  In this paper, we consider a linear quantum network composed of two distantly
separated cavities that are connected via a one-way optical field. When one of
the cavity is damped and the other is undamped, the overall cavity state
obtains a large amount of entanglement in its quadratures. This entanglement
however immediately decays and vanishes in a finite time. That is, entanglement
sudden-death occurs. We show that the direct measurement feedback method
proposed by Wiseman can avoid this entanglement sudden-death, and further,
enhance the entanglement. It is also shown that the entangled state under
feedback control is robust against signal loss in a realistic detector,
indicating the reliability of the proposed direct feedback method in practical
situations.
