Quantum limits encountered in atomic spin measurements

The research program investigating the limits imposed by quantum mechanics on the precision with which a collective atomic spin can be measured is reviewed in this paper. We consider both the cases, when the interaction between atoms and light is weak and strong. In the first case, the precision of the measurement is mainly limited by the quantum noise of light, and the atomic spin noise plays a minor role. In the latter case, we must consider the quantum mechanical measurement induced back action feeding quantum noise of light into the atomic spin system. Here, we show that the precision is limited by atomic quantum spin noise as well as the optical quantum noise. Although it is disturbing for polarization spectroscopy, the measurement induced back action may prove to be useful from a quantum informatics viewpoint, since this enables storage of the polarization state of light in the atomic spin.