Related papers: The four-qubit singlet state and decoherence-free …

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This paper has been withdrawn by the authors.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This paper has been withdrawn due to security loophole. We thank Dr G. P. he for pointing this to us.

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