Multiplicative measures on free groups

We introduce a family of atomic measures on free groups generated by no-return random walks. These measures are shown to be very convenient for comparing "relative sizes" of subgroups, context-free and regular subsets (that, subsets generated by finite automata) of free groups. Many asymptotic characteristics of subsets and subgroups are naturally expressed as analytic properties of related generating functions. We introduce an hierarchy of asymptotic behaviour "at infinity" of subsets in the free groups, more sensitive than the traditionally used asymptotic density, and apply it to normal subgroups and regular subsets.Comment: LaTeX, requires amssymb.sty; 31 pp Version 3: more detail in Example 2 and Tauberian theorem

Photonic realization of a quantum finite automaton

We describe a physical implementation of a quantum finite automaton that recognizes 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