Related papers: Optimal time evolution in (non)hermitian quantum m…

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This paper has been withdrawn by the author. Proofs of two propositions need more details.

This paper has been withdrawn by the author due to a crucial error in eq.59. I apologize for the inconveniences.

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This paper has been withdrawn by the authors. Significantly revised versions of the results of this paper are now available in arXiv:0707.0487v2 and arXiv:0808.3169v1.

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This paper has been withdrawn by the authors, due a oversimplified decoherence model. It will be substituted by a new work.