Universal N-partite d-level pure-state entanglement witness based on realistic measurement settings

Sciara, Stefania and Reimer, Christian and Kues, Michael and Roztocki, Piotr and Cino, Alfonso and Moss, David J. and Caspani, Lucia and Munro, William J. and Morandotti, Roberto (2019) Universal N-partite d-level pure-state entanglement witness based on realistic measurement settings. Physical Review Letters, 122 (12). 120501. ISSN 1079-7114 (https://doi.org/10.1103/PhysRevLett.122.120501)

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Entanglement witnesses are operators that are crucial for confirming the generation of specific quantum systems, such as multipartite and high-dimensional states. For this reason, many witnesses have been theoretically derived which commonly focus on establishing tight bounds and exhibit mathematical compactness as well as symmetry properties similar to that of the quantum state. However, for increasingly complex quantum systems, established witnesses have lacked experimental achievability, as it has become progressively more challenging to design the corresponding experiments. Here, we present a universal approach to derive entanglement witnesses that are capable of detecting the presence of any targeted complex pure quantum system and that can be customized towards experimental restrictions or accessible measurement settings. Using this technique, we derive experimentally optimized witnesses that are able to detect multipartite d-level cluster states, and that require only two measurement settings. We present explicit examples for customizing the witness operators given different realistic experimental restrictions, including witnesses for high-dimensional entanglement that use only two-dimensional projection measurements. Our work enables us to confirm the presence of probed quantum states using methods that are compatible with practical experimental realizations in different quantum platforms.