  Preparing and manipulating quantum states of mechanical resonators is a
highly interdisciplinary undertaking that now receives enormous interest for
its far-reaching potential in fundamental and applied science. Up to now, only
nanoscale mechanical devices achieved operation close to the quantum regime. We
report a new micro-optomechanical resonator that is laser cooled to a level of
30 thermal quanta. This is equivalent to the best nanomechanical devices,
however, with a mass more than four orders of magnitude larger (43 ng versus 1
pg) and at more than two orders of magnitude higher environment temperature (5
K versus 30 mK). Despite the large laser-added cooling factor of 4,000 and the
cryogenic environment, our cooling performance is not limited by residual
absorption effects. These results pave the way for the preparation of 100-um
scale objects in the quantum regime. Possible applications range from
quantum-limited optomechanical sensing devices to macroscopic tests of quantum
physics.
