Quantum simulations of classical random walks and undirected graph   connectivity
  It is not currently known if quantum Turing machines can efficiently simulate probabilistic computations in the space-bounded case. In this paper we show that space-bounded quantum Turing machines can efficiently simulate a limited class of random processes: random walks on undirected graphs. By means of such simulations, it is demonstrated that the undirected graph connectivity problem for regular graphs can be solved by one-sided error quantum Turing machines that run in logspace and halt absolutely. It follows that symmetric logspace is contained in the quantum analogue of randomized logspace. 
John Watrous (University of Montreal)