Succinct quantum testers for closeness and $k$-wise uniformity of probability distributions

Abstract

We explore potential quantum speedups for the fundamental problem of testing the properties of closeness and $k$-wise uniformity of probability distributions. Closeness testing is the problem of distinguishing whether two $n$-dimensional distributions are identical or at least $\varepsilon$-far in $\ell^1$- or $\ell^2$-distance. We show that the quantum query complexities for $\ell^1$- and $\ell^2$-closeness testing are $O(\sqrt{n}/\varepsilon)$ and $O(1/\varepsilon)$, respectively, both of which achieve optimal dependence on $\varepsilon$, improving the prior best results of Gily\'en and Li (2020). $k$-wise uniformity testing is the problem of distinguishing whether a distribution over $\{0, 1\}^n$ is uniform when restricted to any $k$ coordinates or $\varepsilon$-far from any such distributions. We propose the first quantum algorithm for this problem with query complexity $O(\sqrt{n^k}/\varepsilon)$, achieving a quadratic speedup over the state-of-the-art classical algorithm with sample complexity $O(n^k/\varepsilon^2)$ by O'Donnell and Zhao (2018). Moreover, when $k = 2$ our quantum algorithm outperforms any classical one because of the classical lower bound $\Omega(n/\varepsilon^2)$. All our quantum algorithms are fairly simple and time-efficient, using only basic quantum subroutines such as amplitude estimation.