Narrow-band single photons represent an important resource for quantum memories due to their efficient interaction
with atomic resonances. In this paper, we report on the generation of photons with 3 MHz linewidth by
cavity-enhanced parametric down-conversion and demonstrate direct proof of their single-photon character by
detection of heralding idler photons. Compared to a Poissonian source, a suppression of higher-order photon
numbers by nearly two orders of magnitude could be achieved. Moreover, the brightness of our source exceeds
previous realizations by more than a factor of 100.