  We propose and demonstrate a single-photon sensitive technique for optical
vibrometry. It uses high speed photon counting to sample the modulated
backscattering from a vibrating target. Designed for remote vibration sensing
with ultralow photon flux, we show that this technique can detect small
displacements down to 110 nm and resolve vibration frequencies from DC up to
several kilohertz, with less than 0.01 detected photons per pulse. This
single-photon sensitive optical vibrometry may find important applications in
acousto-optic sensing and imaging, especially in photon-starved environments.
