Measuring the internal state of a single atom without energy exchange
  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. 
J\"urgen Volz, Roger Gehr, Guilhem Dubois, J\'er\^ome Est\`eve and   Jakob Reichel
quant-ph