  For broadband quantum noise reduction of gravitational-wave detectors,
frequency-dependent squeezed vacuum states realized using a filter cavity is a
mature technique and will be implemented in Advanced LIGO and Advanced Virgo
from the fourth observation run. To obtain the benefit of frequency-dependent
squeezing, detuning and alignment of the filter cavity with respect to squeezed
vacuum states must be controlled accurately. To this purpose, we suggest a new
length and alignment control scheme, using coherent control sidebands which are
already used to control the squeezing angle. Since both squeezed vacuum states
and coherent control sidebands have the same mode matching conditions and
almost the same frequency, detuning and alignment of the filter cavity can be
controlled accurately with this scheme. In this paper, we show the principle of
this scheme and its application to a gravitational-wave detector.
