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arXiv:0710.2389 · Quantum Information and Computation

Entanglement of formation from optimal decomposition

Limei Chen, Y.-X. Chen

Published Nov 1, 2008• 1 citation
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Abstract

We present a new method of constructing sets of states with known entanglement of formation. The method realizes the optimal decomposition families of states. We also derive the additivity of the entanglement of formation and thus calculate the entanglement cost. These results are used to find out the undistillable entanglement of some states. We illustrate our method by investigating the two-qubit state, the separable state, the maximally correlated state, the isotropic state and the Werner state. We also discuss the realization of Werner state in experiment.

DOI
10.26421/qic8.10-6

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We present a new method of constructing sets of states with known entanglement of formation. The method realizes the optimal decomposition families of states. We also derive the additivity of the entanglement of formation and thus calculate the entanglement cost. These results are used to find out the undistillable entanglement of some states. We illustrate our method by investigating the two-qubit state, the separable state, the maximally correlated state, the isotropic state and the Werner state. We also discuss the realization of Werner state in experiment.
An important measure of bipartite entanglement is the entanglement of formation, which is defined as the minimum average pure state entanglement of all decompositions realizing a given state. A decomposition which achieves this minimum is called an optimal decomposition. However, as for the entanglement of formation, there is not much known about the structure of such optimal decompositions, except for some special cases, like states of two qubits or isotropic states. Here we present a necessary and sufficient condition for a set of pure states of a finite dimensional bipartite system to form an optimal decomposition. This condition is well suited to treat the question, whether the entanglement of formation is additive or not.
We present a conjugate gradient method for calculating the entanglement of formation of arbitrary mixed quantum states of any dimension and with any bipartite division of the Hilbert space. The development of the gradient used by the algorithm, its implications for the number of states required in the optimal decomposition, and the way that conjugate gradient minimization has been adapted for this particular problem are outlined. We have found that the algorithm exhibits linear convergence for general mixed states, and that it correctly reproduces the known results for pairs of qubits and for isotropic states. The results of an example application of the code are discussed: calculating the entanglement of formation of a Psi+ Bell state of two qutrits when one of those qutrits is subject to various decoherence channels. The results for qutrits are contrasted with those for qubits: for the types of decoherence considered here, qutrit entanglement appears to be more robust than qubit entanglement.
Two families of bipartite mixed quantum states are studied for which it is proved that the number of members in the optimal-decomposition ensemble—the ensemble realizing the entanglement of formation—is greater than the rank of the mixed state. We find examples for which the number of states in this optimal ensemble can be larger than the rank by an arbitrarily large factor. In one case the proof relies on the fact that the partial transpose of the mixed state has zero eigenvalues; in the other case the result arises from the properties of product bases that are completable only by embedding in a larger Hilbert space.
Two families of bipartite mixed quantum states are studied for which it is proved that the number of members in the optimal-decomposition ensemble—the ensemble realizing the entanglement of formation—is greater than the rank of the mixed state. We find examples for which the number of states in this optimal ensemble can be larger than the rank by an arbitrarily large factor. In one case the proof relies on the fact that the partial transpose of the mixed state has zero eigenvalues; in the other case the result arises from the properties of product bases that are completable only by embedding in a larger Hilbert space.
Two families of bipartite mixed quantum states are studied for which it is proved that the number of members in the optimal-decomposition ensemble --- the ensemble realizing the entanglement of formation --- is greater than the rank of the mixed state. We find examples for which the number of states in this optimal ensemble can be larger than the rank by an arbitrarily large factor. In one case the proof relies on the fact that the partial transpose of the mixed state has zero eigenvalues; in the other case the result arises from the properties of product bases that are completable only by embedding in a larger Hilbert space.