Pulsed excitation dynamics of an optomechanical crystal resonator near its quantum ground-state of motion

Using pulsed optical excitation and read-out along with single-phonon-counting techniques, we measure the transient backaction, heating, and damping dynamics of a nanoscale silicon optomechanical crystal cavity mounted in a dilution refrigerator at a base temperature of T-f approximate to 11 mK. In addition to observing a slow (approximately 740-ns) turn-on time for the optical-absorption-induced hot-phonon bath, we measure for the 5.6-GHz "breathing" acoustic mode of the cavity an initial phonon occupancy as low as < n > = 0.021 +/- 0.007 (mode temperature T-min approximate to 70 mK) and an intrinsic mechanical decay rate of gamma(0) = 328 +/- 14 Hz (Q(m) approximate to 1.7 x 10(7)). These measurements demonstrate the feasibility of using short pulsed measurements for a variety of quantum optomechanical applications despite the presence of steady-state optical heating.