Electron-spin dynamics induced by photon spins
Strong rotating magnetic fields may cause a precession of the electronʼs spin around the rotation axis of the magnetic field. The superposition of two counterpropagating laser beams with circular polarization and opposite helicity features such a rotating magnetic field component but also carries spin. The laserʼs spin density, which can be expressed in terms of the laserʼs electromagnetic fields and potentials, couples to the electronʼs spin via a relativistic correction to the Pauli equation. We show that the quantum mechanical interaction of the electronʼs spin with the laserʼs rotating magnetic field and with the laserʼs spin density counteract each other in such a way that a net spin rotation remains with a precession frequency that is much smaller than the frequency one would expect from the rotating magnetic field alone. In particular, the frequency scales differently with the laserʼs electric field strength depending on whether relativistic corrections are taken into account or not. Thus, the relativistic coupling of the electronʼs spin to the laserʼs spin density changes the dynamics not only quantitatively but also qualitatively as compared to the nonrelativistic theory. The electronʼs spin dynamics are a genuine quantum mechanical relativistic effect.