  We propose a simple setup for the conversion of multipartite entangled states
in a quantum network with restricted access. The scheme uses nonlocal
operations to enable the preparation of states that are inequivalent under
local operations and classical communication, but most importantly does not
require full access to the states. It is based on a flexible linear optical
conversion gate that uses photons, which are ideally suited for distributed
quantum computation and quantum communication in extended networks. In order to
show the basic working principles of the gate, we focus on converting a
four-qubit entangled cluster state to other locally inequivalent four-qubit
states, such as the GHZ and symmetric Dicke state. We also show how the gate
can be incorporated into extended graph state networks, and can be used to
generate variable entanglement and quantum correlations without entanglement
but nonvanishing quantum discord.
