  In this paper, we consider stochastic master equations describing the
evolution of a multi-qubit system interacting with electromagnetic fields
undergoing continuous-time measurements. By considering multiple z-type (Pauli
z matrix on different qubits) and x-type (Pauli x matrix on all qubits)
measurements and one control Hamiltonian, we provide general conditions on the
feedback controller and the control Hamiltonian ensuring almost sure
exponential convergence to a predetermined Greenberger-Horne-Zeilinger (GHZ)
state, which is assumed to be a common eigenstate of the measurement operators.
We provide explicit expressions of feedback controllers satisfying such
conditions. We also consider the case of only z-type measurements and multiple
control Hamiltonians, and we discuss asymptotic convergence towards a
predetermined GHZ state. Finally, we demonstrate the effectiveness of our
methodology for a three-qubit system through numerical simulations.
