Quantum intermittency for sparse CMV matrices with an application to quantum walks on the half-line

We study the dynamics given by the iteration of a (half-line) CMV matrix with sparse, high barriers. Using an approach of Tcheremchantsev, we are able to explicitly compute the transport exponents for this model in terms of the given parameters. In light of the connection between CMV matrices and quantum walks on the half-line due to Cantero–Grünbaum–Moral–Velázquez, our result also allows us to compute transport exponents corresponding to a quantum walk which is sparsely populated with strong reflectors. To the best of our knowledge, this provides the first rigorous example of a quantum walk that exhibits quantum intermittency, i.e., nonconstancy of the transport exponents. When combined with the CMV version of the Jitomirskaya–Last theory of subordinacy and the general discrete-time dynamical bounds from Damanik–Fillman–Vance, we are able to exactly compute the Hausdorff dimension of the associated spectral measure.