Quantum observers for dissipative interacting qubits in a common environment

We previously developed coherent quantum observers for linear and bilinear quantum plants corresponding to open harmonic oscillators and finite level quantum systems respectively. In this paper we consider the case where two XY-coupled qubits are interacting with a common environment, and a class of coherent quantum observers is designed aimed at tracking variables characterising the two qubits including dipole-dipole product terms in the sense of mean values. The quantum observers proposed here have the potential to be employed in coherent feedback control design involving coupling between qubits.