Adaptive Entanglement Witnessing for Two-Qubit Systems

Authors:Lev Gruber, Ria Haapala, Elio Thadhani, Paco Navarro, Eritas Yang, Benjamin Hartley, Rebecca Verghese, Oscar Scholin, Isabel Godoy, Stuart Kerr, Brayden Mendoza, Alec Roberson, Theresa W. Lynn

Abstract:Entanglement witnesses are a key instrument in determining the entanglement of unknown bipartite qubit systems without requiring a full tomography. Riccardi et al. (2020) proposed a set of two-qubit witnesses, denoted $\{W^3\}$, which require a set of local measurements in three bases. In this work we propose an additional set of witnesses, denoted $\{W^5\}$, that build on $\{W^3\}$ and require measurements in two additional bases. We present an adaptive two-step method that uses the measurement results obtained for $\{W^3\}$ to select which $W^5$ to evaluate. We computationally demonstrate that our adaptive two-step protocol can identify 72% of entangled states generated according to the methods of Roik et al. (2021) while requiring measurements in only five of the nine bases needed for full quantum tomography. Additionally, we experimentally verify the behavior of both sets of witnesses using polarization-entangled photon pairs.

arXiv:2610.16018 [quant-ph]

(or arXiv:2610.16018v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2610.16018 arXiv-issued DOI via DataCite (pending registration)