The only known qubit states that produce maximal quantum violation of a tight Bell inequality with maximally incompatible local measurements are Bell states, Greenberger-Horne-Zeilinger states, and a recently identified three-qubit state called $|S\rangle$. Here we report the results of an experiment for preparing $|S\rangle$ and testing the maximum quantum violation of the corresponding tripartite Bell inequality. Using a heralded source of three entangled photons and three tunable polarization-dependent filters, we experimentally prepare $|S\rangle$ with 0.924 fidelity. Using maximally incompatible measurements for the three parties, we observe a value of $T_{26}=7.30\pm 0.06$, which clearly violates the tight Bell inequality $T_{26}\le 5$. In addition, we show that our setup allows preparing multipartite states that were inaccessible with previous methods and are useful for quantum information and metrology.

• Received 24 August 2017
• Revised 29 January 2019

DOI:https://doi.org/10.1103/PhysRevA.100.022104