  We investigate quantum beats in the arrival-time distribution of single
photons from a single trapped $^{40}$Ca$^+$ ion, revealing their fundamentally
different physical origins in two distinct experimental situations: In a
$\Lambda$-type level scheme the interference of two 854-nm absorption
amplitudes suppresses and enhances the emission process of Raman-scattered
393-nm photons; in a V-type level scheme the interference of two 393-nm
emission amplitudes causes a rotation of their dipole emission pattern
resulting in a temporal modulation of the detected photons. For both cases we
demonstrate coherent control over the quantum-beat phase through the phases of
the atomic and photonic input states, which also allows controlled adjustment
of the total photon detection efficiency.
