Measurement-based quantum feedback

Measurement-based quantum feedback adapts a control input from a measurement record or conditional quantum state estimate. A measurement contributes backaction and generally stochastic dynamics. Exact instantaneous expectation values cannot be read from a single system without these effects. A feedback law derived for deterministic closed-system evolution therefore needs a new conditional dynamical analysis before being applied as measurement feedback.

An observed dissipative channel combined with direct feedback can create a modified Lindblad operator and a compensating Quantum Hamiltonian. Choosing those parameters can make a prescribed pure state stationary and globally attractive. At exceptional parameter values, the dissipator can instead leave an entire family of states stationary, so algebraic target stationarity does not establish stabilization. Checking the Affine Bloch equation and its transverse eigenvalues distinguishes these possibilities.