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Quantum mechanical entanglement is a resource for quantum computation,quantum teleportation, and quantum cryptography. The ability to quantify thisresource correctly has thus become of great interest to those working in thefield of quantum information theory. In this paper, we show that all existingentanglement measures but one fail important tests of fitness when applied to nparticle, m site states of indistinguishable particles, where n,m>=2. Theaccepted method of measuring the entanglement of a bipartite system ofdistinguishable particles is to use the von Neumann entropy of the reduceddensity matrix of one half of the system. We show that expressing the fulldensity matrix using a site-spin occupation number basis, and reducing withrespect to that basis, gives an entanglement which meets all currently knownfitness criteria for systems composed of either distinguishable orindistinguishable particles.  We consider an output state from a previously published thought experiment, astate which is entangled in both spin and spatial degrees of freedom, and showthat the site entropy measure gives the correct total entanglement. We alsoshow how the spin-space entanglement transfer occurring within the apparatuscan be understood in terms of the transfer of probability from single-occupancyto double-occupancy sectors of the density matrix.Comment: 2 figures; added Appendix A; added Figure 2; made changes to take  account of v2 of quant-ph/0105120; some typos remove

Describing mixed spin-space entanglement of pure states of indistinguishable particles using an occupation-number basis