  Quantum mechanical entanglement is a resource for quantum computation,
quantum teleportation, and quantum cryptography. The ability to quantify this
resource correctly has thus become of great interest to those working in the
field of quantum information theory. In this paper, we show that all existing
entanglement measures but one fail important tests of fitness when applied to n
particle, m site states of indistinguishable particles, where n,m>=2. The
accepted method of measuring the entanglement of a bipartite system of
distinguishable particles is to use the von Neumann entropy of the reduced
density matrix of one half of the system. We show that expressing the full
density matrix using a site-spin occupation number basis, and reducing with
respect to that basis, gives an entanglement which meets all currently known
fitness criteria for systems composed of either distinguishable or
indistinguishable particles.
  We consider an output state from a previously published thought experiment, a
state which is entangled in both spin and spatial degrees of freedom, and show
that the site entropy measure gives the correct total entanglement. We also
show how the spin-space entanglement transfer occurring within the apparatus
can be understood in terms of the transfer of probability from single-occupancy
to double-occupancy sectors of the density matrix.
