Ultra-bright narrow-band down-conversion source for atom-photon interaction

In recent years huge progress was made on the exploitation of quantum effects by optical means in quantum communication, quantum computation and quantum imaging. Many of the schemes rely on the availability of single photons or entangled photon pairs. A common way to produce the required states is by spontaneous parametric down-conversion (SPDC) in nonlinear crystals, where the produced photons exhibit a bandwidth from a few hundred GHz to some THz. Certain applications such as the efficient interaction of single photons with trapped single atoms or ensembles of atoms require single photons with a bandwidth corresponding to the linewidth of atomic absorption lines of a few MHz. There has been increasing interest in the generation of such photon pairs using a SPDC source inside an optical cavity to enhance the down-conversion into modes resonant to the cavity, each of the modes having a MHz spectrum. These optical parametric oscillators (OPO) operated far below threshold also provide a well defined spatial mode.