  We present and analyze an effective scheme for preparing squeezed spin states
in a novel spin-mechanical hybrid device, which is realized by a single crystal
diamond waveguide with built-in silicon-vacancy (SiV) centers. After studying
the strain couplings between the SiV spins and the propagating phonon modes, we
show that long-range spin-spin interactions can be achieved under large
detuning condition. We model these nonlinear spin-spin couplings with an
effective one-axis twisting Hamiltonian, and find that the system can be
steered to the squeezed spin states in the practical situations. This work may
have interesting applications in high-precision metrology and quantum
information.
