  We report on the observation of the spin-exchange dynamics of
$^{171}\mathrm{Yb}$ atoms in the ground state $^1\mathrm{S}_0$ and in the
metastable state $^3\mathrm{P}_0$. We implement the mixed-dimensional
two-orbital system using a near-resonant and magic-wavelength optical lattices,
where the $^1\mathrm{S}_0$ and $^3\mathrm{P}_0$ atoms are itinerant in a
one-dimensional tube and localized in three dimensions, respectively. By
exploiting an optical Stern-Gerlach method, we observe the spin depolarization
of the $^1\mathrm{S}_0$ atoms induced by the spin-exchange interaction with the
$^3\mathrm{P}_0$ atom. Our work could open the way to the quantum simulation of
the Kondo effect.

Quantum Gases