[en] This article discusses the role of covariance correlation tensor in the establishment of the criterion of quantum entanglement. It gives a simple example to show the powerfulness in the treatment of quantum dense coding , and illustrates the fact that this method also provides theoretical basis for establishing corresponding knotted pictures.

[en] Two schemes are proposed to realize the controlled remote preparation of an arbitrary four-qubit cluster-type state via a partially entangled channel. We construct ingenious measurement bases at the sender’s and the controller’s locations, which play a decisive role in the proposed schemes. The success probabilities can reach 50% and 100%, respectively. Compared with the previous proposals, the success probabilities are independent of the coefficients of the entangled channel. (paper)

[en] We study the dynamics of entanglement transfer in a system composed of two initially correlated three-level atoms, each located in a cavity interacting with its own reservoir. Instead of tracing out reservoir modes to describe the dynamics using the master equation approach, we consider explicitly the dynamics of the reservoirs. In this situation, we show that the entanglement is completely transferred from atoms to reservoirs. Although the cavities mediate this entanglement transfer, we show that under certain conditions, no entanglement is found in cavities throughout the dynamics. Considering the entanglement dynamics of interacting and noninteracting bipartite subsystems, we found time windows where the entanglement can only flow through interacting subsystems, depending on the system parameters.

[en] Pairwise correlation is really an important property for multi-qubit states. For the two-qubit X states extracted from Dicke states and their superposition states, we obtain a compact expression of the quantum discord by numerical check. We then apply the expression to discuss the quantum correlation of the reduced two-qubit states of Dicke states and their superpositions, and the results are compared with those obtained by entanglement of formation, which is a quantum entanglement measure. (general)

[en] Entanglement-assisted quantum codes are studied from classical matrix-product codes point of view. Four methods to construct Entanglement-assisted quantum codes from matrix-product codes are provided. These constructions are applied to obtain numerous new Entanglement-assisted quantum codes, some of which have good parameters.

[en] Parametric down-conversion has been studied as a source of entangled radiation (Lee et al 2008 J. Phys. B: At. Mol. Opt. Phys . 41 145504). We investigate and quantify the entanglement of this system when the initial cavity modes are taken as two-mode Gaussian states. We study the effect of nonclassicality, purity, noise and leakage through the cavity modes on the two-mode Gaussian state entanglement. (letter)

[en] We study the simplest optomechanical system with a focus on the bistable regime. The covariance matrix formalism allows us to study both cooling and entanglement in a unified framework. We identify two key factors governing entanglement; namely, the bistability parameter (i.e., the distance from the end of a stable branch in the bistable regime) and the effective detuning, and we describe the optimum regime where entanglement is greatest. We also show that, in general, entanglement is a nonmonotonic function of optomechanical coupling. This is especially important in understanding the optomechanical entanglement of the second stable branch.

[en] We show that the restricted shareability and distribution of multiqubit entanglement can be characterized by Tsallis-q entropy. We first provide a class of bipartite entanglement measures named Tsallis-q entanglement, and provide its analytic formula in two-qubit systems for 1≤q≤4. For 2≤q≤3, we show a monogamy inequality of multiqubit entanglement in terms of Tsallis-q entanglement, and we also provide a polygamy inequality using Tsallis-q entropy for 1≤q≤2 and 3≤q≤4.

[en] It is shown that while entanglement ensures difficulty in discriminating a set of mutually orthogonal states perfectly by local operations and classical communication (LOCC), entanglement content does not. In particular, for a class of entangled multiqubit states, the maximum number of perfectly LOCC distinguishable orthogonal states is shown to be independent of the average entanglement of the states, and the spatial configuration with respect to which LOCC operations may be carried out. It is also pointed out that for this class, the makeup of an ensemble, that is whether it consists only of entangled states or not, determines the maximum number of perfectly distinguishable states.

[en] We show that there are six inequivalent $4\times <!--\; ??\; -->4$ unextendible product bases (UPBs) of size eight, when we consider only 4-qubit product vectors. We apply our results to construct positive-partial-transpose entangled states of rank nine. They are at the same time 4-qubit, $2\times <!--\; ??\; -->2\times <!--\; ??\; -->4$ and $4\times <!--\; ??\; -->4$ states, and their ranges have product vectors. One of the six UPBs turns out to be orthogonal to an incompletely genuinely entangled space, in the sense that the latter does not contain $4\times <!--\; ??\; -->4$ product vector in any bipartition of 4-qubit systems. We also show that the multipartite UPB orthogonal to a genuinely entangled space exists if and only if the $n\times <!--\; ??\; -->n\times <!--\; ??\; -->n$ UPB orthogonal to a genuinely entangled space exists for some n. These results help understand an open problem in Demianowicz and Augusiak (Phys Rev A 98:012313, 2018).