We investigate Gaussian quantum states in view of their exceptional role
within the space of all continuous variables states. A general method for
deriving extremality results is provided and applied to entanglement measures,
secret key distillation and the classical capacity of Bosonic quantum channels.
We prove that for every given covariance matrix the distillable secret key rate
and the entanglement, if measured appropriately, are minimized by Gaussian
states. This result leads to a clearer picture of the validity of frequently
made Gaussian approximations. Moreover, it implies that Gaussian encodings are
optimal for the transmission of classical information through Bosonic channels,
if the capacity is additive.