We describe a quantum key-distribution scheme in which two nearly orthogonal coherent states carry the key, and the superposition of these states (cat states) protects the communication channel from eavesdropping. Any eavesdropping activity can be detected from the disappearance of the interferential fringes in the distribution of the outcome when a certain quadrature component is measured through homodyne detection. This scheme is secure against conclusive-measurement attack and generalized beamsplitter attack, both believed to be a potential risk when multi-photon states are used as a quantum signal.
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