A novel scheme for secure direct communication between Alice and Bob is proposed, where there is no need for establishing a shared secret key. The communication is based on Einstein-PodolskyRosen pairs and teleportation between Alice and Bob. After insuring the security of the quantum channel (EPR pairs), Bob encodes the secret message directly on a sequence of particle states and transmits them to Alice by teleportation. In this scheme teleportation transmits Bob's message without revealing any information to a potential eavesdropper. Alice can read out the encoded messages directly by the measurement on her qubits. Because there is not a transmission of the qubit which carry the secret message between Alice and Bob, it is completely secure for direct secret communication if perfect quantum channel is used.PACS numbers: 03.67. Dd, 42.79.Sz Since languages become the tool for communication, the desire and need to transmit secret messages from one person to another begin. Then human have the cryptography -an art to transmit information so that it is unintelligible and therefore useless to those who are not meant to have access to it. It is generally believed that cryptography schemes are only completely secure when the two communicating parties, Alice and Bob, establish a shared secret key before the transmission of a message. This means they first have to create a random bit sequence, which is not known to anyone else, and which is of the same length as the message. In order to communicate, Alice then multiplies the bits of the message one by one with the key bits. When she announces the result to Bob, or even publicly, then he is the only one who can interpret it and deduce Alice's message.But it is difficult to distribute securely the secret key through a classical channel. Quantum key distribution (QKD), the approach using quantum mechanics principle for distribution of secret key is the only proven protocol for secure key distribution.Since Bennett and Brassard proposed the standard BB84 QKD protocol [1] in 1984, it has been developed quickly. Up to now there have already been a lot of theoretical QKD schemes, for instance in Refs. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18].Different from key distribution whose object is to establish a common random key between two parties, a secure direct communication is to communicate important messages directly without first establishing a random key to encrypt them. As classical message can be copied fully, it is impossible to transmit secret messages directly through classical channels. Recently Beige et al. [19] proposed a quantum secure direct communication (QSDC) scheme. In this scheme the message can be read out only after a transmission of an additional classical information for each qubit. Boström and Felbinger put forward a Ping-Pong QSDC scheme [20]. It is secure for key distribution, quasi-secure for direct secret communication if perfect quantum channel is used. But it is insecure if it is operated in a noisy quantum channel, as shown by Wójcik [21]. T...

Quantum coherence is a fundamental manifestation of the quantum superposition principle. Recently, Baumgratz et al. [ Phys. Rev. Lett. 113, 140401 (2014)] presented a rigorous framework to quantify coherence from the view of theory of physical resource. Here we propose a new valid quantum coherence measure which is a convex roof measure, for a quantum system of arbitrary dimension, essentially using the generalized Gell-Mann matrices. Rigorous proof shows that the proposed coherence measure, coherence concurrence, fulfills all the requirements dictated by the resource theory of quantum coherence measures. Moreover, strong links between the resource frameworks of coherence concurrence and entanglement concurrence is derived, which shows that any degree of coherence with respect to some reference basis can be converted to entanglement via incoherent operations. Our work provides a clear quantitative and operational connection between coherence and entanglement based on two kinds of concurrence. This new coherence measure, coherence concurrence, may also be beneficial to the study of quantum coherence.

We present a deterministic secure direct communication scheme via entanglement swapping, where a set of ordered maximally entangled three-particle states (GHZ states), initially shared by three spatially separated parties, Alice, Bob and Charlie, functions as a quantum information channel. After ensuring the safety of the quantum channel, Alice and Bob apply a series local operations on their respective particles according to the tripartite stipulation and the secret message they both want to send to Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is able to infer the secret messages directly. The secret messages are faithfully transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ states without revealing any information to a potential eavesdropper. Since there is not a transmission of the qubits carrying the secret message between any two of them in the public channel, it is completely secure for direct secret communication if perfect quantum channel is used.

We propose a quantum secret sharing protocol between multi-party (m members in group 1) and multi-party (n members in group 2) using a sequence of single photons. These single photons are used directly to encode classical information in a quantum secret sharing process. In this protocol, all members in group 1 directly encode their respective keys on the states of single photons via unitary operations, then the last one (the m th member of group 1) sends 1/n of the resulting qubits to each of group 2. Thus the secret message shared by all members of group 1 is shared by all members of group 2 in such a way that no subset of each group is efficient to read the secret message, but the entire set (not only group 1 but also group 2) is. We also show that it is unconditionally secure. This protocol is feasible with present-day techniques.

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In this paper, we present a measure of multipartite entanglement (k-nonseparable), k-ME concurrence C k−ME (ρ) that unambiguously detects all k-nonseparable states in arbitrary dimensions, where the special case, 2-ME concurrence C2−ME(ρ), is a measure of genuine multipartite entanglement. The new measure k-ME concurrence satisfies important characteristics of an entanglement measure including entanglement monotone, vanishing on k-separable states, convexity, subadditivity and strictly greater than zero for all k-nonseparable states. Two powerful lower bounds on this measure are given. These lower bounds are experimentally implementable without quantum state tomography and are easily computable as no optimization or eigenvalue evaluation is needed. We illustrate detailed examples in which the given bounds perform better than other known detection criteria.

We consider the concept of "the permutationally invariant (PI) part of a density matrix," which has proven very useful for both efficient quantum state estimation and entanglement characterization of N-qubit systems. We show here that the concept is, in fact, basis dependent but that this basis dependence makes it an even more powerful concept than has been appreciated so far. By considering the PI part ρ(PI) of a general (mixed) N-qubit state ρ, we obtain (i) strong bounds on quantitative nonseparability measures, (ii) a whole hierarchy of multipartite separability criteria (one of which entails a sufficient criterion for genuine N-partite entanglement) that can be experimentally determined by just 2N+1 measurement settings, (iii) a definition of an efficiently measurable degree of separability, which can be used for quantifying a novel aspect of decoherence of N qubits, and (iv) an explicit example that shows there are, for increasing N, genuinely N-partite entangled states lying closer and closer to the maximally mixed state. Moreover, we show that if the PI part of a state is k nonseparable, then so is the actual state. We further argue to add as requirement on any multipartite entanglement measure E that it satisfy E(ρ)≥E(ρ(PI)), even though the operation that maps ρ→ρ(PI) is not local.