1. Center for NanoScience and Sektion Physik, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München, Germany
2. Materials Department and QUEST, University of California, Santa Barbara, California 93106 , USA
3. Present address: Department of Physics, Heriot-Watt University, Edinburgh EH14 4AS, UK

Correspondence to: R. J. Warburton^1,2 Correspondence and requests for materials should be addressed to R.J.W. (e-mail: Email: R.J.Warburton@hw.ac.uk).

Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers^{1, 2, 3}; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters⁴, storage devices^{5, 6, 7} and fluorescence markers⁸. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.