  Linearly polarized light tuned slightly below the optical transition of the
negatively charged exciton (trion) in a single quantum dot causes the
spontaneous nuclear spin polarization (self-polarization) at a level close to
100%. The effective magnetic field of spin-polarized nuclei brings the optical
transition energy into resonance with photon energy. The resonantly enhanced
Overhauser effect sustains the stability of the nuclear self-polarization even
in the absence of spin polarization of the quantum dot electron. As a result
the optically selected single quantum dot represents a tiny magnet with the
ferromagnetic ordering of nuclear spins - the nuclear spin nanomagnet.
