Norbert
Lütkenhaus
Chen, J. ., Ji, Z. ., Kribs, D. ., Lütkenhaus, N. ., & Zeng, B. . (2014). Symmetric Extension of Two-Qubit States. Physical Review A, 90. http://doi.org/10.1103/PhysRevA.90.032318 (Original work published September 2014)