Asymptotic velocity of a position-dependent quantum walk

Abstract

We consider a position-dependent coined quantum walk on $\mathbb{Z}$ and assume that the coin operator $C(x)$ satisfies \[ \|C(x) - C_0 \| \leq c_1|x|^{-1-\epsilon}, \quad x \in \mathbb{Z} \] with positive $c_1$ and $\epsilon$ and $C_0 \in U(2)$. We show that the Heisenberg operator $\hat x(t)$ of the position operator converges to the asymptotic velocity operator $\hat v_+$ so that \[ \mbox{s-}\lim_{t \to \infty} {\rm exp}\left( i \xi \frac{\hat x(t)}{t} \right) = \Pi_{\rm p}(U) + {\rm exp}(i \xi \hat v_+) \Pi_{\rm ac}(U) \] provided that $U$ has no singular continuous spectrum. Here $\Pi_{\rm p}(U)$ (resp. $\Pi_{\rm ac}(U)$) is the orthogonal projection onto the direct sum of all eigenspaces (resp. the subspace of absolute continuity) of $U$. We also prove that for the random variable $X_t$ denoting the position of a quantum walker at time $t \in \mathbb{N}$, $X_t/t$ converges in law to a random variable $V$ with the probability distribution \[ \mu_V = \|\Pi_{\rm p}(U)\Psi_0\|^2\delta_0 + \|E_{\hat v_+}(\cdot) \Pi_{\rm ac}(U)\Psi_0\|^2, \] where $\Psi_0$ is the initial state, $\delta_0$ the Dirac measure at zero, and $E_{\hat v_+}$ the spectral measure of $\hat v_+$.