Quantum key distribution with continuous variables

Summary form only given. Quantum cryptographic schemes use fundamental properties of quantum mechanics to ensure the protection of random number keys. In particular the act of measurement in quantum mechanics inevitably disturbs the system. Furthermore, for single quanta such as a photon, simultaneous measurements of non-commuting variables are forbidden. It is of practical and fundamental interest to quantum information research to investigate links between discrete variable, single photon phenomena and continuous variable, multiphoton effects. This motivates a consideration of quantum cryptography using multi-photon light modes. We consider encoding key information as small signals carried on the amplitude and phase quadratures of a light beam. Although simultaneous measurements of these noncommuting observables can be made, the information that can be obtained is strictly limited by the generalized uncertainty pritlciple. Hence any eavesdropper will inevitably introduce errors into the transmission.