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Complexity Lower Bounds for Computing the Approximately-Commuting Operator Value of Non-Local Games to High Precision

Authors Matthew Coudron , William Slofstra

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Matthew Coudron
  • Institute for Quantum Computing, University of Waterloo, Waterloo, Canada
William Slofstra
  • Institute for Quantum Computing and Department of Pure Mathematics, University of Waterloo, Waterloo, Canada


We thank Zhengfeng Ji, Alex Bredariol Grilo, Anand Natarajan, Thomas Vidick, and Henry Yuen for helpful discussions.

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Matthew Coudron and William Slofstra. Complexity Lower Bounds for Computing the Approximately-Commuting Operator Value of Non-Local Games to High Precision. In 34th Computational Complexity Conference (CCC 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 137, pp. 25:1-25:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)


We study the problem of approximating the commuting-operator value of a two-player non-local game. It is well-known that it is NP-complete to decide whether the classical value of a non-local game is 1 or 1- epsilon, promised that one of the two is the case. Furthermore, as long as epsilon is small enough, this result does not depend on the gap epsilon. In contrast, a recent result of Fitzsimons, Ji, Vidick, and Yuen shows that the complexity of computing the quantum value grows without bound as the gap epsilon decreases. In this paper, we show that this also holds for the commuting-operator value of a game. Specifically, in the language of multi-prover interactive proofs, we show that the power of MIP^{co}(2,1,1,s) (proofs with two provers, one round, completeness probability 1, soundness probability s, and commuting-operator strategies) can increase without bound as the gap 1-s gets arbitrarily small. Our results also extend naturally in two ways, to perfect zero-knowledge protocols, and to lower bounds on the complexity of computing the approximately-commuting value of a game. Thus we get lower bounds on the complexity class PZK-MIP^{co}_{delta}(2,1,1,s) of perfect zero-knowledge multi-prover proofs with approximately-commuting operator strategies, as the gap 1-s gets arbitrarily small. While we do not know any computable time upper bound on the class MIP^{co}, a result of the first author and Vidick shows that for s = 1-1/poly(f(n)) and delta = 1/poly(f(n)), the class MIP^{co}_delta(2,1,1,s), with constant communication from the provers, is contained in TIME(exp(poly(f(n)))). We give a lower bound of coNTIME(f(n)) (ignoring constants inside the function) for this class, which is tight up to polynomial factors assuming the exponential time hypothesis.

Subject Classification

ACM Subject Classification
  • Theory of computation → Quantum complexity theory
  • Quantum complexity theory
  • Non-local game
  • Multi-prover interactive proof
  • Entanglement


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