A Modified Protocol of Quantum Key Distribution Based on Entanglement Swapping

By comparing Cabello＇s addendum to his quantum key distribution protocol [Phys. Rev. A 64 （2001） 024301], we propose a more convenient modified protocol based on the entanglement swapping which is secure against the eavesdropping strategy addressed by Zhang et al. [Phys. Rev. A 63 （2001）036301] and other existing types of attack.     

Multiparty Quantum Secret Sharing of Quantum States Using Entanglement States

A multi-partite-controlled quantum secret sharing scheme using several non-orthogonal entanglement states is presented with unconditional security. In this scheme, the participants share the secret quantum state by exchanging the secret polarization angles of the disordered travel particles. The security of the secret quantum state is also guaranteed by the non-orthogonal multi-partite-controlled entanglement states, the participants＇ secret polarizations, and the disorder of the travelling particles. Moreover, the present scheme is secure against the particle-number splitting attack and the intercept-and-resend attack. It may be still secure even if the distributed quantum state is embedded in a not-so-weak coherent-state pulse.     

Quantum Bidirectional Secure Direct Communication via Entanglement Swapping

In light of a quantum secure direct communication （QSDC） scheme using entanglement swapping [Chin. Phys. Lett. 22 （2005）18], by introducing additional local operations for encoding, we propose a bidirectional QSDC （BQSDC） protocol, in which two legitimate users can simultaneously exchange their respective messages. The rule for the users to retrieve his/her partner＇s messages is derived explicitly in the most general case. Eve＇s commonly used attack method has been discussed and can be detected with the security checking process.     

Cryptanalysis of Multiparty Quantum Secret Sharing of Quantum State Using Entangled States

Security of a quantum secret sharing of quantum state protocol proposed by Guo et al. [Chin. Phys. Lett. 25 （2008） 16] is reexamined. It is shown that an eavesdropper can obtain some of the transmitted secret information by monitoring the classical channel or the entire secret by intercepting the quantum states, and moreover, the eavesdropper can even maliciously replace the secret message with an arbitrary message without being detected. Finally, the deep reasons why an eavesdropper can attack this protocol are discussed and the modified protocol is presented to amend the security loopholes.     

Secure Quantum Secret Sharing Based on Reusable GHZ States as Secure Carriers

We show that a potential eavesdropper can eavesdrop whole secret information when the legitimate users use secure carrier to encode and decode classical information repeatedly in the protocol proposed by Bagherinezhad S and Karimipour V [Phys. Rev. A 67（2003）044302]. Then we present a revised quantum secret sharing protocol by using the Greenberger-Horne-Zeilinger state as secure carrier. Our protocol can resist Eve＇s attack.     

Improvement of Security of Three-Party Quantum Secure Direct Communication Based on GHZ States

A protocol for three-party quantum secure direct communication based on Greenberger-Horne-Zeilinger （GHZ） states was recently proposed by 3in et al. [Phys. Lett. A 354 （2006） 67] By analysing the protocol we find some security loopholes, e.g. one bit of secret messages of a party （Alice in the original paper） can always be leaked straight to the public without any eavesdropping. These problems suggested previously are discussed and possible solutions are presented to improve the security of the original protocol.     

Partial Hermitian Conjugate Separability Criteria for Pure Quantum States

We propose a criterion for the separability of quantum pure states using the concept of a partial Hermitian conjugate. It is equivalent to the usual positive partial transposition criteria, with a simple physical interpretation.     

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.     

One Atom in an Optical Cavity： Entanglement and Applications to Quantum Computation

We propose a scheme to implement two-bit .quantum phase gates and one-bit unitary gates by using the two- mode two-photon Jaynes Cummings model. The entanglement between the atom and cavity is also investigated in the presence of phase decoherence. It is found that there is stationary entanglement that is sensitive with the detuning     

Quantum Key Distribution Network Based on Differential Phase Shift

Using a series of quantum correlated photon pairs, we propose a theoretical scheme for any-to-any multi-user quantum key distribution network based on differential phase shift. The differential phase shift and the different detection time slots ensure the security of our scheme against eavesdropping. We discuss the security under the intercept-resend attack and the source replacement attack.     

Multiparty Quantum Remote Secret Conference

We present two schemes for multiparty quantum remote secret conference in which each legitimate conferee can read out securely the secret message announced by another, but a vicious eavesdropper can get nothing about it. The first one is based on the same key shared efficiently and securely by all the parties with Greenberger-Horne- Zeilinger （GHZ） states, and each conferee sends his secret message to the others with one-time pad crypto-system. The other one is based on quantum encryption with a quantum key~ a sequence of GHZ states shared among all the conferees and used repeatedly after confirming their security. Both these schemes are optimal as their intrinsic efficiency for qubits approaches the maximal value.     

Threshold Quantum Secret Sharing of Secure Direct Communication

We propose a （L, n）-threshold quantum secret sharing protocol of secure direct communication following some ideas of Zhang＇s protocol [Phys. Lett. A 342 （2005） 60] and Tokunaga et al.＇s protocol [Phys. Rev. A 71 （2005） 012314]. The sender distributes the classical secret shares to his or her n agents and each agent owns a secret share in advance. The sender＇s secure direct communication message can be extracted by an agent subset by collaboration in such a way that at least t or more agents can obtain the secret message with the mutual assistances but any t - 1 or fewer agents cannot. In contrast to the previous multiparty quantum secret sharing protocols in which the sender＇s secret message can be recovered only if all the agents collaborate, our protocol is more practical and more flexible.     

Entanglement transformation between two-qubit mixed states by LOCC

Based on a new set of entanglement monotones of two-qubit pure states, we give sufficient and necessary conditions that one two-qubit mixed state is transformed into another one by local operations and classical communication (LOCC). This result can be viewed as a generalization of Nielsen's theorem Nielsen (1999) [1]. However, we find that it is more difficult to manipulate the entanglement transformation between single copy of two-qubit mixed states than to do between single copy of two-qubit pure ones.     

Teleportation of a general two-mode coherent-state superposition via attenuated quantum channels with ideal and/or threshold detectors

Three novel probabilistic yet conclusive schemes are proposed to teleport a general two-mode coherent-state superposition via attenuated quantum channels with ideal and/or threshold detectors. The calculated total success probability is highest (lowest) when only ideal (threshold) detectors are used.     

Generation of free-travelling four-mode cluster-type entangled coherent states

A new scheme is proposed to generate free-travelling four-mode cluster-type entangled coherent states. Compared with other previously proposed schemes ours is very simple in execution using only one π-cross-Kerr medium, two 50:50 beam-splitters and two π/2-phase-shifters. Despite the setup simplicity, our scheme, unlike the others which are all probabilistic, is efficient with 100% success probability since no measurements are involved at all.     

Boundary Quantum Entanglement of the XXZ Spin Chain with Boundary Impurities

The boundary quantum entanglement for the s = 1/2 X X Z spin chain with boundary impurities is studied via the density matrix renormalization group （DMRG） method. It is shown that the entanglement entropy of the boundary bond （the impurity and the chain spin next to it） behaves differently in different phases. The relationship between the singular points of the boundary entropy and boundary quantum critical points is discussed.     

Pairwise Entanglement and Quantum Phase Transitions in Spin Systems

We examine several well-known quantum spin models and categorize the behaviour of pairwise entanglement at quantum phase transitions. A unitied picture on the connection between the entanglement and quantum phase transition in spin systems is presented.     

Quantum Nonlocality and Entanglement of Pair Cat States with Phase Decoherence

We investigate the entanglement of pair cat states in the phase damping channel by adopting the log-negativity and then study the possible violations of Bell＇s inequalities for the pair cat states in terms of the Wigner representation in phase space based upon parity measurement and displacement operation.     

Possible Realization of Cluster States and Quantum Information Transfer in Cavity QED via Raman Transition

We present a scheme to generate cluster states with many scheme, no transfer of quantum information between the atoms in cavity QED via Raman transition. In this atoms and cavities is required, the cavity fields are only virtually excited and thus the cavity decay is suppressed during the generation of cluster states. The atoms are always populated in the two ground states. Therefore, the scheme is insensitive to the atomic spontaneous emission and cavity decay. We also show how to transfer quantum information from one atom to another.     

A New Quantum Secure Direct Communication Protocol Using Decoherence-Free Subspace

A new quantum secure direct communication （QSDC） protocol is proposed by using decoherence free subspace （DFS） to avoid insecurity of the present QSDC protocols in a quantum noise channel. This protocol makes it easily for Bob and Alice to find eavesdropping in channel because the collective dephasing noise disappears in DFS. The probability of successful attack by Eve in this protocol is smaller than in BB84 protocol. Thus this protocol realizes secure QSDC and is feasible with present-day technology.