Cryptanalysis and Improvement of a Multi-User Quantum Communication Network Using χ-Type Entangled States

The security of a multi-user quantum communication network protocol using χ-type entangled states (Chang et al., J. Korean Phys. Soc. 61:1–5, 2012) is analyzed. We find that, by using one χ-type state in this protocol, two participants can only share 2 bits of information, not 4 bits as the authors stated. In addition, we give a special attack strategy by which an eavesdropper can elicit half of the secret information without being detected. Finally, we improve the protocol to be secure against all the present attacks.     

A Blind Quantum Signature Scheme with χ-type Entangled States

A blind quantum signature scheme with χ-type entangled states is proposed, which can be applied to E-voting system. In this scheme, the particles in χ-type state sequence are used for quantum key distribution first, and then for quantum signature. Our scheme is characterized by its blindness, impossibility of forgery, impossibility of disavowal. In addition, our scheme can perform an audit program with respect to the validity of the verification process in the light of actual requirements. The security of the scheme is also analyzed.     

Efficient Quantum Cryptography Network without Entanglement and Quantum Memory

An efficient quantum cryptography network protocol is proposed with d-dimensional polarized photons, without resorting to entanglement and quantum memory. A server on the network, say Alice, provides the service for preparing and measuring single photons whose initial state are ｜0〉. The users code the information on the single photons with some unitary operations. To prevent the untrustworthy server Alice from eavesdropping the quantum lines, a nonorthogonal-coding technique is used in the process that the quantum signal is transmitted between the users. This protocol does not require the servers and the users to store the quantum states and almost all of the single photons can be used for carrying the information, which makes it more convenient for application than others with present technology. We also discuss the case with a faint laser pulse.     

Probabilistic Preparation of N—particle Cat States via Entanglement Swapping and Entanglement Concentration

We discuss two different schemes for the probabilistic preparation of N-particle cat states using pure multiparticle entangled states via entanglement swapping and entanglement concentration.At the centre of distribution A,Alice performs all of the operations required to achieve our goal.     

Solution to Information Leakage in a Quantum Network System of QSS-QDC Using χ-Type Entangled States

Recently, Hong et al. (Chin. Phys. Lett. 29:050303, 2012) put forward two quantum secret sharing (QSS) protocols of quantum direct communication (QDC) by using χ-type entangled states. Later, some studies (Gao et al. in Chin. Phys. Lett. 29:110305, 2012; Chin. Phys. Lett. 30:079904, 2013; Liu et al. in Chin. Phys. Lett. 30:039901, 2013; Hong and Yang in Chin. Phys. Lett. 30:069901, 2013; Liu and Chen in Chin. Phys. Lett. 30:079903, 2013) made up for the drawbacks of Hong et al.’s two protocols to some extent. However, the information leakage weakness is still not thoroughly solved. In this Letter, the author analyzes the inner reason of information leakage weakness in detail at first. And then he suggests an effective encoding rule to avoid this weakness.     

Quantum Private Comparison of Equality with χ-Type Entangled States

In this paper, a quantum private comparison protocol with χ-type entangled states is proposed. In the protocol, two users can determine whether their secret inputs are equal, with the help of a semi-trusted third party. Here, the only constraint condition of this third party is that he could not collude with one user. Although the third party takes part in the whole process of the presented protocol, a feature of χ-type entangled state is utilized to design a eavesdropping check method to stand against his attack. Security analysis showed that the presented protocol is secure against some well-known attacks. Meanwhile, all the particles undergo only a one-way trip, which improves the efficiency and security of the presented protocol. Furthermore, only two-particle measurement is required in the implementation of the protocol, which make it more feasible in technique.     

Quantum Hypothesis Testing for Gaussian States: Quantum Analogues of χ2, t-, and F-Tests

We consider quantum counterparts of testing problems for which the optimal tests are the χ², t-, and F-tests. These quantum counterparts are formulated as quantum hypothesis testing problems concerning Gaussian state families, and they contain nuisance parameters, which have group symmetry. The quantum Hunt-Stein theorem removes some of these nuisance parameters, but other difficulties remain. In order to remove them, we combine the quantum Hunt-Stein theorem and other reduction methods to establish a general reduction theorem that reduces a complicated quantum hypothesis testing problem to a fundamental quantum hypothesis testing problem. Using these methods, we derive quantum counterparts of the χ², t-, and F-tests as optimal tests in the respective settings.     

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.     

Constructing Entanglement Witnesses for States in?Infinite-Dimensional Bipartite Quantum Systems

In this paper, two approaches of constructing entanglement witnesses for finite- or infinite-dimensional bipartite quantum systems are presented. Let H _A and H _B be complex Hilbert spaces and {E _k } and {F _k } be sequences of self-adjoint Hilbert-Schmidt operators on H _A and H _B , respectively, such that Tr(E^f_kE_l)=Tr(F^f_kF_l)=d_kl\mathrm{Tr}(E^{\dag}_{k}E_{l})=\mathrm{Tr}(F^{\dag}_{k}F_{l})=\delta_{kl}. Then W=I−∑ _k E _k ⊗F _k is an entanglement witness on H _A ⊗H _B if W\not 3 0W\not\geq 0. If ρ is an entangled state and τ ₀ is the nearest separable state to ρ under the Hilbert-Schmidt norm, then W=c ₀ I+τ ₀−ρ with c ₀=Tr[τ ₀(ρ−τ ₀)] is an entanglement witness.     

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.     

Explicit Protocol for Deterministic Entanglement Concentration

We present an explicit protocol for extraction of an EPR pair from two partially entangled pairs in a deterministic fashion via local operations and classical communication. This protocol is constituted by a local measurement described by a positive operator-valued measure (POVM), one-way classical communication, and a corresponding local unitary operation or a choice between the two pairs. We explicitly construct the required POVM by the analysis of the doubly stochastic matrix connecting the initial and the final states. Our scheme might be useful in future quantum communication.     

Multipartite Entanglement Preparation and Quantum Communication with Atomic Ensembles

We propose an experimentally feasible scheme to generate Greenberger-Horne-Zeilinger type of maximal entanglement among three atomic ensembles, and show one of the applications, controlled secure direct communication. The scheme involves laser manipulation of atomic ensembles, quarter- and half-wave plates, beam splitters, polarizing beam splitters and single-photon detectors, which are within the reach of current experimental technology.     

Concentration of Non-maximally Entangled States via Entanglement Swapping in Cavity QED

Using the highly detuned interaction between Λ-type three-level atoms and coherent optical fields in cavity QED, we can create a maximally entangled state between an atom and a cavity mode from a non-maximally entangled atomic state and a non-maximally entangled coherent state via entanglement swapping. The averaged output entanglement of the scheme is the product of the initial two non-maximal entanglements rather than the sum of them. The impact made by the spontaneous emission from the atomic excited levels has been canceled here, which makes the current protocol more feasible.     

Quantum Private Comparison Based on χ-Type Entangled States

A two-party quantum private comparison (QPC) protocol is constructed with χ-type entangled states in this paper. The proposed protocol employs a semi-honest third party (TP) that is allowed to misbehave on his own but cannot conspire with the adversary. The proposed protocol need perform Bell basis measurements and single-particle measurements but neither unitary operations nor quantum entanglement swapping technology. The proposed protocol possesses good security toward both the outside attack and the participant attack. TP only knows the comparison result of the private information from two parties in the proposed protocol.

     

Quantum Secure Direct Communication Network with Two-Step Protocol

An efficient quantum secure direct communication network protocol with the two-step scheme is proposed by using the Einstein-Podolsky-Rosen （EPR） pair block as the quantum information carrier. The server, say Alice, prepares and measures the EPR pairs in the quantum communication and the users perform the four local unitary operations to encode their message. Anyone of the legitimate users can communicate another one on the network securely. Since almost all of the instances in this scheme are useful and each EPR pair can carry two bits of information, the efficiency for qubits and the source capacity both approach the maximal values.     

Separability of Pure and Mixed States of the Quantum Network of Four Nodes

This article discusses the separability of the pure and mixed states of the quantum network of four nodes by means of the criterion of entanglement in terms of the covariance correlation tensor in quantum network theory.     

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.     

Complete Analysis of Four-Photon χ-Type Entangled State via Cross-Kerr Nonlinearity

We propose an efficient method to construct an optical four-photon |χ state analyzer via the cross-Kerr nonlinearity combined with linear optical elements. In this protocol, two four-qubit parity-check gates and two controlled phase gates are employed. We show that all the 16 orthogonal four-qubit |χ states can be completely discriminated with our apparatus. The scheme is feasible and realizable with current technology. It may have useful potential applications in quantum information processing which based on |χ state.