  We report on the selective addressing of an individual atom in a pair of
single-atom microtraps separated by $3\;\mu$m. Using a tunable light-shift, we
render the selected atom off-resonant with a global Rydberg excitation laser
which is resonant with the other atom, making it possible to selectively block
this atom from being excited to the Rydberg state. Furthermore we demonstrate
the controlled manipulation of a two-atom entangled state by using the
addressing beam to induce a phase shift onto one component of the wave function
of the system, transferring it to a dark state for the Rydberg excitation
light. Our results are an important step towards implementing quantum
information processing and quantum simulation with large arrays of Rydberg
atoms.
