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Quantum Cryptography Based on Two Mixed States

1996; American Physical Society; Volume: 77; Issue: 10 Linguagem: Inglês

10.1103/physrevlett.77.2137

1092-0145

Masato Koashi, Nobuyuki Imoto,

Quantum Computing Algorithms and Architecture

Although it is known that any two nonorthogonal pure states can constitute a secure quantum cryptosystem, the generalization of this scheme to the use of two mixed states is not trivial. It is shown here that even if a condition corresponding to the nonorthogonality in the pure-state case is satisfied, the mixed-state cryptosystem is still vulnerable to attack by an eavesdropper. A necessary and sufficient condition for the secure communication is derived. It states that the two mixed states must be connected by a rotation operator with a nonorthogonal angle.