The Department of Physics at the University of Illinois at Urbana-Champaign is known for its collegial style of research—the “Urbana style”—that often involves collaborations across research areas and among theoretical and experimental physicists. Here is an inside glimpse of what some of our leading physicists are working on.

# Spectrum - FALL 2018

###### The Department of Physics at the University of Illinois at Urbana-Champaign is known for its collegial style of research—the “Urbana style”—that often involves collaborations across research areas and among theoretical and experimental physicists. Here is an inside glimpse of what some of our leading physicists are working on.

BARRY BRADLYN

Theoretical Condensed Matter Physics**TAYLOR HUGHES, VIDYA MADHAVAN, AND DALE VAN HARLINGEN**

Condensed Matter Physics Collaboration

GILBERT HOLDER

Theoretical Astrophysics and Cosmology

ALEXEY BEZRYADIN

Experimental Condensed Matter Physics

**BARRY BRADLYN**

Broadly speaking, my research interests lie in studying the interplay between geometry and topology in condensed matter systems. Geometry enters into condensed matter physics in the guise of order parameters for symmetry breaking, conserved quantities, crystal symmetry groups, and elasticity theory. Topology, on the other hand, enters when we try to describe properties of systems which are robust to perturbations, such as quantized response functions, surface states, and vortex-like excitations. Since the initial discovery of systems with topologically protected behavior, the influence of topology has spread across all areas of condensed matter physics. It is this—in addition to individual realizations of topological phases—that is in my opinion the biggest boon of this new paradigm. Topology now stands alongside abstract algebra (as it pertains, for instance, to symmetry groups) as one of our main tools for exploring quantum phenomena in solids and liquids.

My current research focuses on using symmetries to learn about the topological properties of materials. As a theoretical physicist, one way that I do this is by asking about how different systems respond to geometric deformations like shears and strains. The electrons in topological materials behave like a fluid, and can be assigned properties like density and viscosity, which can be both constrained by symmetry and dependent on the topological phase of the system. A second focus of my research is in using the tools of crystal symmetry to design new topological materials. I am developing tools to better understand the role of chemical bonding in determining the topological properties of materials, and I apply these tools to discover new topological properties in previously synthesized materials.

**TAYLOR HUGHES, VIDYA MADHAVAN, AND DALE VAN HARLINGEN**

We are combining forces to bring complementary experimental and theoretical expertise to bear on a new topic in condensed matter physics. Our group collaboration was recently awarded seed funding through the I-MRSEC iSuperSEED program to study higher-order topological phases of matter.

While the three of us have worked together in the past, we are now able to really unite our strengths of materials growth, quantum devices and measurement, and theoretical modeling to approach this nascent field of condensed matter physics. The work on this new topic is a natural outgrowth of our previous research efforts, in which the three of us have separately established strong track records in the study of topological matter. Dale has made breakthroughs in topological superconductivity, Vidya pioneered experimental efforts on topological crystalline insulators, and Taylor has made a number of important theoretical contributions, including the initial work that spawned the field that is now the focus of our joint work.

Our collaborative work begins with quantum materials growth that will take place in Vidya’s lab. After joining the faculty at the U of I, she has quickly established a flourishing materials-growth capability in addition to her primary expertise in scanning tunneling microscopy (STM). Vidya and Taylor are working together to design the precise materials needed to create higher-order topological insulators, starting with films of bismuth and moving on to more exotic materials from there. Once the materials are grown, it is Taylor’s and Dale’s job to help design the devices and experiments needed to characterize these new phases of matter. Vidya is planning to carry out STM measurements, and Dale will build devices that interface these materials with superconductors, which will enable us to explore a wide-range of exciting quantum phenomena.

We are all excited to have the opportunity to work on such a well defined, focused project together. Typically, awards for groups of faculty investigators would have a diverse set of projects, but in our case we have one goal, and if we are successful, there is a chance we could expand our effort into a broad range of interconnected scientific disciplines and departments. It’s not often the case that you have the chance to explore a high-impact and possibly risky project with a strong coordinated effort, and we are eagerly anticipating our next few years of research.

**GILBERT HOLDER**

How did the universe begin? What is the universe made of? How did the rich structure we see around us arise? Long the domain of philosophers and theologians, these are now physics questions. I am always looking for new ways to find answers to these questions.

As we look out in the universe, we are looking back in time. This allows us to trace the evolution of galaxies and galaxy clusters, to see how the visible matter has evolved. We can use these measurements to learn about the expansion history of the universe, which appears to be entering a phase of accelerated expansion driven by a mysterious form of energy density that has been dubbed “dark energy.” We can also use deflections of light by fluctuations in the gravitational potentials to trace the behavior of the total mass, which is found to be dominated by “dark matter,” a form of matter which is well studied but still not detected in the lab. We map this dark matter on a wide variety of scales, ranging from the scale of the entire universe down to scales much smaller than individual galaxies, searching for clues about the nature of dark matter. Ideas that we are investigating include self-interacting dark matter, warm dark matter, and dark matter that is actually acting as a Bose-Einstein condensate on small scales.

Going back further in time, the cosmic microwave background was formed when the universe was only one-thousandth its current size, the earliest moment from which we can measure electromagnetic radiation. I am currently a member of a team with a telescope at the geographic South Pole (the South Pole Telescope), which is currently making the most detailed maps of this background to date, and am looking forward to the next generation experiment, CMB-S4. These data provide a wealth of knowledge about our universe, with perhaps the most exciting possibility being to use the universe as a whole as a gravitational wave detector. Whatever happened at the beginning of time, it is likely to have roiled spacetime in a way that produces gravitational waves that have been propagating through the universe ever since. These waves will slightly distort the cosmic microwave background; measuring this effect would let us reconstruct the very first moments in time, when the universe as we know it began.

In the course of these measurements, there are always new discoveries, and it is important to be aware of possible serendipitous discoveries. For example, there has been recent evidence of a possible extra planet in the outer solar system, more than a hundred times farther from the Sun than the Earth, with a mass five to ten times the mass of the Earth. Such a planet would be easily discovered if it happened to be in the field of view of the South Pole Telescope, as would any other dwarf planets (like Pluto) that remain to be discovered. Other serendipitous sources include accreting black holes and gamma-ray bursts. While the motivation for our surveys are big questions about the structure and evolution of the universe, we may also be discovering new answers to the oldest question in astronomy, our solar system.

**ALEXEY BEZRYADIN**

My fields of interest include condensed matter physics, nanotechnology, and quantum physics. In my lab, we fabricate and study nanometer-scale superconducting devices, such as memory elements made of superconducting nanowires, superconducting qubits and nanocapacitors. Arguably, the spectacular, very rapid evolution of the computer and information processing technologies is happening because of the critical contributions of researchers working in nanotechnology and quantum physics. Quantum mechanics (QM), originating from Planck’s discovery of the energy quantization followed by Heisenberg’s discovery of an exact mathematical description of quantum phenomena, was originally invented to describe very small (elementary) particles, such as electrons and protons. After a hundred years of evolution, QM can now describe macroscopic systems. The idea of macroscopic quantum mechanics, originally proposed by our colleague A. Leggett, has been realized in the form of qubits, or quantum bits. These devices mimic electrons and atoms in the sense that they can be prepared in quantum superpositions of macroscopically distinct states, and their evolution is governed by Schrödinger’s equation rather than by the Newtonian equations that usually apply to macroscopic systems.

My group aims to develop new qubits having longer coherence times—a necessity for the creation of practical quantum computers. We have collaborated with J. Eckstein’s group and developed and tested transmon qubits with impressive characteristics, namely a very long relaxation time of about 50 microseconds. Yet this is still not enough to realize practical quantum computers. For that reason, we are focusing now on developing topological qubits—hybrid devices combining topological insulators and superconductors. Topological qubits are predicted to harbor so-called non-Abelian particles, namely Majorana particles. These particles are peculiar in the sense that they remember their mutual history of mutual exchange operations, where “exchange” means an exchange or swapping of the positions of two particles. Thus, quantum information can be encoded, at least in theory, in an ensemble of non-Abelian particles by performing braiding operations, or more simply put, by moving the particles around one another. Each such motion changes the ground state of the system into a new independent ground state. Unfortunately, non-Abelian systems have never been observed. If realized in the laboratory, they would give rise to topologically protected qubits, which would hold their quantum coherence for a long time. As of today, utilization of Majorana zero modes represent one of the main routes for the realization of quantum computers.