We present a tunable, frequency-stabilized, narrow-bandwidth source of frequency-degenerate, entangled photon pairs. The source is based on spontaneous parametric downconversion in periodically poled KTiOPO(4). Its wavelength can be stabilized to 850 or 854 nm, thus allowing to address two transitions in (40)Ca(+) 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).