Experimental measurement of collective nonlinear entanglement witness for two qubits

We present a proof-of-principle experiment demonstrating measurement of the collectibility, a nonlinear entanglement witness proposed by L. Rudnicki et al. [Phys. Rev. Lett. 107, 150502 ( 2011)]. This entanglement witness works for both mixed and pure two-qubit states. In the latter 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.We present a proof-of-principle experiment demonstrating measurement of the collectibility, a nonlinear entanglement witness proposed by L. Rudnicki et al. [Phys. Rev. Lett. 107, 150502 ( 2011)]. This entanglement witness works for both mixed and pure two-qubit states. In the latter 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.