  Quantum amplifier is an essential device in quantum information processing.
As in the classical (non-quantum) case, its characteristic uncertainty needs to
be suppressed by feedback, and in fact such a control theory for a single
quantum amplifier has recently been developed. This letter extends this result
to the case of cascaded quantum amplifier. In particular, we consider two types
of structures: the case where controlled amplifiers are connected in series,
and the case where a single feedback control is applied to the cascaded
amplifier. Then, we prove that the latter is better in the sense of sensitivity
to the uncertainty. A detailed numerical simulation is given to show actual
performance of these two feedback schemes.
