Random graph-state overlap conjecture for Pauli-diagonal product states

Let GG be sampled uniformly from graph states, and let d4afd4af be the set of eigenstates of the operators X+YX+Y and XYX-Y. For an nn-qubit product state, write d4afd4af\textsuperscriptnd4af^{d4af\textsuperscript{⊗}n} for the set of tensor products of nn states from d4afd4af. Random graph-state overlap conjecture. There exist constants c,C>0c,C>0 such that

P[maxα\ind4afnαG22n+nc]C.\mathbb{P}\left[\max_{\lvert\alpha\rangle\ind4af^{\otimes n}}\left\lvert\langle\alpha\mid G\rangle\right\rvert^2\geq 2^{-n+n^c}\right]\geq C.

Here the probability is taken uniformly over graph states. The conjecture asserts that the overlap with these highly magical product states is substantially larger than the baseline 2n2^{-n} with constant probability, which would rule out the proposed classical simulation strategy based on replacing X+YX+Y and XYX-Y measurements by coin flips; the paper states that this remains unproved.

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Primary source

Soumik Ghosh, Dominik Hangleiter and Jonas Helsen, “Random regular graph states are complex at almost any depth”, arXiv:2412.07058 (2025).

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