On the Simulation Cost of Quantum Finite Automata

TL;DR

This paper quantifies the quantum advantage in finite automata by establishing exact simulation costs, revealing a hierarchy of quantum benefits through sharp laws and a prepare-test framework.

Contribution

It introduces precise simulation cost laws for quantum finite automata and develops a prepare-test framework to analyze quantum advantage.

Findings

One-way automaton with classical states and quantum register has simulation cost Θ(cq^2).

Measure-once quantum automaton has worst-case cost Θ(n^2).

Results clarify the hierarchy of quantum advantage in finite automata.

Abstract

This paper identifies exact probabilistic simulation cost as the natural quantitative measure of quantum advantage for finite automata under strict cutpoints. It gives sharp simulation laws for two representative models. A one-way finite automaton with $c$ classical states and a $q$-dimensional quantum register has exact probabilistic simulation cost $\Theta(cq^2)$, while an $n$-dimensional measure-once one-way quantum finite automaton has worst-case cost $\Theta(n^2)$. The proofs develop a prepare--test framework, in which prefixes generate the relevant real operator degrees of freedom and suffixes convert them into strict-cutpoint tests. The same obstruction is recast through finite sign-rank matrices, clarifying the role of Forster's spectral method. Placed beside the surrounding two-way separations, these results give a clean hierarchy of finite-automata quantum advantage.