Experimental Demonstration of Adaptive Entanglement Witnessing for Two-Qubit Pure and Mixed States

Efficient detection of quantum entanglement is vital for secure quantum communication and quantum information applications. We experimentally verify the efficacy of an adaptive entanglement witnessing procedure on two qubits that utilizes local measurements in fewer bases than are necessary for full state tomography. We begin with six classes of witnesses, {W}, formulated by Riccardi et al (Phys. Rev. A 101, 062319, 2020), and construct 3 additional triplet sets of optimal entanglement witnesses, {W'}, that each require local measurements in two further bases. Our group found that the adaptive choice method detects 72% of computationally generated random two-qubit entangled states while requiring only 56% of a full tomography. Using polarization-entangled photons generated via spontaneous parametric down conversion, we create and measure both pure and mixed two-qubit states. Here, we demonstrate the success of each {W'} triplet in the detection of states which go undetected by the {W} witnesses, showing the success of our two-step adaptive procedure for efficiently detecting entanglement.