  Engineering a sensor system for detecting an extremely tiny signal such as
the gravitational-wave force is a very important subject in quantum physics. A
major obstacle to this goal is that, in a simple detection setup, the
measurement noise is lower bounded by the so-called standard quantum limit
(SQL), which is originated from the intrinsic mechanical back-action noise.
Hence, the sensor system has to be carefully engineered so that it evades the
back-action noise and eventually beats the SQL. In this paper, based on the
well-developed geometric control theory for classical disturbance decoupling
problem, we provide a general method for designing an auxiliary (coherent
feedback or direct interaction) controller for the sensor system to achieve the
above-mentioned goal. This general theory is applied to a typical
opto-mechanical sensor system. Also, we demonstrate a controller design for a
practical situation where several experimental imperfections are present.
