Absorption problems for quantum walks in one dimension

This paper treats absorption problems for the one-dimensional quantum walk determined by a 2 × 2 unitary matrix U on a state space {0, 1, ..., N} where N is finite or infinite by using a new path integral approach based on an orthonormal basis P, Q, R and S of the vector space of complex 2 × 2 matrices. Our method studied here is a natural extension of the approach in the classical random walk.