Abstract:
We give a definition of a quantum branching program. A well-known Boolean function
$\operatorname{MOD}_p$ is considered. We prove that any deterministic branching program and
any probabilistic ordered stable branching program which $(1/2+\varepsilon)$-compute the function
$\operatorname{MOD}_p$ are of width no less than $p$. We construct a stable quantum branching program of width $O(\log p)$ which computes the function $\operatorname{MOD}_p$ with one-sided error $\varepsilon>0$.