  We present a proposal for entanglement purification of the
continuous-variable quantum state of two propagating optical fields. The scheme
is based on letting each field interact with a local node, an atomic ensemble,
whose internal collective excitation plays the role of an ancillary continuous
variable resource. Entanglement purification is achieved by a dichotomic
measurement, representing the required non-Gaussian element, and which consists
in detecting the presence or absence of collective excitations in the atomic
ensemble. This scheme can be extended to networks, where the nodes are single
trapped atoms, and constitutes an important building block for the
implementation of a continuous-variable quantum repeater.
