We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with (^3)He in a magnetic field. A rigorous quantum theory for spin-changing transitions is developed and applied to calculate the spin exchange and spin relaxation rates of Li and K atoms in cryogenic (^3)He gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for Li-(^3)He and K-(^3)He, which are in agreement with the present theory. Our calculations demonstrate that the alkali-metal atoms have extremely slow spin depolarization rates, suggesting a number of potential applications in precision spectroscopy and quantum optics.
