Once the periodic properties of elements were unveiled, chemical behaviour could be
understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum
dots, and quantum computation could be performed by merely controlling the outer-shell
electrons of dot-based qubits. Imperfections in semiconductor materials disrupt this analogy,
so real devices seldom display a systematic many-electron arrangement. We demonstrate
here an electrostatically confined quantum dot that reveals a well defined shell structure. We …