  Global conservation laws imply superselection rules (SSR) which restrict the
operations that are possible on any given state. Imposing the additional
constraint of local operations and classical communication (LOCC) forbids the
transfer of quantum systems between spatially separated sites. In the case of
particle conservation this imposes a SSR for local particle number. That is,
the coherences between subspaces of fixed particle number at each site are not
accessible and any state is therefore equivalent to its projection onto these
subspaces. The accessible entanglement under the SSR is less than (or equal to)
that available in the absence of the SSR. An ancilla can be used as a reference
system to increase the amount of accessible entanglement. We examine the
relationship between local particle number uncertainty and the accessible
entanglement and consider the optimal reference states for recovering
entanglement from certain systems. In particular we derive the optimal ancilla
state for extracting entanglement for a single shared particle and make steps
towards the optimum for general systems.
