Implementation of one-dimensional quantum walks on spin-orbital angular momentum space of photons

Photons can carry spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used to realize a qubit system and a high-dimension system, respectively. This spin-orbital system is very suitable for implementing one-dimensional discrete-time quantum random walks. We propose a simple scheme of quantum walks on the spin-orbital angular momentum space of photons, where photons walk on the infinite OAM space controlled by their SAM. By employing the recent invention of an optical device, the so-called ``q-plate,'' our scheme is more simple and efficient than others because there is no Mach-Zehnder interferometer in the scheme.