  During the past decade the interaction of light with multi-atom ensembles has
attracted a lot of attention as a basic building block for quantum information
processing and quantum state engineering. The field started with the
realization that optically thick free space ensembles can be efficiently
interfaced with quantum optical fields. By now the atomic ensemble - light
interfaces have become a powerful alternative to the cavity-enhanced
interaction of light with single atoms. We discuss various mechanisms used for
the quantum interface, including quantum nondemolition or Faraday interaction,
quantum measurement and feedback, Raman interaction and electromagnetically
induced transparency. The paper provides a common theoretical frame for these
processes, describes basic experimental techniques and media used for quantum
interfaces, and reviews several key experiments on quantum memory for light,
quantum entanglement between atomic ensembles and light, and quantum
teleportation with atomic ensembles. We discuss the two types of quantum
measurements which are most important for the interface: homodyne detection and
photon counting. The paper concludes with an outlook on the future of atomic
ensembles as an enabling technology in quantum information processing.
