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This paper has been withdrawn by the author due to a crucial problem associated with Figs. 2 and 3.

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This paper has been withdrawn by the author due to some errors.

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This paper has been withdrawn due to that the same conclusion has been proposed before.

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This paper has been withdrawn by the authors, due a oversimplified decoherence model.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.

This paper has been withdrawn by the author due to some errors.

The paper has been withdrawn by the author because the result obtained has been reported earlier by other authors.

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This paper has been withdrawn by the authors because the paper is largely revised and improved, and to appear in Mechanics Research Communications.

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This paper has been withdrawn by the author(s), due to double submission. You can find it under: physics/0208019

This paper has been withdrawn by the author due to the incorrect argument for the security.

This paper has been withdrawn by the authors.