  Quantum key distribution (QKD) enables the establishment of secret keys
between users connected via a channel vulnerable to eavesdropping, with
information-theoretic security, that is, independently of the power of a
malevolent party. QKD systems based on the encoding of the key information on
continuous variables (CV), such as the values of the quadrature components of
coherent states, present the major advantage that they only require standard
telecommunication technology. However, the most general security proofs for
CV-QKD required until now the use of Gaussian modulation by the transmitter,
complicating practical implementations. Here, we experimentally implement a
protocol that allows for arbitrary, Gaussian-like, discrete modulations, whose
security is based on a theoretical proof that applies very generally to such
situations. These modulation formats are compatible with the use of powerful
tools of coherent optical telecommunication, allowing our system to reach a
performance of tens of megabit per second secret key rates over 25 km.
