Scalable tests of quantum contextuality from stabilizer-testing nonlocal games

TL;DR

This paper develops scalable methods to test quantum contextuality using stabilizer-testing nonlocal games, providing bounds on classical performance and demonstrating how to witness contextuality with minimal fidelity in large quantum states.

Contribution

Introduces new bounds on classical success probabilities in stabilizer-testing games, including asymptotically tight bounds for cyclic cluster states, advancing scalable quantum contextuality tests.

Findings

Classical value of stabilizer-testing games is at most 7/8 for all codes.

Tight bounds established for GHZ, toric-code, and cyclic cluster states.

Exponential fidelity suffices to witness contextuality in large states.

Abstract

Soon after the dawn of quantum error correction, DiVincenzo and Peres observed that stabilizer codewords could give rise to simple proofs of quantumness via contextuality. This discovery can be recast in the language of nonlocal games: every $n$-qubit stabilizer state defines a specific "stabilizer-testing" $n$-player nonlocal game, which quantum players can win with probability one. If quantum players can moreover outperform all possible classical players, then the state is contextual. However, the classical values of stabilizer-testing games are largely unknown for scalable examples beyond the $n$-qubit GHZ state. We introduce several new methods for upper-bounding the classical values of these games. We first prove a general coding-theory bound for all stabilizer-testing games: if the classical value $p_{\mathrm{cl}}^* < 1$, then $p_{\mathrm{cl}}^* \leq 7/8$, i.e., there is no classical strategy that can perform as well as the optimal quantum strategy even in an asymptotic sense. We then show how to tighten this bound for the most common scalable examples, namely GHZ, toric-code and cyclic cluster states. In particular, we establish an asymptotically tight upper bound for cyclic cluster states using transfer-matrix methods. This leads to the striking conclusion that measuring an exponentially small fidelity to the cyclic cluster state will suffice to witness its contextuality.