Abstract
The non-local game scenario provides a powerful framework to study the limitations of classical and quantum correlations, by studying the upper bounds of the wining probabilities those correlations offer in cooperation games where communication between players is prohibited. Building upon results presented in the seminal work of Cleve et al. (in: 19th IEEE annual conference on computational complexity, 2004), a straightforward construction to compute the Tsirelson bounds for simple two-player XOR games is presented. The construction is applied explicitly to some examples, including the Entanglement Assisted Orientation in Space (EAOS) game of Brukner et al. (Int J Quant Inf 4(2):365–370), proving for the first time that their proposed quantum strategy is in fact the optimal, as it reaches the Tsirelson bound.
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Notes
These strictly speaking are the stages of just one round of a game and some games could have more than one round - since this work deals exclusively with one round games this description fully characterizes the evolution of such games.
Note that \(Q^{x}\) does not mean the set of all possible questions for the xth player, like \(Q_i\) meant for the ith player. It means the xth element of Q, whatever it might be according to some arbitrary order. Likewise \(A^{y}\) means the yth element of A.
This is equivalent to saying that the average over a set of positive numbers is never greater than the highest number of the set.
We make the implicit assumption that Alice and Bob always try to win the game with the highest possible probability.
The quantum/classical bias is the difference between the quantum/classical value and the winning probability offered by a trivial random answer - it is a standard way to measure the quantum over classical advantage in non-local games.
It is this exact rule that makes this a simple two-player XOR game. If there would be no restrictions on the questions, then (26) would be ever harder to solve for increasing values of n.
References
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Acknowledgements
The author would like to thank N. Paunković, who first introduced him to the EAOS game, and also the support of SQIG (Security and Quantum Information Group), the Instituto de Telecomunicações (IT) Research Unit, Ref. UID/EEA/50008/2019, funded by Fundação para a Ciência e Tecnologia (FCT). The author acknowledges funding from FCT through the DP-PMI Grant PD/BD/128636/2017.
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Faleiro, R. Quantum strategies for simple two-player XOR games. Quantum Inf Process 19, 229 (2020). https://doi.org/10.1007/s11128-020-02717-2
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DOI: https://doi.org/10.1007/s11128-020-02717-2