Electron transport and coherence in semiconductor quantum dots and rings

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A number of experiments on electron transport and coherence in semiconductor vertical and lateral quantum dots and semiconductor rings is described. Quantum dots are often referred to as "artificial atoms", because of their similarities with real atoms. Examples of such atom-like properties that have been studied, are spin-singlet-triplet transitions and the Kondo effect. A strong Kondo effect is observed where Coulomb blockade is overcome completely and the conductance reaches the unitary-limit value at 2e2/h. It is shown that phase-coherent transport through a Kondo quantum dot is possible, by measuring electron interference in an Aharonov-Bohm ring with the dot embedded in one of its arms.Where single quantum dots are regarded as "artificial atoms", two quantum dots can be coupled to form an "artificial molecule". Motivated by their relevance for realizing solid-state quantum bits, electron transport experiments on two lateral quantum dots coupled in series are reviewed. Finally, an electro-magnetic Aharonov-Bohm effect in a 2D electron gas ring is studied. A new method is developed to measure the non-equilibrium electron dephasing time with a focus on the role of electron-electron interactions.