  We bring the set of linear quantum operations, important for many fundamental
studies in photonic systems, to the material domain of collective excitations
known as spin waves. Using the ac Stark effect we realize quantum operations on
single excitations and demonstrate a spin-wave analogue of Hong-Ou-Mandel
effect, realized via a beamsplitter implemented in the spin wave domain. Our
scheme equips atomic-ensemble-based quantum repeaters with quantum information
processing capability and can be readily brought to other physical systems,
such as doped crystals or room-temperature atomic ensembles.
