Emergence of the persistent spin helix

The spin-orbit interaction is a relativistic effect: an electron moving in an external electric field sees a magnetic field in its rest frame. In a semiconductor the interaction causes an electron's spin to precess as it moves through the material; this is the basis of various proposed "spintronic" devices. In nano-structures, quantum confinement can change the symmetry of the spin-orbit interaction; under special circumstances a new, higher symmetry is predicted to create a conserved, helical spin wave at a wavelength of a few microns: the "persistent spin helix" (PSH). I describe the theory of this effect, and the optical experiments that have recently measured it in semiconductor quantum wells.