  We propose the use of quantum polarization spectroscopy for detecting
multi-particle entanglement of ultracold atoms in optical lattices. This
method, based on a light-matter interface employing the quantum Farady effect,
allows for the non destructive measurement of spin-spin correlations. We apply
it to the specific example of a one dimensional spin chain and reconstruct its
phase diagram using the light signal, readily measurable in experiments with
ultracold atoms. Interestingly, the same technique can be extended to detect
quantum many-body entanglement in such systems.
