  Generating entangled states is one of the most important tasks in quantum
information technology. However, in reality any entanglement generator must
contain some characteristic uncertainty, and as a result the produced entangled
state becomes an undesirable mixed state. This paper develops a coherent
feedback control scheme that suppresses the characteristic uncertainty of a
typical entanglement generator (non-degenerate optical parametric oscillator)
for producing robust Gaussian entangled states. In particular, we examine a
two-mode squeezed state and Gaussian four-mode cluster states to demonstrate
the effectiveness of the proposed control method.
