Invisible transmission in quantum cryptography using continuous variables: A proof of Eve’s vulnerability

By analogy to classical cryptography, we develop a quantum cryptographic scheme in which the two public and private keys consist in each of two entangled beams of squeezed light. An analog secret information is encrypted by modulating the phase of the beam sent in public. The knowledge of the degree of nonclassical correlation between the beam quadratures measured in private and in public allows only the receiver to decrypt the secret information. Finally, in a view towards absolute security, we formally prove that any external intervention of an eavesdropper makes him vulnerable to any subsequent detection.