Quantum dots or rings are artificial nanometre-sized clusters that confine
electrons in all three directions. They can be fabricated in a semiconducto
r system by embedding an island of low-bandgap material in a sea of materia
l with a higher bandgap. Quantum dots are often referred to as artificial a
toms because, when filled sequentially with electrons, the charging energie
s are pronounced for particular electron numbers(1-3); this is analogous to
Hund's rules in atomic physics. But semiconductors also have a valence ban
d with strong optical transitions to the conduction band. These transitions
are the basis for the application of quantum dots as laser emitters(4), st
orage devices(5-7) and fluorescence markers(8). Here we report how the opti
cal emission (photoluminescence) of a single quantum ring changes as electr
ons 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 o
f the jumps reveal a shell structure.