  We describe a direct method for experimental determination of the negativity
of an arbitrary two-qubit state with 11 measurements performed on multiple
copies of the two-qubit system. Our method is based on the experimentally
accessible sequences of singlet projections performed on up to four qubit
pairs. In particular, our method permits the application of the Peres-Horodecki
separability criterion to an arbitrary two-qubit state. We explicitly
demonstrate that measuring entanglement in terms of negativity requires three
measurements more than detecting two-qubit entanglement. The reported minimal
set of interferometric measurements provides a complete description of
bipartite quantum entanglement in terms of two-photon interference. This set is
smaller than the set of 15 measurements needed to perform a complete quantum
state tomography of an arbitrary two-qubit system. Finally, we demonstrate that
the set of 9 Makhlin's invariants needed to express the negativity can be
measured by performing 13 multicopy projections. We demonstrate that these
invariants are both a useful theoretical concept for designing specialized
quantum interferometers and that their direct measurement within the framework
of linear optics does not require performing complete quantum state tomography.
