  Recently, Meyer and Wallach [D.A. Meyer and N.R. Wallach (2002), J. of Math.
Phys., 43, pp. 4273] proposed a measure of multi-qubit entanglement that is a
function on pure states. We find that this function can be interpreted as a
physical quantity related to the average purity of the constituent qubits and
show how it can be observed in an efficient manner without the need for full
quantum state tomography. A possible realization is described for measuring the
entanglement of a chain of atomic qubits trapped in a 3D optical lattice.
