  Spin-exchange collisions in alkali vapors have been at the basis of several
fundamental and applied investigations, like nuclear structure studies and
tests of fundamental symmetries, ultra-sensitive atomic magnetometers, magnetic
resonance and bio-magnetic imaging. Spin-exchange collisions cause loss of spin
coherence, and concomittantly produce spin noise, both phenomena being central
to quantum metrology. We here develop the quantum trajectory picture of
spin-exchange collisions, consistent with their long-standing ensemble
description using density matrices. We then use quantum trajectories to reveal
the nature of spin-noise correlations that spontaneously build up in
multi-species atomic vapors, frequently utilized in the most sensitive spin
measurements.
