Abstract
Entangled games are a quantum analog of constraint satisfaction problems and have had important applications to quantum complexity theory, quantum cryptography, and the foundations of quantum mechanics. Given a game, the basic computational problem is to compute its entangled value: the supremum success probability attainable by a quantum strategy. We study the complexity of computing the (commutingoperator) entangled value omega^* of entangled XOR games with any number of players. Based on a duality theory for systems of operator equations, we introduce necessary and sufficient criteria for an XOR game to have omega^* = 1, and use these criteria to derive the following results:
1) An algorithm for symmetric games that decides in polynomial time whether omega^* = 1 or omega^* < 1, a task that was not previously known to be decidable, together with a simple tensorproduct strategy that achieves value 1 in the former case. The only previous candidate algorithm for this problem was the NavascuésPironioAcín (also known as noncommutative Sum of Squares or ncSoS) hierarchy, but no convergence bounds were known.
2) A family of games with three players and with omega^* < 1, where it takes doubly exponential time for the ncSoS algorithm to witness this. By contrast, our algorithm runs in polynomial time.
3) Existence of an unsatisfiable phase for random (nonsymmetric) XOR games. We show that there exists a constant C_k^{unsat} depending only on the number k of players, such that a random kXOR game over an alphabet of size n has omega^* < 1 with high probability when the number of clauses is above C_k^{unsat} n.
4) A lower bound of Omega(n log(n)/log log(n)) on the number of levels in the ncSoS hierarchy required to detect unsatisfiability for most random 3XOR games. This is in contrast with the classical case where the (3n)^{th} level of the sumofsquares hierarchy is equivalent to bruteforce enumeration of all possible solutions.
BibTeX  Entry
@InProceedings{benewatts_et_al:LIPIcs:2018:10103,
author = {Adam Bene Watts and Aram W. Harrow and Gurtej Kanwar and Anand Natarajan},
title = {{Algorithms, Bounds, and Strategies for Entangled XOR Games}},
booktitle = {10th Innovations in Theoretical Computer Science Conference (ITCS 2019)},
pages = {10:110:18},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {9783959770958},
ISSN = {18688969},
year = {2018},
volume = {124},
editor = {Avrim Blum},
publisher = {Schloss DagstuhlLeibnizZentrum fuer Informatik},
address = {Dagstuhl, Germany},
URL = {http://drops.dagstuhl.de/opus/volltexte/2018/10103},
URN = {urn:nbn:de:0030drops101032},
doi = {10.4230/LIPIcs.ITCS.2019.10},
annote = {Keywords: Nonlocal games, XOR Games, Pseudotelepathy games, Multipartite entanglement}
}
Keywords: 

Nonlocal games, XOR Games, Pseudotelepathy games, Multipartite entanglement 
Collection: 

10th Innovations in Theoretical Computer Science Conference (ITCS 2019) 
Issue Date: 

2018 
Date of publication: 

08.01.2019 