  Demonstrating and exploiting the quantum nature of larger, more macroscopic
mechanical objects would help us to directly investigate the limitations of
quantum-based measurements and quantum information protocols, as well as test
long standing questions about macroscopic quantum coherence. The field of
cavity opto- and electro-mechanics, in which a mechanical oscillator is
parametrically coupled to an electromagnetic resonance, provides a practical
architecture for the manipulation and detection of motion at the quantum level.
Reaching this quantum level requires strong coupling, interaction timescales
between the two systems that are faster than the time it takes for energy to be
dissipated. By incorporating a free-standing, flexible aluminum membrane into a
lumped-element superconducting resonant cavity, we have increased the single
photon coupling strength between radio-frequency mechanical motion and resonant
microwave photons by more than two orders of magnitude beyond the current
state-of-the-art. A parametric drive tone at the difference frequency between
the two resonant systems dramatically increases the overall coupling strength.
This has allowed us to completely enter the strong coupling regime. This is
evidenced by a maximum normal mode splitting of nearly six bare cavity
line-widths. Spectroscopic measurements of these 'dressed states' are in
excellent quantitative agreement with recent theoretical predictions. The basic
architecture presented here provides a feasible path to ground-state cooling
and subsequent coherent control and measurement of the quantum states of
mechanical motion.
