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Quantum Generalizations of the Polynomial Hierarchy with Applications to QMA(2)

Authors Sevag Gharibian, Miklos Santha, Jamie Sikora, Aarthi Sundaram, Justin Yirka

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Sevag Gharibian
  • University of Paderborn, Paderborn, North Rhine-Westphalia, Germany, and Virginia Commonwealth University, Richmond, Virginia, USA
Miklos Santha
  • CNRS, IRIF, Université Paris Diderot, Paris, France and Centre for Quantum Technologies, National University of Singapore, Singapore
Jamie Sikora
  • Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada
Aarthi Sundaram
  • Joint Center for Quantum Information and Computer Science, University of Maryland, College Park, Maryland, USA
Justin Yirka
  • Virginia Commonwealth University, Richmond, Virginia, USA

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Sevag Gharibian, Miklos Santha, Jamie Sikora, Aarthi Sundaram, and Justin Yirka. Quantum Generalizations of the Polynomial Hierarchy with Applications to QMA(2). In 43rd International Symposium on Mathematical Foundations of Computer Science (MFCS 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 117, pp. 58:1-58:16, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2018)


The polynomial-time hierarchy (PH) has proven to be a powerful tool for providing separations in computational complexity theory (modulo standard conjectures such as PH does not collapse). Here, we study whether two quantum generalizations of PH can similarly prove separations in the quantum setting. The first generalization, QCPH, uses classical proofs, and the second, QPH, uses quantum proofs. For the former, we show quantum variants of the Karp-Lipton theorem and Toda's theorem. For the latter, we place its third level, Q Sigma_3, into NEXP using the Ellipsoid Method for efficiently solving semidefinite programs. These results yield two implications for QMA(2), the variant of Quantum Merlin-Arthur (QMA) with two unentangled proofs, a complexity class whose characterization has proven difficult. First, if QCPH=QPH (i.e., alternating quantifiers are sufficiently powerful so as to make classical and quantum proofs "equivalent"), then QMA(2) is in the Counting Hierarchy (specifically, in P^{PP^{PP}}). Second, unless QMA(2)= Q Sigma_3 (i.e., alternating quantifiers do not help in the presence of "unentanglement"), QMA(2) is strictly contained in NEXP.

Subject Classification

ACM Subject Classification
  • Theory of computation → Quantum complexity theory
  • Theory of computation → Complexity classes
  • Theory of computation → Semidefinite programming
  • Complexity Theory
  • Quantum Computing
  • Polynomial Hierarchy
  • Semidefinite Programming
  • QMA(2)
  • Quantum Complexity


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