@ARTICLE {4165,
author={D. Qiu Qiu and S. Yu},
doi={10.1016/j.tcs.2009.03.040},
journal={Theoretical Computer Science},
title={Hierarchy and equivalence of multi-letter quantum finite automata},
year={2009},
month={January},
volume={410},
number={30-32},
pages={3006-3017},
ISSN={0304-3975}
}