Magnetic bistability of a single atom

Summary form only given. Photon antibunching is a prime example of quantum fluctuations of the electrodynamic field of resonance fluorescence. In the entangled atom-photon system it is complementally accompanied by transient atomic Rabi oscillations. Polarization resolved resonance fluorescence reveals an analogous phenomenon which can be interpreted as magnetic bistability at the microscopic level. We have subjected a single trapped Cs atom to a 3d-/spl sigma//sup +//spl sigma//sup -/-light field with local linear polarization. An atom in steady state at rest has vanishing magnetization, (m)=0. The observation of very strong anticorrelations for left/right handed photons with a contrast up to 62% and relaxation times of several /spl mu/s can be interpreted as a bistable, spontaneous formation of magnetization in the atom. The detection of the first photon causes projection onto a new state with (m)>0 which creates an unbalanced coupling to the circularly polarized components forming the linearly polarized field. Subsequently the atom is preferentially pumped into a stable stretched m-state. Unidirectional sequential photon absorption is responsible for acceleration (heating), while motion induced optical pumping causes a redistribution of Zeeman sublevels. Relaxation times are compatible with temperatures describing the time scale of atomic motion over a wavelength.