  We propose a novel and experimentally feasible approach to achieve
high-efficiency ground-state cooling of a mechanical oscillator in an
optomechanical system under the deeply unresolved sideband condition with the
assistance of both intracavity and extracavity squeezing. In the scheme, a
degenerate optical parametric amplifier is placed inside the optical cavity,
generating the intracavity squeezing; besides, the optical cavity is driven by
externally generated squeezing light, namely the extracavity squeezing. The
quantum interference effect generated by intracavity squeezing and extracavity
squeezing can completely suppress the non-resonant Stokes heating process while
greatly enhancing the anti-Stokes cooling process. Therefore, the
joint-squeezing scheme is capable of cooling the mechanical oscillators to
their quantum ground state in a regime far away from the resolved sideband
condition. Compared with other traditional optomechanical cooling schemes, the
single-photon cooling rate in this joint-squeezing scheme can be tremendously
enlarged by nearly three orders of magnitude. At the same time, the coupling
strength required to achieve ground-state cooling can be significantly reduced.
This scheme is promising for cooling large-mass and low-frequency mechanical
oscillators, which provides a prerequisite for preparing and manipulating
non-classical states in macroscopic quantum systems and lays a significant
foundation for quantum manipulation.
