Twoway finite automata with quantum and classical states
Abstract
We introduce 2way finite automata with quantum and classical states (2qcfa's). This is a variant on the 2way quantum finite automata (2qfa) model which may be simpler to implement than unrestricted 2qfa's; the internal state of a 2qcfa may include a quantum part that may be in a (mixed) quantum state, but the tape head position is required to be classical. We show two languages for which 2qcfa's are better than classical 2way automata. First, 2qcfa's can recognize palindromes, a language that cannot be recognized by 2way deterministic or probabilistic finite automata. Second, in polynomial time 2qcfa's can recognize {a^n b^n  n>=0}, a language that can be recognized classically by a 2way probabilistic automaton but only in exponential time.
 Publication:

arXiv eprints
 Pub Date:
 November 1999
 arXiv:
 arXiv:cs/9911009
 Bibcode:
 1999cs.......11009A
 Keywords:

 Computer Science  Computational Complexity;
 Quantum Physics;
 F.1.1
 EPrint:
 11 pages