Quantum mechanical complementarity ensures the security of thekey-distribution scheme reported by Brassard and Bennet in 1984 (BB84), butdoes not prohibit use of multi-photons as a signal carrier. We describe a novelBB84 scheme in which two nearly orthogonal coherent states carry the key, andthe superposition of these states (cat states) protects the communicationchannel from eavesdropping. Information leakage to eavesdroppers can bedetermined from the visibility of the interferential fringes in thedistribution of the outcome when a certain quadrature component is measuredthrough homodyne detection. The effect of channel loss and detectorinefficiency is discussed.
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