We use cryogenic scanning tunneling microscopy (STM) to create individual nanostructures on semiconductor surfaces and explore their quantum properties by scanning tunneling spectroscopy. The structures are assembled from native indium adatoms on an indium arsenide surface. The adatoms are positively charged and can be repositioned by the STM tip, making it possible to engineer the electrostatic surface potential landscape and confine surface-state electrons on the atomic length scale and with atomic precision. In this way, quantum dots with a perfectly defined level structure can be constructed, as well as quantum-dot molecules whose coupling has no intrinsic variation but can nonetheless be tuned over a wide range.
This experimental approach opens the way to create and tune quantized electronic states in designed nanoscale structures such as quantum rings, single-molecule transistors, and dimerized quantum-dot chains exhibiting topological boundary modes. Atom manipulation in combination with scanning tunneling spectroscopy provides detailed insight into the behavior of electrons in reduced dimensions. This insight is important both for fundamental science and future quantum technologies.