Gharibian S, Santha M, Sikora J, Sundaram A, Yirka J. Quantum Generalizations of the Polynomial Hierarchy with Applications to QMA(2). In: Potapov I, Spirakis P, Worrell J, eds. 43rd International Symposium on Mathematical Foundations  of Computer Science (MFCS 2018). Vol 117. Leibniz International Proceedings in Informatics (LIPIcs). Schloss Dagstuhl - Leibniz-Zentrum fuer Informatik; 2018:58:1-58:16. doi:10.4230/LIPIcs.MFCS.2018.58

Gharibian S, Kempe J. Hardness of approximation for quantum problems. Quantum Information &; Computation. 2014;14(5-6):517-540.