Verification Complexity and Extension of Classical Shadows
TL;DR
This work shows CSV is efficiently solvable for permutation-invariant shadows, QMA-hard for real, fermionic, and bosonic shadows, and both coNP-hard and QMA-hard when the observable family consists of all $n-qubit Pauli strings.
Abstract
Classical shadows are an influential framework for compressing copies of a given quantum state $\rho$ into classical data $S$, enabling many properties of $\rho$ to be predicted from relatively few copies. In this work, we study two natural questions involving shadows: (1) Given $S$, when can one efficiently verify that $S$ came from a genuine $n$-qubit state? This is called the Classical Shadow Validity (CSV) problem, introduced by Karaiskos, Rudolph, Meyer, Eisert, and Gharibian [ICALP 2026]. (2) Given $S$ that allows one to capture 2-local properties of $\rho$, can one fake or spoof a shadow $S'$ which predicts 3-local properties of some state? For (1), we show CSV is efficiently solvable for permutation-invariant shadows, QMA-hard for real, fermionic, and bosonic shadows, and both coNP-hard and QMA-hard when the observable family consists of all $n$-qubit Pauli strings. A result of independent interest along the way is a new upper bound qc-$\Sigma_2$ $\subseteq$ $\mathrm{P}^{\mathrm{PP}}$, where qc-$\Sigma_2$ is a quantum analogue of the second level of the polynomial hierarchy in which the first proof is quantum. For (2), we show intractability: Given the 2-local marginals $S$ of a quantum state $\rho$, estimating the 3-local marginals of $\rho$ is intractable unless QCMA $\subseteq$ BPP, even if the state $\rho$ is the unique state consistent with $S$.