We discuss the use of active control to reduce mirrors position fluctuations at the quantum level. Recent experiments have shown that it is possible to reduce the thermal motion of a mirror by cold damping. The mirror motion is measured with an optomechanical sensor based on an high-finesse optical cavity, and corrected by a feedback loop. We show that this approach can be extended to lock the mirror motion at the quantum level and we propose to use this quantum locking technique to reduce the noise in interferometric measurements such as gravitational-waves detectors. We analyze the back-action effects of the optomechanical sensor and show that quantum limits can be transferred from the sensor measurement to the interferometric one. This simple technique allows one to suppress the quantum effects of radiation pressure in the interferometer and to greatly enhance its sensitivity. The effect is furthermore insensitive to losses in the interferometer.