  Split-step quantum walks are models of supersymmetric quantum walk, and thus
their Witten indices can be defined. We prove that the Witten index of a
split-step quantum walk coincides with the difference between the winding
numbers of functions corresponding to the right-limit of coins and the
left-limit of coins. As a corollary, we give an alternative derivation of the
index formula for split-step quantum walks, which is recently obtained by
Suzuki and Tanaka.
