  We present a tunable, frequency-stabilized, narrow-bandwidth source of
frequency-degenerate, entangled photon pairs. The source is based on
spontaneous parametric downconversion (SPDC) in periodically-poled KTiOPO4
(PPKTP). Its wavelength can be stabilized to 850 or 854 nm, thus allowing to
address two D-P transitions in 40Ca+ ions. Its output bandwidth of 22 MHz
coincides with the absorption bandwidth of the calcium ions. Its spectral power
density is 1.0 generated pairs/(s MHz mW).
