Gate-defined quantum dots in silicon/silicon-germanium are widely regarded as a promising platform for scalable quantum computing. They also have particularly rich physics, because of two features of the silicon band structure. First, electrons are relatively heavy in silicon, with a mass of 0.19 the free electron mass, making it surprisingly easy to reach the limit of strong interactions. I will present experiments in which these interactions lead to the formation of two-electron Wigner molecule states. One consequence of such strong interactions and Wigner-molecule physics is a reduction in the gap between the two-electron excited states, enabling coherent control of many different resonances in a very convenient microwave frequency range. The second important feature of the silicon band structure is its six conduction band minima, or valleys. In strained-silicon quantum dots, two of these valleys are lower in energy than the other four. I will discuss two new types of structures that incorporate modulations of germanium concentration directly in the quantum well leading to an increased splitting between these two low-lying valley valleys in silicon, which has important consequences for the development of qubits in silicon quantum dots.
