  So-called direct measurements of entanglement are collective measurements on
multiple copies of a (bipartite or multipartite) quantum system that directly
provide one a value for some entanglement measure, such as the concurrence for
bipartite states. Multiple copies are needed since the entanglement of a mixed
state is not a linear function of the density matrix. Unfortunately, so far all
experimental implementations of direct measurements made unverified assumptions
about the form of the states, and, therefore, do not qualify as entanglement
verification tests. I discuss how a direct measurement can be turned into a
quantitative entanglement verification test by exploiting a recent theorem by
Renner (R. Renner, Nature Physics 3, 645 (2007)).

Quantum Physics