  We have cooled the motion of a radio-frequency nanomechanical resonator by
parametric coupling to a driven microwave frequency superconducting resonator.
Starting from a thermal occupation of 480 quanta, we have observed occupation
factors as low as 3.8$\pm$1.2 and expect the mechanical resonator to be found
with probability 0.21 in the quantum ground state of motion. Cooling is limited
by random excitation of the microwave resonator and heating of the dissipative
mechanical bath.

Quantum Physics