Topological quantum materials are characterized by nontrivial features in their electronic band structure that give rise to robust boundary states and unconventional quantum phenomena, offering opportunities for quantum information science and advanced electronics. Realizing and controlling these states requires precise tuning of crystal structure, strain, dimensionality, and interfaces. In this talk, I will discuss our efforts to use molecular beam epitaxy as a platform for controlling topological quantum phases, with a focus on elemental Sn and Bi–Sb materials.

Elemental Sn provides a unique system in which α-Sn hosts tunable topological phases, while β-Sn is superconducting. We demonstrate selective stabilization of these structural phases through epitaxial growth and lattice strain and investigate the strain-dependent electronic structure and quantum transport of α-Sn. Building on this capability, we are developing α-Sn/β-Sn heterostructures with atomically controlled interfaces as a platform for exploring the interplay between topology and superconductivity. I will also discuss dimensionality-driven topological transitions in Sb and Bi thin films, where quantum confinement provides another route to tuning electronic states. Finally, I will describe our efforts to integrate topological materials into nanoscale and hybrid device architectures through selective-area epitaxy and superconductor integration, and briefly discuss emerging freestanding quantum-material membrane platforms for strain control and heterogeneous integration.

Electrons in solids can do two remarkable things: they can pair up and flow without resistance, and they can collectively behave as fractionally charged particles. Predicting these phenomena remains a major challenge because the physics spans enormous spatial scales and exponentially large quantum state spaces. I will show how my group uses AI to understand electron pairing in realistic materials: machine-learned Hamiltonians reproduce moiré electronic structures with meV accuracy for systems of up to millions of atoms, enabling studies of superconductivity from weak to strong coupling in twisted semiconductors.

To understand fractionalization, we turn to quantum processors. Constant-depth circuits prepare 18 fractional quantum Hall states on up to 156 superconducting qubits; remarkably, some of the most exotic states are also the easiest to prepare. Together, these efforts show how AI can extend quantum-materials modeling to realistic scales, while quantum processors open access to highly entangled states beyond classical reach.

No colloquium: Labor Day Holiday.

Our understanding of the Red Planet is evolving as we continue to receive data from the ongoing activities of NASA’s Curiosity and Perseverance rovers. In this talk, we will review the questions driving our exploration of the martian surface, take a look at the some of the basic data derived from rover science, and explore some of the fundamental difficulties of interpreting data from the surface of another planet. We will particularly focus on issues familiar to all of science: our current state of knowledge, intrinsic bias and the assumptions we make and the questions we ask, and the necessity of integrating information across both discipline and scale.

TBA

The discovery of gravitation by Isaac Newton in the 17th century is often referred to as the beginning of physics as we know it. More than two centuries later, the value of the gravitational constant, “the big G,” serves as the fundamental basis for understanding large-scale cosmic structures, while providing the essential theoretical foundation for general relativity. Yet, despite centuries of experimentation, the global dataset for G remains frustratingly inconsistent. Compared to the strength of the electromagnetic interaction, known to a relative uncertainty of 0.2 parts-per-billion, G is only known to 22 parts-per-million. This staggering five-order-of-magnitude gap highlights the unique stubbornness of gravity. Two primary factors contribute to this: (1) unlike its dimensionless electromagnetic counterpart, G is a dimensioned quantity tied to our macroscopic units, and (2) the gravitational force is extremely weak compared to other fundamental forces and impossible to shield. In this talk, I will discuss the recent NIST measurement that we undertook to better understand the global dataset. I will attempt to answer three central questions: Why is G so difficult to pin down? How did we measure it? And what now for the future of the gravitational constant?

No colloquium; Fall Break.

Global celebration greeted the 2012 discovery at CERN’s Large Hadron Collider of a particle that matches the textbook description of the Higgs boson. That achievement validated a remarkable chain of theoretical reasoning that combined the notion that symmetries dictate interactions with lessons from superconductivity. It was enabled by triumphs of accelerator art and experimental technique, and by human resourcefulness and collaboration on a global scale.

Some imagine that, once the keystone of the standard model of particle physics has been set, our subject is over. Others worry that we may be at an impasse, without sharp clues to a more complete understanding. I prefer to savor the challenge that our description of nature is incomplete, that we have so much more to learn.

This colloquium surveys themes that explore the breadth and depth of opportunities to enhance our understanding of the physical world, illustrated by examples from my research.

Quantum fluids and solids (QFS) exhibit remarkable macroscopic quantum phenomena and provide exceptionally clean environments for exploring fundamental physics and developing emerging quantum technologies. Superfluid helium-4 (He II), for example, supports frictionless superflow and quantized vortices, while cryogenic surfaces such as liquid helium and solid neon can naturally trap electrons in vacuum with minimal environmental disturbance. In this colloquium, I will discuss our efforts to explore and harness these unusual properties of cryogenic quantum matter. I will begin by introducing He II as a model quantum fluid and describing advanced flow-visualization techniques that allow us to directly probe quantized vortex dynamics, quantum turbulence, and related nonequilibrium phenomena. I will highlight our recent studies of vortex reconnections, quantum dissipation associated with vortex rings, and coherent vortex structures. I will then turn to emerging opportunities for using quantum fluids as platforms for quantum sensing, including our recent work with magnetically levitated He II droplets. Finally, I will discuss our efforts to develop qubits based on single electrons trapped on solid neon surfaces. Recent electron-on-neon experiments have demonstrated excellent coherence, but scaling the platform to larger and more controllable qubit arrays remains a major challenge. I will describe a new architecture based on magnetically levitated solid-neon microparticles that may offer a pathway toward scalable electron-qubit systems. Together, these studies illustrate how quantum fluids and solids can bridge fundamental discoveries in macroscopic quantum physics with emerging opportunities in quantum science and technology.