Quantum key distribution, first proposed by Bennett and Brassard, provides apossible key distribution scheme whose security depends only on the quantumlaws of physics. So far the protocol has been proved secure even under channelnoise and detector faults of the receiver, but is vulnerable if the photonsource used is imperfect. In this paper we propose and give a concrete designfor a new concept, {\it self-checking source}, which requires the manufacturerof the photon source to provide certain tests; these tests are designed suchthat, if passed, the source is guaranteed to be adequate for the security ofthe quantum key distribution protocol, even though the testing devices may notbe built to the original specification. The main mathematical result is astructural theorem which states that, for any state in a Hilbert space, ifcertain EPR-type equations are satisfied, the state must be essentially theorthogonal sum of EPR pairs.
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