(Physics.aps.org) Anthony Leverrier at Inria in France and colleagues have provided theoretical arguments for the security of continuous-variable (CV) QKD, a version that relies on interferometric detection schemes widely used in high-speed telecom networks. The result suggests that CVQKD holds potential for large-scale deployment in such networks.
Most protocols for quantum key distribution (QKD)—a quantum cryptography technique—encode an encryption key in discrete quantum states, such as the two orthogonal polarizations of a photon. Instead of discrete polarization states, CVQKD uses coherent quantum states that can be encoded in continuous variables like the amplitude and phase of an optical signal traveling through a fiber.
Leverrier and his team analyzed a CVQKD protocol that uses only four quantum states, which can be reliably prepared with available optical technology. Their analysis shows that this protocol could securely transmit a secret key over distances ranging from 50 to 150 km, depending on the noise in the transmission channel.

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