  Entanglement generation and preservation is a key task in quantum information
processing, and a variety of protocols exist to entangle remote qubits via
measurement of their spontaneous emission. We here propose feedback methods,
based on monitoring the fluorescence of two qubits and using only local
pi-pulses for control, to increase the yield and/or lifetime of entangled
two-qubit states. Specifically, we describe a protocol based on photodetection
of spontaneous emission (i.e. using quantum jump trajectories) which allows for
entanglement preservation via measurement undoing, creating a limit cycle
around a Bell states. We then demonstrate that a similar modification can be
made to a recent feedback scheme based on homodyne measurement (i.e. using
diffusive quantum trajectories), [L. S. Martin and K. B. Whaley,
arXiv:1912.00067] in order to increase the lifetime of the entanglement it
creates. Our schemes are most effective for high measurement efficiencies, and
the impact of less-than-ideal measurement efficiency is quantified. The method
we describe here combines proven techniques in a novel way, complementing
existing protocols, and offering a pathway towards generating and protecting
entangled states so that they may be used in various applications on demand.
