  We present a protocol for generating multipartite quantum correlations across
a quantum network with a continuous-variable architecture. An arbitrary number
of users possess two-mode entangled states, keeping one mode while sending the
other to a central relay. Here a suitable multipartite Bell detection is
performed which conditionally generates a cluster state on the retained modes.
This cluster state can be suitably manipulated by the parties and used for
tasks of quantum communication in a fully optical scenario. More interestingly,
the protocol can be used to create a purely-mechanical cluster state starting
from a supply of optomechanical systems. We show that detecting the optical
parts of optomechanical cavities may efficiently swap entanglement into their
mechanical modes, creating cluster states up to 5 modes under suitable
cryogenic conditions.
