On the von Neumann entropy of certain quantum walks subject to decoherence

Abstract

In this paper, we consider a discrete-time quantum walk on theN-cycle governed by the condition that at every time step of the walk, the option persists, with probabilityp, of exercising a projective measurement on the coin degree of freedom. For a bipartite quantum system of this kind, we prove that the von Neumann entropy of the total density operator converges to its maximum value. Thus, when influenced by decoherence, the mutual information between the two subsystems corresponding to the space of the coin and the space of the walker must eventually diminish to zero. Put plainly, any level of decoherence greater than zero forces the system to become completely ‘disentangled’ eventually.