  The cascaded decay in a four-level quantum emitter is a well established
mechanism to generate polarization entangled photon pairs, the building blocks
of many applications in quantum technologies. The four most prominent maximally
entangled photon pair states are the Bell states. In a typical experiment based
on an undriven emitter only one type of Bell state entanglement can be observed
in a given polarization basis. Other types of Bell state entanglement in the
same basis can be created by continuously driving the system by an external
laser. In this work we propose a protocol for time-dependent entanglement
switching in a four-level quantum emitter--cavity system that can be operated
by changing the external driving strength. By selecting different two-photon
resonances between the laser-dressed states, we can actively switch back and
forth between the different types of Bell state entanglement in the same basis
as well as between entangled and nonentangled photon pairs. This remarkable
feature demonstrates the possibility to achieve a controlled, time-dependent
manipulation of the entanglement type that could be used in many innovative
applications.
