  In this paper, we consider the feedback stabilization problem for N-level
quantum angular momentum systems undergoing continuous-time measurements. By
using stochastic and geometric control tools, we provide sufficient conditions
on the feedback control law ensuring almost sure exponential convergence to a
predetermined eigenstate of the measurement operator. In order to achieve these
results, we establish general features of quantum trajectories which are of
interest by themselves. We illustrate the results by designing a class of
feedback control laws satisfying the above-mentioned conditions and finally we
demonstrate the effectiveness of our methodology through numerical simulations
for three-level quantum angular momentum systems.
