Quantum finite automata using ancilla qubits

We introduce a new model of quantum finite automata. By using ancilla qubits, it becomes possible to recognize any regular language with certainty. Some nonregular languages can be recognized with one-sided unbounded error. We analyze a class of languages that can be recognized in this model in terms of a cascade composition of automata. This also allows to treat the case of an automaton with both classical and quantum states.