4 research outputs found
Multipartite entanglement in 2 x 2 x n quantum systems
We classify multipartite entangled states in the 2 x 2 x n (n >= 4) quantum
system, for example the 4-qubit system distributed over 3 parties, under local
filtering operations. We show that there exist nine essentially different
classes of states, and they give rise to a five-graded partially ordered
structure, including the celebrated Greenberger-Horne-Zeilinger (GHZ) and W
classes of 3 qubits. In particular, all 2 x 2 x n-states can be
deterministically prepared from one maximally entangled state, and some
applications like entanglement swapping are discussed.Comment: 9 pages, 3 eps figure
Entanglement, Mixedness, and Spin-Flip Symmetry in Multiple-Qubit Systems
A relationship between a recently introduced multipartite entanglement
measure, state mixedness, and spin-flip symmetry is established for any finite
number of qubits. It is also shown that, within those classes of states
invariant under the spin-flip transformation, there is a complementarity
relation between multipartite entanglement and mixedness. A number of example
classes of multiple-qubit systems are studied in light of this relationship.Comment: To appear in Physical Review A; submitted 14 May 200
A multi-photon Stokes-parameter invariant for entangled states
We consider the Minkowskian norm of the n-photon Stokes tensor, a scalar
invariant under the group realized by the transformations of stochastic local
quantum operations and classical communications (SLOCC). This invariant is
offered as a candidate entanglement measure for n-qubit states and discussed in
relation to measures of quantum state entanglement for certain important
classes of two-qubit and three-qubit systems. This invariant can be directly
estimated via a quantum network, obviating the need to perform laborious
quantum state tomography. We also show that this invariant directly captures
the extent of entanglement purification due to SLOCC filters.Comment: 9 pages, 0 figures, Accepted for publication in Physical Review
Tensor of coherences parameterization of multiqubit density operators and its use in detecting entanglement
The tensor of coherences is a real parameterization obtained by juxtaposition of affine Bloch vectors for multiqubit densities. The Stokes tensorial structure highlights pattern of classical and quantum correlations between subsystems. The tensorial formalism provides interpretation of NPT and bound entanglement corresponding to linear combinations of tensor products in which one of the factors is not well-defined density as its Bloch vector is very high. The explicit mixtures for families of separable states is feasible for few-qubit symmetric densities and provides evidence of entanglement for such densities