Bell states within the linewidth of SPDC and applications.

In this paper we review our recent works on the generation of different Bell states within the lineshape of parametric down-conversion (SPDC) and their possible applications. Indeed, for polarization-entangled two-photon states produced via SPDC, the frequency-angular lineshape allowed by phase matching is considered. It is shown that there are always different Bell states generated for different mismatch values within the natural bandwidth. Consideration is made for two different methods of polarization entanglement preparation, based on type-II SPDC and on SPDC in two type-I crystals producing orthogonally polarized photon pairs. Different Bell states can be filtered out by either frequency selection or angular selection, or both. Our theoretical calculations are confirmed by a series of experiments, performed for the two above-mentioned ways of producing polarization-entangled photon pairs and with two kinds of measurements: frequency-selective and angular-selective. Finally, we mention possible application to quantum communication with fibers.