  We propose a quantum mechanical method of detecting weak vibrational
disturbances inspired by the protocol of entanglement farming. We consider a
setup where pairs of atoms in their ground state are successively sent through
an optical cavity. It is known that in this way it is possible to drive that
cavity toward a stable fixed-point state. Here we study how that fixed-point
state depends on the time interval between pairs of atoms and on the distance
between the cavity's mirrors. Taking advantage of an extremely precise
resonance effect, we find that there are special values of these parameters
where the fixed-point state is highly sensitive to perturbations, even harmonic
vibrations with frequencies several orders of magnitude below the cavity's
natural frequency. We propose that this sensitivity may be useful for high
precision metrology.
