Entanglement preservation of two coupled modes

A scheme to control the evolution of any initial state in subspace {|1〉⊗|0〉,|0〉⊗|1〉}{|1〉⊗|0〉,|0〉⊗|1〉} is presented. The physical system considered is the one of two coupled modes sharing one excitation. The scheme is based on unitary interactions with an auxiliary subsystem, and it can be used to preserve the initial entanglement of the system. A proposal for implementation of the scheme in the context of microwave cavity is presented.