Limits on QED-Induced Decoherence in the Stern-Gerlach Experiment

The Stern–Gerlach experiment reveals the quantum mechanical nature of spin through the force experienced by a non-relativistic neutral atom in a static magnetic field gradient. A key open question concerns when, along the atom’s trajectory, the spin state collapses into an eigenstate and by what mechanism. We analyze the influence of quantum electrodynamic (QED) vacuum fluctuations on spin dynamics and show that they induce spin self-interactions, leading to decoherence and memory effects. However, the magnitude of these effects is shown to be too small to account for the deterministic collapse of the spin wavefunction.