We present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with ${}^3\mathrm{He}$ 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 ${}^3\mathrm{He}$ gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for $\mathrm{Li}\text{−}{}^3\mathrm{He}$ and $\mathrm{K}\text{−}{}^3\mathrm{He}$, 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.

• Received 3 October 2008

DOI:https://doi.org/10.1103/PhysRevA.78.060703