Perfect play with spatial quantum strategies is Σ⁰₂-complete

When does a finite nonlocal game admit perfect spatial quantum play? We prove that this decision problem is complete for the second existential level of the arithmetical hierarchy. The same classification holds when a perfect strategy must have finite marginal entropy. Hardness persists among games whose finite-dimensional value is one and which have no finite-dimensional perfect strategy. A finite clock-phase order supplies the existential witness: normality forces a finite order on a nonzero sector, while the strategy chooses the order. For the upper bound, we normalize the largest Schmidt coefficient and use compact operator sandwiches. Fixing a lower bound on the largest Schmidt probability gives a co-recursively enumerable stratum. No computable bound selects a sufficient stratum for every game. Exact attainment of a rational Bell bound, of an unspecified supremum, and of a specified scalar value is also Sigma^0_2-complete. The hardness proof extends the modular-shift and rational-row toolkit of our earlier entropy-game paper to four calibrated bridges and a deterministic computation on rooted finite squares.

Authors

Publication Details

Journal
Zenodo (CERN European Organization for Nuclear Research)
Published
2026-11-03
DOI
https://doi.org/10.5281/zenodo.23910986
Citations
2
Primary Topic
Quantum Information and Cryptography
Type
preprint
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preprint

Perfect play with spatial quantum strategies is Σ⁰₂-complete

Nidhal Mghirbi, Seth Douglas
2 citations
Zenodo (CERN European Organization for Nuclear Research)
Quantum Information and Cryptography
preprint

Perfect play with spatial quantum strategies is Σ⁰₂-complete

Nidhal Mghirbi, Seth Douglas
preprint en
2 citations

Abstract

When does a finite nonlocal game admit perfect spatial quantum play? We prove that this decision problem is complete for the second existential level of the arithmetical hierarchy. The same classification holds when a perfect strategy must have finite marginal entropy. Hardness persists among games whose finite-dimensional value is one and which have no finite-dimensional perfect strategy. A finite clock-phase order supplies the existential witness: normality forces a finite order on a nonzero sector, while the strategy chooses the order. For the upper bound, we normalize the largest Schmidt coefficient and use compact operator sandwiches. Fixing a lower bound on the largest Schmidt probability gives a co-recursively enumerable stratum. No computable bound selects a sufficient stratum for every game. Exact attainment of a rational Bell bound, of an unspecified supremum, and of a specified scalar value is also Sigma^0_2-complete. The hardness proof extends the modular-shift and rational-row toolkit of our earlier entropy-game paper to four calibrated bridges and a deterministic computation on rooted finite squares.

Zenodo (CERN European Organization for Nuclear Research)
Quantum Information and Cryptography
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Perfect play with spatial quantum strategies is Σ⁰₂-complete — Nidhal Mghirbi, Seth Douglas · Zenodo (CERN European Organization for Nuclear Research) (2026) | TGRS Research Map | TGRS