1,316 research outputs found
Entanglement molecules
We investigate the entanglement properties of multiparticle systems,
concentrating on the case where the entanglement is robust against disposal of
particles. Two qubits -belonging to a multipartite system- are entangled in
this sense iff their reduced density matrix is entangled. We introduce a family
of multiqubit states, for which one can choose for any pair of qubits
independently whether they should be entangled or not as well as the relative
strength of the entanglement, thus providing the possibility to construct all
kinds of ''Entanglement molecules''. For some particular configurations, we
also give the maximal amount of entanglement achievable.Comment: 4 pages, 1 figur
Interior points of the completely positive cone.
A matrix A is called completely positive if it can be decomposed as A = BB^T with an entrywise nonnegative matrix B. The set of all such matrices is a convex cone. We provide a characterization of the interior of this cone as well as of its dual
Separable states to distribute entanglement
It was shown that two distant particles can be entangled by sending a third
particle never entangled with the other two [T. S. Cubitt et al., Phys. Rev.
Lett. 91, 037902 (2003)]. In this paper, we investigate a class of three-qubit
separable states to distribute entanglement by the same way, and calculate the
maximal amount of entanglement which two particles of separable states in the
class can have after applying the way.Comment: 4 pages, no figures, Revised argumen
Equivalence classes of non-local unitary operations
We study when a multipartite non--local unitary operation can
deterministically or probabilistically simulate another one when local
operations of a certain kind -in some cases including also classical
communication- are allowed. In the case of probabilistic simulation and
allowing for arbitrary local operations, we provide necessary and sufficient
conditions for the simulation to be possible. Deterministic and probabilistic
interconversion under certain kinds of local operations are used to define
equivalence relations between gates. In the probabilistic, bipartite case this
induces a finite number of classes. In multiqubit systems, however, two unitary
operations typically cannot simulate each other with non-zero probability of
success. We also show which kind of entanglement can be created by a given
non--local unitary operation and generalize our results to arbitrary operators.Comment: (1) 9 pages, no figures, submitted to QIC; (2) reference added, minor
change
Quantum simulation of classical thermal states
We establish a connection between ground states of local quantum Hamiltonians
and thermal states of classical spin systems. For any discrete classical
statistical mechanical model in any spatial dimension, we find an associated
quantum state such that the reduced density operator behaves as the thermal
state of the classical system. We show that all these quantum states are unique
ground states of a universal 5-body local quantum Hamiltonian acting on a
(polynomially enlarged) system of qubits arranged on a 2D lattice. The only
free parameters of the quantum Hamiltonian are coupling strengthes of two-body
interactions, which allow one to choose the type and dimension of the classical
model as well as the interaction strength and temperature.Comment: 4 pages, 1 figur
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