  Real quantum measurements almost always cause a much stronger back action
than required by the laws of quantum mechanics. In particular, free-space
optical detection methods for single atoms and ions such as the shelving
technique, though being among the most sensitive detection methods in quantum
physics, inevitably require spontaneous scattering, even in the dispersive
regime. This causes heating, a limitation for atom-based quantum information
processing where it obviates straightforward reuse of the qubit. No such energy
exchange is required by quantum mechanics. Here we experimentally demonstrate
optical detection of an atomic qubit with significantly less than one
spontaneous scattering event. We measure transmission and reflection of an
optical cavity containing the atom. In addition to the qubit detection itself,
we quantitatively measure how much spontaneous scattering has occurred. This
allows us to relate the information gained to the amount of spontaneous
emission, and we obtain a detection error below 10% while scattering less than
0.2 photons on average. Furthermore, we perform a quantum Zeno type experiment
to quantify the measurement back action and find that every incident photon
leads to an almost complete state collapse. Together, these results constitute
a full experimental characterization of a quantum measurement in the "energy
exchange-free" regime below a single spontaneous emission event. Besides its
fundamental interest, this means significant simplification for proposed
neutral-atom quantum computation schemes and may enable sensitive detection of
molecules and atoms lacking closed transitions.
