Quantum capacity of lossy channel with additive classical Gaussian noise : a perturbation approach

For a quantum channel of additive Gaussian noise with loss, in the general case of $n$ copies input, we show that up to first order perturbation, any non-Gaussian perturbation to the product thermal state input has a less quantum information transmission rate when the input energy tend to infinitive.Comment: 4 page

Quantum capacity of channel with thermal noise

The quantum capacity of thermal noise channel is studied. The extremal input state is obtained at the postulation that the coherent information is convex or concave at its vicinity. When the input energy tends to infinitive, it is verified by perturbation theory that the coherent information reaches its maximum at the product of identical thermal state input. The quantum capacity is obtained for lower noise channel and it is equal the one shot capacity.Comment: 5 page

Perfect A/D conversion of entanglement

We investigate how entanglement can be perfectly transfered between continuous variable and qubits system. We find that a two-mode squeezed vacuum state can be converted to the product state of an infinitive number of two-qubit states while keeping the entanglement. The reverse process is also possible. The interaction Hamitonian is a kind of non-linear Jaynes-Cumings Hamiltonian.Comment: 3 pages, 1 figur

Gaussian relative entropy of entanglement

For two gaussian states with given correlation matrices, in order that relative entropy between them is practically calculable, I in this paper describe the ways of transforming the correlation matrix to matrix in the exponential density operator. Gaussian relative entropy of entanglement is proposed as the minimal relative entropy of the gaussian state with respect to separable gaussian state set. I prove that gaussian relative entropy of entanglement achieves when the separable gaussian state is at the border of separable gaussian state set and inseparable gaussian state set. For two mode gaussian states, the calculation of gaussian relative entropy of entanglement is greatly simplified from searching for a matrix with 10 undetermined parameters to 3 variables. The two mode gaussian states are classified as four types, numerical evidence strongly suggests that gaussian relative entropy of entanglement for each type is realized by the separable state within the same type.For symmetric gaussian state it is strictly proved that it is achieved by symmetric gaussian state.Comment: 12 pages, 3 figure

Locally inequivalent four qubit hypergraph states

Hypergraph states as real equally weighted pure states are important resources for quantum codes of non-local stabilizer. Using local Pauli equivalence and permutational symmetry, we reduce the 32768 four qubit real equally weighted pure states to 28 locally inequivalent hypergraph states and several graph states. The calculation of geometric entanglement supplemented with entanglement entropy confirms that further reduction is impossible for true hypergraph states.Comment: 6 pages,2 figures, Accepted by J. Phys.

On Graded Bialgebra Deformations

We introduce the graded bialgebra deformations, which explain Andruskiewitsch-Schneider's liftings method. We also relate this graded bialgebra deformation with the corresponding graded bialgebra cohomology groups, which is the graded version of the one due to Gerstenhaber-Schack.Comment: Presented in AsiaLink Conference and Mistakes changed. To appear in Alg. Coll

Entanglement of stabilizer codewords

The geometric measure, the logarithmic robustness and the relative entropy of entanglement are proved to be equal for a stabilizer quantum codeword. The entanglement upper and lower bounds are determined with the generators of code. The entanglement of dual-containing CSS codes, Gottesman codes and the related codes are given. An iterative algorithm is developed to determine the exact value of the entanglement when the two bounds are not equal.Comment: 9 page

A hierarchy of entanglement criteria for four qubit symmetric Greenberger-Horne-Zeilinger diagonal states

With a two step optimization method of entanglement witness, we analytically propose a set of necessary and sufficient entanglement criteria for four qubit symmetric Greenberger-Horne-Zeilinger (GHZ) diagonal states. The criterion set contains four criteria. Two of them are linear with density matrix elements. The other two criteria are nonlinear with density matrix elements. The criterion set has a nest structure. A proper subset of the criteria is necessary and sufficient for the entanglement of a proper subset of the states. We illustrate the nest structure of criterion set with the general Werner state set and its superset the highly symmetric GHZ diagonal state set, they are subsets of the symmetric GHZ diagonal state set.Comment: 10 pages, 3 figure

Matched witness for multipartite entanglement

We transform the way of finding entanglement criterion into two steps: to obtain necessary criterion of separability by maximizing an algebra function for a set of characteristic variables of the witness operator and the given number of partitions, then to obtain the sufficient criterion by minimizing an algebra function with respect to the characteristic variables for a given quantum state. Our method avoids the semi-definite program calculation in the witness operator entanglement detection. The necessary and sufficient criterion of separability for the three qubit X shaped state is given to illustrate the procedure of finding the criterion. We give the necessary and sufficient criteria of the three partite and full separabilities for the four qubit noisy GHZ state and the four qubit noisy cluster state.Comment: 8 pages, no figure

The entanglement of some non-two-colorable graph states

We exactly evaluate the entanglement of a six vertex and a nine vertex graph states which correspond to non ''two-colorable'' graphs. The upper bound of entanglement for five vertices ring graph state is improved to 2.9275, less than upper bound determined by LOCC. An upper bound of entanglement is proposed based on the definition of graph state.Comment: 6 pages, 1 figur