Continuous variable quantum cryptography using a bidirectional quantum channel

In recent years, quantum cryptography has been developed into the continuous variable framework where it has been shown to fully exploit the potentialities of quantum optics. In this framework, we introduce novel “transform and measure” protocols which generalize, and are proven to outperform, the previous “prepare and measure” protocols. In these new protocols the secret information is encoded via random unitary transformations onto a quantum state which is transmitted forward and backward by the trusted parties. Thanks to this multiple quantum communication, Alice and Bob make an iterated use of the uncertainty principle which may lead to a security enhancement. Our analysis investigates the simplest and non-trivial transform and measure protocol, i.e. the ones based on a bidirectional use of a quantum channel, and show a superadditive behavior of the security threshold.