The Quantum Query Complexity of Finding a Tarski Fixed Point on the 2D Grid

Reed Phillips

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arXiv · 2604.08223

The Quantum Query Complexity of Finding a Tarski Fixed Point on the 2D Grid

Reed Phillips

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Abstract

Tarski's theorem states that every monotone function from a complete lattice to itself has a fixed point. We specifically consider the two-dimensional lattice $\mathcal{L}^2_n$ on points $\{1, \ldots, n\}^2$ and where $(x_1, y_1) \leq (x_2, y_2)$ if $x_1 \leq x_2$ and $y_1 \leq y_2$. We show that the quantum query complexity of finding a fixed point given query access to a monotone function on $\mathcal{L}^2_n$ is $\Omega((\log n)^2)$, matching the classical deterministic upper bound. The proof consists of two main parts: a lower bound on the quantum query complexity of a composition of a class of functions including ordered search, and an extremely close relationship between finding Tarski fixed points and nested ordered search.

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Reed Phillips. 2026-04-09. The Quantum Query Complexity of Finding a Tarski Fixed Point on the 2D Grid. https://arxiv.org/abs/2604.08223

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