  We demonstrate feedback control of a superconducting transmon qubit using
discrete, projective measurement and conditional coherent driving. Feedback
realizes a fast and deterministic qubit reset to a target state with 2.4% error
averaged over input superposition states, and cooling of the transmon from 16%
spurious excitation to 3%. This closed-loop qubit control is necessary for
measurement-based protocols such as quantum error correction and teleportation.
