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The two qubit (spin 1/2) singlet state has invariant perfect EPR correlations: if two observers measure {\em any} identical observables the results are always perfectly (anti-)correlated. We show that no such correlations exists in pure or mixed bipartite quantum states if . This points that two qubit singlets are high-quality quantum information carriers in long distance transmission: their correlations are unaffected by identical random unitary transformations of both qubits. Pairs of entangled quits, , do not have this property. Thus, quit properties should be rather exploited only in protocols in which transmission is not a long distance one.

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