Engineering macroscopic quantum state of light is an important goal in quantum optics. However, generating such photonic states in a deterministic way turns out to be hard, as photon-photon and atom-photon interactions are usually weak. By coupling light with a single atom inside an optical resonator, atom-light interactions can be effectively controlled. We demonstrate the on-demand generation of entangled atom-light Schrödinger's cat states and a deterministic quantum gate between a single atom and an optical cat state.