  Optically addressable spins are actively investigated in quantum
communication, processing and sensing. Optical and spin coherence lifetimes,
which determine quantum operation fidelity and storage time, are often limited
by spin-spin interactions, which can be decreased by polarizing spins in their
lower energy state using large magnetic fields and/or mK range temperatures.
Here, we show that optical pumping of a small fraction of ions with a fixed
frequency laser, coupled with spin-spin interactions and spin diffusion, leads
to substantial spin polarization in a paramagnetic rare earth doped crystal,
$^{171}$Yb$^{3+}$:YSO. Indeed, up to more than 90 % spin polarizations have
been achieved at 2 K and zero magnetic field. Using this spin polarization
mechanism, we furthermore demonstrate an increase in optical coherence lifetime
from 0.3 ms to 0.8 ms, due to a strong decrease in spin-spin interactions. This
effect opens the way to new schemes for obtaining long optical and spin
coherence lifetimes in various solid-state systems such as ensembles of rare
earth ions or color centers in diamond, which is of interest for a broad range
of quantum technologies.
