  A phase-controlled ultralow-threshold phonon laser is proposed by using
tunable optical amplifiers in coupled-cavity-optomechanical system. Giant
enhancement of coherent photon-phonon interactions is achieved by engineering
the strengths and phases of external parametric driving. This in turn enables
single-photon optomechanics and low-power phonon lasing, opening up novel
prospects for applications, e.g. quantum phononics and ultrasensitive motion
detection.
