Long Distance Continuous-Variable Quantum Key Distribution with a Gaussian Modulation

We designed high-efficiency error correcting codes allowing to extract an errorless secret key in a continuous-variable quantum key distribution protocol using a Gaussian modulation of coherent states and a homodyne detection. These codes are available for a wide range of signal-to-noise ratios on an AWGN channel with a binary modulation and can be combined with a multidimensional reconciliation method proven secure against arbitrary collective attacks. This improved reconciliation procedure considerably extends the secure range of a continuous-variable quantum key distribution with a Gaussian modulation, giving a secret key rate of about 10^{-3} bit per pulse at a distance of 120 km for reasonable physical parameters.