  Controlling the motion of macroscopic oscillators in the quantum regime has
been the subject of intense research in recent decades. In this direction,
opto-mechanical systems, where the motion of micro-objects is strongly coupled
with laser light radiation pressure, have had tremendous success. In
particular, the motion of levitating objects can be manipulated at the quantum
level thanks to their very high isolation from the environment under ultra-low
vacuum conditions. To enter the quantum regime, schemes using single long-lived
atomic spins, such as the electronic spin of nitrogen-vacancy (NV) centers in
diamond, coupled with levitating mechanical oscillators have been proposed. At
the single spin level, they offer the formidable prospect of transferring the
spins' inherent quantum nature to the oscillators, with foreseeable
far-reaching implications in quantum sensing and tests of quantum mechanics.
Adding the spin degrees of freedom to the experimentalists' toolbox would
enable access to a very rich playground at the crossroads between condensed
matter and atomic physics. We review recent experimental work in the field of
spin-mechanics that employ the interaction between trapped particles and
electronic spins in the solid state and discuss the challenges ahead. Our focus
is on the theoretical background close to the current experiments, as well as
on the experimental limits, that, once overcome, will enable these systems to
unleash their full potential.
