Asymptotic reduced density matrix of discrete-time quantum walks

Abstract

In this article we show that for any quantum walker with \textit{m}-dimensional coin subspace, we have $m^2\times m^2$ specific constant matrix $\mathcal{C}$ where it completely determines the asymptotic reduced density matrix of the walker. We show that for any initial state with $P_0$ projector, reduced density matrix, can be obtained by $Tr_1\left(P_0\otimes I\;\mathcal{C}\right)$ or equivalently $Tr_2\left(I\otimes P_0\;\mathcal{C}\right)$. It is worth to mention that characteristic matrix $\mathcal{C}$ is independent of the initial state and just depends on coin operator, so by finding this matrix for specific type of QW the long-time behavior of it, such as local state of the coin after a long time walking and asymptotic entanglement between coin and position will be completely known for any initial state. We have found the characteristic matrix $\mathcal{C}$ for general coin operator, $U\left(2\right)$, as well as exact form of this matrix for local initial state.