6 research outputs found
Optimal teleportation with a mixed state of two qubits
We consider a single copy of a mixed state of two qubits and derive the
optimal trace-preserving local operations assisted by classical communication
(LOCC) such as to maximize the fidelity of teleportation that can be achieved
with this state. These optimal local operations turn out to be implementable by
one-way communication, and always yields a teleportation fidelity larger than
2/3 if the original state is entangled. This maximal achievable fidelity is an
entanglement measure and turns out to quantify the minimal amount of mixing
required to destroy the entanglement in a quantum state.Comment: 5 pages, expanded version of part II of quant-ph/0203073(v2
Four qubits can be entangled in nine different ways
We consider a single copy of a pure four-partite state of qubits and
investigate its behaviour under the action of stochastic local quantum
operations assisted by classical communication (SLOCC). This leads to a
complete classification of all different classes of pure states of four-qubits.
It is shown that there exist nine families of states corresponding to nine
different ways of entangling four qubits. The states in the generic family give
rise to GHZ-like entanglement. The other ones contain essentially 2- or 3-qubit
entanglement distributed among the four parties. The concept of concurrence and
3-tangle is generalized to the case of mixed states of 4 qubits, giving rise to
a seven parameter family of entanglement monotones. Finally, the SLOCC
operations maximizing all these entanglement monotones are derived, yielding
the optimal single copy distillation protocol
Causal structure of the entanglement renormalization ansatz
We show that the multiscale entanglement renormalization ansatz (MERA) can be
reformulated in terms of a causality constraint on discrete quantum dynamics.
This causal structure is that of de Sitter space with a flat spacelike
boundary, where the volume of a spacetime region corresponds to the number of
variational parameters it contains. This result clarifies the nature of the
ansatz, and suggests a generalization to quantum field theory. It also
constitutes an independent justification of the connection between MERA and
hyperbolic geometry which was proposed as a concrete implementation of the
AdS-CFT correspondence
The Origin of Separable States and Separability Criteria from Entanglement-breaking Channels
In this paper, we show that an arbitrary separable state can be the output of
a certain entanglement-breaking channel corresponding exactly to the input of a
maximally entangled state. A necessary and sufficient separability criterion
and some sufficient separability criteria from entanglement-breaking channels
are given.Comment: EBCs with trace-preserving and EBCs without trace-preserving are
separately discusse
Optimal enhancement of the fidelity of two qubits
We consider a single copy of a mixed state of two qubits and prove tight lower and upper bounds for its fidelity in terms of its concurrence or negativity. Next we derive the optimal filtering operations and the optimal trace-preserving local operations assisted by classical communication (LOCC) such as to maximize the fidelity of a given copy of a mixed state of two qubits. The optimal LOCC operations turn out to be very simple and implementable by 1-LOCC protocols, and always yield a fidelity larger then 1/2 if the original state was entangled