  We describe a physical implementation of a quantum finite automaton
recognizing a well known family of periodic languages. The realization exploits
the polarization degree of freedom of single photons and their manipulation
through linear optical elements. We use techniques of confidence amplification
to reduce the acceptance error probability of the automaton. It is worth
remarking that the quantum finite automaton we physically realize is not only
interesting per se, but it turns out to be a crucial building block in many
quantum finite automaton design frameworks theoretically settled in the
literature.
