An optimal quantum strategy for the $2$-hat game

Abstract

We study the two-player hat game as a two-input, three-output non-local game. We calculate the quantum commuting value and show that it coincides with the quantum one. In particular, we show that \[ω_{\rm q}(H_2)=ω_{\mathrm{qc}}(H_2)=\frac{15+\sqrt{33}}{36}\approx 0.576238.\] We achieve the optimal quantum advantage using a shared non-maximally entangled qutrit state. To establish optimality, we exploit the game's symmetry under the dihedral group $D_8$ and apply its representation theory to substantially reduce the complexity of the underlying matrix problem. We also determine the no-signalling value ${\omega }_{\mathrm{ns}}(H_2)=2/3$.