Physics Colloquia

Quantum computing is based on the manipulation of quantum bits – qubits – that are two-level systems meeting a set of stringent requirements known as the DiVincenzo criteria. As much as possible, qubits must be isolated from their environment in order to preserve quantum coherence. Remarkably, the techniques used to make classical silicon CMOS devices can be used to make qubits with excellent performance. The operation of these devices, on the other hand – from the required temperatures to the number of electrons comprising a typical qubit – is very different from what is found in even the most advanced classical integrated circuits. In this talk I will present both a short historical overview of how quantum computing in silicon has developed, as well as the latest results from both our group at Wisconsin and from around the world. I will emphasize the role of integration, including 3D integration, which enables readout of qubits formed in Si/SiGe heterostructures by measuring the microwave transmission of a superconducting resonator that is hosted on a separate substrate. That resonator chip is flip-chip bonded to the qubit chip, creating a stacked set of integrated semiconductor and superconductor circuits. I will close this talk by discussing very recent results demonstrating the remarkable properties of silicon quantum wells containing short wavelength oscillations in the concentration of added germanium atoms. Advances like these have, in just the last few years, demonstrated that a future quantum computing technology in silicon will likely integrate sophisticated techniques and knowledge cutting across many disciplines, not only from physics and computer science, but also from electrical engineering and materials science – a feature that makes it an incredibly dynamic (and fun!) field of science and technology.

Collective motion of active matter occurs in many living systems, such as bacterial communities, epithelial cell populations, bird flocks, and fish schools. A remarkable example can be found in the soil-dwelling bacterium Myxococcus xanthus. Key to the life cycle of M. xanthus cells is the formation of collective groups: they feed on prey in swarms and aggregate upon starvation. However, the physical mechanisms that keep M. xanthus cells together remains unclear. I’ll present a computational model to explore the role that capillary forces play in bacterial collective dynamics. The modeling results, combined with experiments, show that water menisci forming around bacteria mediate strong capillary attraction between cells. The model accounts for a variety of previously observed phases of collective dynamics as the result of a competition between cell-cell capillary attraction and cell motility. Finally, I’ll discuss the large-scale self-organization of bacterial populations and highlight the importance of capillary force in this process. Together, these results suggest that cell-cell capillary attraction provides a generic mechanism underpinning bacterial collective dynamics.