  Controlling the motion of macroscopic oscillators in the quantum regime has
been the subject of intense research over the past decades. In this direction,
opto-mechanical systems, where the motion of micro-objects is strongly coupled
to 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 environments under very ultra-low vacuum.
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 to
levitating mechanical oscillators have been proposed. At the single spin level,
they offer the formidable prospect of transferring the inherent quantum nature
of electronic spins to the oscillators, with foreseeable far-reaching
implications in quantum sensing and tests of quantum mechanics. Adding spin
degrees of freedom to the toolbox would enable accessing a very rich playground
at the crossroad between condensed matter and atomic physics and with prospects
for exploring quantum effects and improved sensors based on the laws of quantum
mechanics. 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 the
experimental limits that, once overcome, will enable these systems to unleash
their full potential.
