  The effect of a coupling laser field on the entanglement of a three-level
quantum system and its spontaneous emission is investigated via reduced quantum
entropy. We consider two schemes, the upper- and lower-level couplings. By
calculating the degree of entanglement (DEM) for both systems, it is shown that
the entanglement between atom and its spontaneous emission can be controlled by
the coupling laser field. This field, however, affects the entanglement
differently in the two schemes; it is only the lower-level coupling scheme that
shows a nonzero steady state DEM which can be controlled by the intensity and
detuning of the coupling laser field.
