## Feedback stabilization of quantum ensembles: a global convergence analysis on complex flag manifolds

### Claudio Altafini

* IEEE Transactions on Automatic Control*, 52(11):2019-2028, 2007.

In an N-level quantum mechanical system, the problem of unitary feedback stabilization of mixed density operators to periodic orbits admits a natural Lyapunov-based time-varying feedback design.
A global description of the domain of attraction of the closed-loop system can be provided based on a ``root-space''-like structure of the space of density operators.
This convex set foliates as a complex flag manifold where each leaf is identified with the coadjoint orbit of the eigenvalues of the density operator.
The converging conditions are time-independent but depend from the topology of the flag manifold: it is shown that the closed loop must have a number of equilibria at least equal to the Euler characteristic of the manifold, thus imposing obstructions of topological nature to global stabilizability.

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