Single-electron transport measurements have been the main experimental technique for investigating electron tunneling into QDs ( 5). Tremendous progress has been made not only in understanding the properties of single electrons in QDs but also in controlling their quantum states, which is an essential prerequisite for quantum computation ( 4). The ability to confine single charges at discrete energy levels makes semiconductor quantum dots (QDs) promising candidates as a platform for quantum computation ( 1, 2) and single-photon sources ( 3). Relative coupling strengths can be estimated from these images of grouped coupled dots. Scanning the surface shows that several quantum dots may reside on what topographically appears to be just one. The spectra show clear level degeneracies in isolated quantum dots, supported by the quantitative measurement of predicted temperature-dependent shifts of Coulomb blockade peaks. An electron addition spectrum results from a change in cantilever resonance frequency and dissipation when an electron tunnels on/off a dot. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. Because the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. Strong confinement of charges in few-electron systems such as in atoms, molecules, and quantum dots leads to a spectrum of discrete energy levels often shared by several degenerate states.
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