Quantum Bernstein's Theorem and the Hyperoctahedral Quantum Group

We study an extension of Bernstein's theorem to the setting of quantum groups. For a d-tuple of free, identically distributed random variables we consider a problem of preservation of freeness under the action of a quantum subset of the free orthogonal quantum group. For a subset not contained in the hyperoctahedral quantum group we prove that preservation of freeness characterizes Wigner's semicircle law. We show that freeness is always preserved if the quantum subset is contained in the hyperoctahedral quantum group. We provide examples of quantum subsets which show that our result is an extension of results known in the literature.