  We present a proof-of-principle experiment demonstrating measurement of the
collectibility, a nonlinear entanglement witness proposed by Rudnicki et al.
[Phys. Rev. Lett. 107, 150502 (2011)]. This entanglement witness works for both
mixed and pure two-qubit states. In the later case it can be used to measure
entanglement in terms of the negativity. We measured the collectibility for
three distinct classes of photonic polarization-encoded two-qubit states, i.e.,
maximally entangled, separable and maximally mixed states. We demonstrate that
the measurement procedure is feasible and robust against typical experimental
shortcomings such as imperfect two-photon indistinguishability.
