Making use of self-assembly techniques, we demonstrate the realization of nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit. We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic field perpendicular to the plane of the rings, we find that when approximately one flux quantum threads the interior of each ring, a change in the ground state from angular momentum $ell = 0$ to $ell = -1$ takes place. This ground state transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic field dispersion of the single-electron charging energy.
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