free space outside the cavity.     

Efficient Three-Party Quantum Secret Sharing with Single Photons

A scheme for three-party quantum secret sharing of a private key is presented with single photons. The agent Bob first prepares a sequence of single photons with two biased bases and then sends them to the boss Alice who checks the security of the transmission with measurements and produces some decoy photons by rearranging the orders of some sample photons. Alice encodes her bits with two unitary operations on the photons and then sends them to the other agent. The security of this scheme is equivalent to that in the modified Bennett Brassard 1984 quantum key distribution protocol. Moreover, each photon can carry one bit of the private key and the intrinsic efficiency for qubits and the total efficiency both approach the maximal value 100% when the number of the bits in the key is very large.     

Photonic Four-qubit Entangled Decoherence-free States Assisted by Cavity-QED System

We propose an efficient preparation of photonic four-qubit entangled decoherence-free states assisted by the cavity-QED system. By using the optical selection rule derived by a single electron charged self-assembled GaAs/InAs quantum dot in a micropillar resonator, two photons are used to generate four-qubit entangled decoherence-free states. Compared with previous entanglement based photonic protocols, the present one requires single-photon resources and is deterministic. These states may be applied to long-distance communications because only two photons are transmitted.