International Journal of Quantum Information

We describe a method for private database queries using exchange of quantum states with bits encoded in mutually incompatible bases. For technology with limited coherence time, the database vendor can announce the encoding after a suitable delay to allow the user to privately learn one of two items in the database without the ability to also definitely infer the second item. This quantum approach also allows the user to choose to learn other functions of the items, such as the exclusive-or of their bits, but not to gain more information than equivalent to learning one item, on average. This method is especially useful for items consisting of a few bits by avoiding the substantial overhead of conventional cryptographic approaches.

Properties of the max-relative entropy of entanglement, defined in Ref. 10, are investigated, and its significance as an upper bound to the one-shot rate for perfect entanglement dilution, under a particular class of quantum operations, is discussed. It is shown that it is in fact equal to another known entanglement monotone, namely the log robustness, defined in Ref. 7. It is known that the latter is not asymptotically continuous and it is not known whether it is weakly additive. However, by suitably modifying the max-relative entropy of entanglement we obtain a quantity which is seen to satisfy both these properties. In fact, the modified quantity is shown to be equal to the regularized relative entropy of entanglement.

We propose a cavity-quantum-electrodynamics scheme for one-step generation of the special configuration of W-class state which can implement deterministic teleportation, superdense coding, quantum-information splitting, and phase-covariant telecloning. We also present a method for one-step realization of a nontrivial unitary transformation which can transform a standard W state into a fully separable state. The operation plays a key role in recently proposed quantum-information processing tasks. Both the schemes are robust against decoherence. In addition, they can endure the error of controlling the interaction time between atoms and cavity. Our ideas can also be generalized to other systems.

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

An exact analytical solution to the optimal unambiguous state discrimination involving two finite-dimensional coherent states that occur with given prior probabilities is presented using the Lewenstein–Sanpera decomposition method. Furthermore, a numerical method is advised for efficient solving of the unambiguous state discrimination of the two finite-dimensional coherent states with the same dimensions. In this manner, it is shown that the maximum success rate for the unambiguous states discrimination of the two finite-dimensional coherent states with the arbitrary prior probability is decreased by increasing the dimensionality of the finite-dimensional coherent states. Also, the success rate for the unambiguous states discrimination of the two coherent states satisfies the upper bound proportional to the fidelity of the states for a given prior probability.

Using differential phase shift and photon polarization, we propose two quantum key distribution schemes, i.e. a double-coding BB84 protocol and a double-coding BBM92 protocol. The information capacity of these protocols is twice of the original protocols and their security is ensured by the use of detection time slots, random choice of bases and quantum entanglement. These double-coding protocols are feasible with present technology.

We investigate the Lorentz transformation of the reduced helicity density matrix for a pair of massive spin 1/2 particles. The corresponding Wootters concurrence shows no invariant meaning, which implies that we can generate helicity entanglement simply by the transformation from one reference frame to another. The difference between the helicity and spin case is also discussed.

In this paper, we study the thermal entanglement in a two-qubit Heisenberg XYZ system with different Dzyaloshinskii–Moriya (DM) couplings. We show that different DM coupling parameters have different influences on the entanglement and the critical temperature. In addition, we find that when J_i (i-component spin coupling interaction) is the largest spin coupling coefficient, D_i (i-component DM interaction) is the most efficient DM control parameter, which can be obtained by adjusting the direction of DM interaction.

In this work we describe a protocol by which one can secretly broadcast W-type state among three distant partners. This work is interesting in the sense that we introduce a new kind of local cloning operation to generate two mixed three qubit entangled states between these partners from a W-type state initially shared by them.

We present a quantum deletion algorithm that deletes a marked basis-state from an even superposition of all N basis-states. This algorithm uses only a single query and achieves exponential speed-up compared with classical analog where Θ(N) queries are required.

We study the teleportation of the entangled state in quantum dots system coupled by a single-mode cavity field. By the use of the single-photon detection, spin measurement and Faraday rotation in QDs-cavity system, we perform a teleportation of tripartite GHZ-Like state encoded in the electron spins of a three-quantum-dot system. The success probability of the scheme can reach unity if the cavities are switchable to choice the appropriate Faraday rotation angle. The scheme of the teleportation can be easily generalized to an N-quantum-dot system.