1. Department of Physics, Cornell University, Ithaca, New York 14853, USA


2. Department of Physics, University of Maryland, College Park, Maryland 20742, USA


3. Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada


4. Applied Physics, Caltech, Pasadena, California 91125, USA


5. These authors contributed equally to this work.

Correspondence to: K. C. Schwab⁴ Correspondence and requests for materials should be addressed to K.C.S (Email: schwab@caltech.edu).

Cold, macroscopic mechanical systems are expected to behave contrary to our usual classical understanding of reality; the most striking and counterintuitive predictions involve the existence of states in which the mechanical system is located in two places simultaneously. Various schemes have been proposed to generate and detect such states^{1, 2}, and all require starting from mechanical states that are close to the lowest energy eigenstate, the mechanical ground state. Here we report the cooling of 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 ± 1.3 and expect the mechanical resonator to be found with probability 0.21 in the quantum ground state of motion. Further cooling is limited by random excitation of the microwave resonator and heating of the dissipative mechanical bath. This level of cooling is expected to make possible a series of fundamental quantum mechanical observations including direct measurement of the Heisenberg uncertainty principle and quantum entanglement with qubits.