  Quantum fluctuations of the electromagnetic vacuum impose an observable
quantum limit to the lowest temperatures that can be reached with conventional
laser cooling techniques. As laser cooling experiments continue to bring
massive mechanical systems to unprecedented temperatures, this quantum limit
takes on increasingly greater practical importance in the laboratory.
Fortunately, vacuum fluctuations are not immutable, and can be "squeezed"
through the generation of entangled photon pairs. Here we propose and
experimentally demonstrate that squeezed light can be used to sideband cool the
motion of a macroscopic mechanical object below the quantum limit. To do so, we
first cool a microwave cavity optomechanical system with a coherent state of
light to within 15% of this limit. We then cool by more than 2 dB below the
quantum limit using a squeezed microwave field generated by a Josephson
Parametric Amplifier (JPA). From heterodyne spectroscopy of the mechanical
sidebands, we measure a minimum thermal occupancy of 0.19 phonons. With this
novel technique, even low frequency mechanical oscillators can in principle be
cooled arbitrarily close to the motional ground state, enabling the exploration
of quantum physics in larger, more massive systems.
