Polynomial-time additive-error estimation of output probabilities for shallow quantum circuits

Abstract: We give a deterministic classical algorithm that estimates $|\langle x|U|0<sup>n\rangle|<sup>2$ to additive error $\varepsilon$ in $\mathrm{poly}(n, 1/\varepsilon)$ time, where $U$ is a constant-depth quantum circuit comprised of gates with bounded fan-in and arbitrary connectivity, and $x$ is an arbitrary $n$-bit output string. This improves over prior state-of-the-art algorithms that takes $n<sup>{O(log(n))}$ time for the same task, $n<sup>{O(log(log(n))}$ when $U$ is geometrically local, and $n<sup>{O(1)}$ for 2D geometrically-local circuits.