  We demonstrate the lossless state-selective detection of a single rubidium 87
atom trapped in an optical tweezer. This detection is analogous to the one used
on trapped ions. After preparation in either a dark or bright state, we probe
the atom internal state by sending laser light that couples an excited state to
the bright state only. The laser induced fluorescence is collected by a high
numerical aperture lens. The single-shot fidelity of the detection is 98.6(0.2)
% and is presently limited by the dark count noise of the detector. The
simplicity of this method opens new perspectives in view of applications to
quantum manipulations of neutral atoms.
