Research, teaching and outreach in Physics at UW–Madison
Condensed matter
Keith Bechtol, Victor Brar promoted to Associate Professors
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Congratulations to Keith Bechtol and Victor Brar, who were both promoted to associate professors of physics with tenure!
Keith Bechtol
Bechtol is an observational cosmologist with research interests in dark matter and dark energy, using the whole Universe as a lab to understand the fundamental physics of nature. He is part of the Dark Energy Survey (DES) that has cataloged more 500 million galaxies and thousands of supernovae to understand the nature of dark energy. He and his group are also working on the construction and commissioning of the Vera C. Rubin Observatory in preparation for the Legacy Survey of Space and Time (LSST). LSST is expected to catalog more stars, more galaxies and more solar system objects during its first year of operations than all previous telescopes combined.
“Professor Bechtol plays a leading role in the Vera C. Rubin Observatory, which is now poised to enable a major leap in the data available for understanding the development of our universe,” says Mark Eriksson, Chair and John Bardeen Professor of Physics.
Bechtol was a co-convener of the DES’s Science Release Working Group for four years and a co-convener of the Milky Way Working Group for two years. He is now serving as Technical Coordinator for the LSST Dark Energy Science Collaboration. In 2022, he was selected to the Department of Energy’s Early Career Research Program. He also proposed and is the faculty lead for the physics department’s Thaxton Fellowship, whose goal is to provide more equitable access to physics research experiences for undergraduates.
Victor Brar
Brar, the Van Vleck professor of physics and a member of the Wisconsin Quantum Institute, is an experimental condensed matter physicist with a research focus on quantum materials and novel imaging techniques. His group works on developing metamaterials such as 2D materials for use in laser sailing or fabricating graphene structures for use in telecommunications. They also use scanning tunneling microscopy and scanning tunneling potentiometry to understand the physical and electrical properties of materials.
“The experiments performed by Professor Brar and his research team have enabled measurements of completely new regimes for electron transport in 2D materials,” Eriksson says.
Brar was awarded a Moore Inventor Fellowship in 2018, a Sloan Fellowship in 2021, and a National Science Foundation CAREER award in 2023. He has additionally received two UW–Madison Research Forward awards.
Alex Levchenko honored with H.I. Romnes Fellowship
Physics professor Alex Levchenko was one of thirty-five of the University of Wisconsin–Madison faculty to be awarded fellowships from the Office of the Vice Chancellor for Research and Graduate Education for 2023-24. The awardees span the four divisions on campus: arts and humanities, physical sciences, social sciences and biological sciences.
“These awards recognize our faculty research, academic and outreach successes and provide an opportunity for continued development of their outstanding research programs,” says Steve Ackerman, vice chancellor for research and graduate education. “I’m grateful that we are able to recognize invest in these faculty in this way, and I look forward to seeing the results of their imaginative use of these funds.”
The awards are possible due to the research efforts of UW–Madison faculty and staff. Technology that arises from these efforts is licensed by the Wisconsin Alumni Research Foundation and the income from successful licenses is returned to the OVCRGE, where it’s used to fund research activities and awards throughout the divisions on campus.
Eighteen faculty, including Levchenko, have been honored with the H.I. Romnes Fellowships to recognize faculty with exceptional research contributions within their first six years from promotion to a tenured position. The award is named in recognition of the late WARF trustees president H.I. Romnes and comes with $60,000 that may be spent over five years.
Levchenko studies fundamental aspects of condensed matter physics with a focus on electronic phases of matter and quantum transport. Specific areas of expertise include superconductivity, topological order, and nanoscale systems such as graphene and other van der Waals materials. He is a Fellow of the American Physical Society and of the Alexander von Humboldt Foundation, and recipient of an early career grants from the National Science Foundation and the Binational Science Foundation. His teaching covers all levels of undergraduate and graduate education, and he serves on multiple professional review panels internationally.
Victor Brar, Wisconsin Center for Semiconductor Thermal Photonics earn UW Research Forward funding
The Wisconsin Center for Semiconductor Thermal Photonics will explore fundamental science at the intersection of semiconductor technology and radiative heat transfer. This cross-disciplinary center will explore thermal radiation in unconventional semiconductor materials, in nanostructures, and in extreme conditions, and achieve control of the directionality and timing of radiative heat transfer at unprecedented scales. New technologies will emerge from these fundamental studies, including low-cost spectrometers, imaging and ranging, and energy harvesting and active cooling.
The project is led by ECE associate professor and physics affiliate professor Mikhail Kats as Principal Investigator, with Physics associate professor Victor Brar as one of the co-PIs.
Research Forward, a competition sponsored by the Office of the Vice Chancellor for Research and Graduate Education (OVCRGE), is intended to stimulate and support highly innovative and groundbreaking research at UW–Madison.
The initiative is supported by the Wisconsin Alumni Research Foundation (WARF) and will provide funding for 1–2 years, depending on the needs and scope of the project.
Smooth sailing for electrons in graphene
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A heatmap of electron location in graphene shows that at the lower temperature (left panel), the electrons are more likely to bump into impurities (circles), with relatively fewer making it through the channel between impurities. At higher temperatures (right panel), electron flow shifts to being fluid-like. Fewer are stuck at the impurities and more flow through the channels. UNIVERSITY OF WISCONSIN–MADISON
Physicists at the University of Wisconsin–Madison directly measured, for the first time at nanometer resolution, the fluid-like flow of electrons in graphene. The results, which will appear in the journal Science on Feb. 17, have applications in developing new, low-resistance materials, where electrical transport would be more efficient.
Graphene, an atom-thick sheet of carbon arranged in a honeycomb pattern, is an especially pure electrical conductor, making it an ideal material to study electron flow with very low resistance. Here, researchers intentionally added impurities at known distances and found that electron flow changes from gas-like to fluid-like as temperatures rise.
Zach Krebs
“All conductive materials contain impurities and imperfections that block electron flow, which causes resistance. Historically, people have taken a low-resolution approach to identifying where resistance comes from,” says Zach Krebs, a physics graduate student at UW–Madison and co-lead author of the study. “In this study, we image how charge flows around an impurity and actually see how that impurity blocks current and causes resistance, which is something that hasn’t been done before to distinguish gas-like and fluid-like electron flow.
The researchers intentionally introduced obstacles in the graphene, spaced at controlled distances and then applied a current across the sheet. Using a technique called scanning tunneling potentiomentry (STP), they measured the voltage with nanometer resolution at all points on the graphene, producing a 2D map of the electron flow pattern.
No matter the obstacle spacing, the drop in voltage through the channel was much lower at higher temp (77 kelvins) vs lower temp (4 K), indicating lower resistance with more electrons passing through.
At temperatures near absolute zero, electrons in graphene behave like a gas: they diffuse in all directions and are more likely to hit obstacles than they are to interact with each other. Resistance is higher, and electron flow is relatively inefficient. At higher temperatures — 77 K, or minus 196 C — the fluid-like behavior of electron flow means they are interacting with each other more than they are hitting obstacles, flowing like water between two rocks in the middle of a stream. It is as if the electrons are communicating information about the obstacle to each other and diverting around the rocks.
“We did a quantitative analysis [of the voltage map] and found that at the higher temperature, the resistance is much lower in the channel. The electrons were flowing more freely and fluid-like,” Krebs says. “Graphene is so clean that we’re forcing the electrons to interact with each other before they interact with anything else, and that is crucial in getting them to behave like a fluid.”
Former UW–Madison graduate student Wyatt Behn is a co-first author on this study conducted in physics professor Victor Brar’s group. Funding was provided by the U.S. Department of Energy (DE-SC00020313), the Office of Naval Research (N00014-20-1-2356) and the National Science Foundation (DMR-1653661).
Finding some wiggle room in semiconductor quantum computers
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Classical computers rarely make mistakes, thanks largely to the digital behavior of semiconductor transistors. They are either on or they’re off, corresponding to the ones and zeros of classical bits.
On the other hand, quantum bits, or qubits, can equal zero, one or an arbitrary mixture of the two, allowing quantum computers to solve certain calculations that exceed the capacity of any classical computer. One complication with qubits, however, is that they can occupy energy levels outside the computational one and zero. If those additional levels are too close to one or zero, errors are more likely to occur.
“In a classical computer, all the aspects of a transistor are super uniform,” says UW–Madison Distinguished Scientist Mark Friesen, an author on both papers. “Silicon qubits are in many ways like transistors, and we’ve gotten to the stage where we can control the qubit properties very well, except for one.”
That one property, known as the valley splitting, is the buffer between the computational one-zero energy levels and the additional energy levels, helping to reduce quantum computing errors.
In two papers published in Nature Communications in December, researchers from the University of Wisconsin–Madison, the University of New South Wales and TU-Delft showed that tweaking a qubit’s physical structure, known as a silicon quantum dot, creates sufficient valley splitting to reduce computing errors. The findings turn conventional wisdom on its head by showing that a less perfect silicon quantum dot can be beneficial.
UW–Madison physics professor Alex Levchenko has been named a Humboldt Fellow for Experienced Researchers. Sponsored by the Alexander von Humboldt Foundation, the fellowship enabled highly-qualified scientists and scholars from abroad to spend time conducting research at a partner university in Germany.
Levchenko was nominated by the Max Planck Institute for Solid State Research in Stuttgart, where he will be affiliated with the Quantum Many Body Theory Department. His research topic will be “Effects of Strong Coupling Fluctuations, Criticality, and Topology in Superconductors.”
Bucket brigades and proton gates: Researchers shed new light on water’s role in photosynthesis
Photosystem II is a protein in plants, algae and cyanobacteria that uses sunlight to break water down into its atomic components, unlocking hydrogen and oxygen. A longstanding question about this process is how water molecules are funneled into the center of Photosystem II, where water is split to produce the oxygen we breathe. A better understanding of this process could inform the next generation of artificial photosynthetic systemsthat produce clean and renewable energy from sunlight and water.
In a paper published last week in Nature Communications, an international collaboration between scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (LBNL), SLAC National Accelerator Laboratory and several other institutions uncovers how the protein takes in water and how hydrogen is removed in order to release the oxygen molecules.
Uwe Bergmann
“Plants use the energy from sunlight to split two water molecules and produce the oxygen we breath. The study shows for the first time atomic-resolution snapshots of the likely channel and gate, where the water molecules arrive to the catalytic center to be split apart, and the channel where the protons are shuttled out during the splitting,” says Uwe Bergmann, the Martin L. Perl professor in ultrafast x-ray science at UW–Madison. “This information will help our understanding of one of the most fundamental reactions on earth, and how we might use sunlight in the future to create fuels.”
At SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, the team illuminated samples from cyanobacteria with ultrafast pulses of X-rays to collect both X-ray crystallography and spectroscopy data to simultaneously map the protein structure and how electrons flow in the protein. Through this technique, they are able to test competing theories of how Photosystem II splits water into oxygen. Over the past few years, the team has used this method to observe various steps of this water-splitting cycle at the temperature at which it occurs in nature.
Scientists at UW–Madison have been instrumental to developing these and related x-ray imaging methods over the last decade.
The center of the protein acts as a catalyst, which drives certain chemical reactions to happen in a highly efficient manner. This research seeks to unlock how nature has optimized this catalytic process over millions of years of evolution. A cluster of four manganese atoms and one calcium atom are connected by oxygen atoms, and surrounded by water and the outer layers of the protein. In the step the scientists looked at, water flows through a pathway into the center of the protein, where one water molecule ultimately forms a bridge between a manganese atom and a calcium atom. The researchers showed that this water molecule likely provides one of the oxygen atoms in the oxygen molecule produced at the end of the cycle.
The proposed proton gate around D1-E65, D2-E312, and D1-R334 in the open and closed state. | In Nature Communications, https://doi.org/10.1038/s41467-021-26781-z
Last year, the researchers discovered that Photosystem II ferries water into the center as if through a bucket brigade: Water molecules move in many small steps from one end of the pathway to the other. They also showed that the calcium atom within the center could be involved in shuttling the water in. In this most recent study, the researchers pinpoint, for the first time, the exact pathway where this process unfolds.
“This might prevent water from interacting with the center prematurely, resulting in unwanted intermediates such as peroxide that can cause damage to the enzyme,” said Jan Kern, staff scientist at LBNL and one of the corresponding authors.
The researchers also showed that there is another pathway dedicated to removing hydrogen protons generated during the water-splitting reaction. In the proton pathway, they discovered the existence of a “proton gate,” which blocks the proton from coming back to the center.
“These results show where and how the water molecules enter the catalytic site, and where the protons are released, advancing our understanding of how two waters may come together to form the oxygen we breathe,” said Junko Yano, senior scientist at LBNL and one of the corresponding authors. “It demonstrates that it is just not enough to determine the structure of the main catalytic center, but it is also important to understand how the entire protein carries out the reaction.”
In addition to SLAC and LBNL, the collaboration includes researchers from Uppsala University in Sweden; Humboldt University of Berlin; and the University of Wisconsin-Madison.
LCLS is a DOE Office of Science user facility. This research was supported by the Office of Science.
New 3D integrated semiconductor qubit saves space without sacrificing performance
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Small but mighty, semiconducting qubits are a promising area of research on the road to a fully functional quantum computer. Less than one square micron, thousands of these qubits could fit into the space taken up by one of the current industry-leading superconducting qubit platforms, such as IBM’s or Google’s.
For a quantum computer on the order of tens or hundreds of qubits, that size difference is insignificant. But to get to the millions or billions of qubits needed to use these computers to model quantum physical processes or fold a protein in a matter of minutes, the tiny size of the semiconducting qubits could become a huge advantage.
Except, says Nathan Holman, who graduated from UW–Madison physics professor Mark Eriksson’s group with a PhD in 2020 and is now a scientist with HRL Laboratories, “All those qubits need to be wired up. But the qubits are so small, so how do we get the lines in there?”
In a new study published in NPJ Quantum Information on September 9, Holman and colleagues applied flip chip bonding to 3D integrate superconducting resonators with semiconducting qubits for the first time, freeing up space for the control wires in the process. They then showed that the new chip performs as well as non-integrated ones, meaning that they solved one problem without introducing another.
If quantum computers are to have any chance of outperforming their classical counterparts, their individual qubit units need to be scalable so that millions of qubits can work together. They also need an error correction scheme such as the surface code, which requires a 2D qubit grid and is the current best-proposed scheme.
Proposed approach: the 3D integrated device consists of a superconducting die (top layer) and a semiconducting qubit die (middle layer) brought together though a technique known as flip chip integration. The bottom layer, proposed but not studied experimentally in this work, will serve to enable wiring and readout electronics. This study is the first time that semiconducting qubits (middle layer) and superconducting resonators (top layer) have been integrated in this way, and it frees up space for the wiring needed to control the qubits. | Credit: Holman et al., in NPJ Quantum Information
To attain any 2D tiled structure with current semiconducting devices, it quickly gets to the point where 100% of available surface area is covered by wires — and at that point, it is physically impossible to expand the device’s capacity by adding more qubits.
To try to alleviate the space issue, the researchers applied a 3D integration method developed by their colleagues at MIT. Essentially, the process takes two silicon dies, attaches pillars of the soft metal indium placed onto one, aligns the two dies, and then presses them together. The result is that the wires come in from the top instead of from the side.
“The 3D integration helps you get some of the wiring in in a denser way than you could with the traditional method,” Holman says. “This particular approach has never been done with semiconductor qubits, and I think the big reason why it hadn’t is that it’s just a huge fabrication challenge.”
Mark ErikssonNathan Holman
In the second part of their study, the researchers needed to confirm that their new design was functional — and that it didn’t add disadvantages that would negate the spacing success.
The device itself has a cavity with a well-defined resonant frequency, which means that when they probe it with microwave photons at that frequency, the photons transmit through the cavity and are registered by a detector. The qubit itself is coupled to the cavity, which allows the researchers to determine if it is functioning or not: a functioning qubit changes the resonant frequency, and the number of photons detected goes down.
They probed their 3D integrated devices with the microwave photons, and when they expected their qubits to be working, they saw the expected signal. In other words, the new design did not negatively affect device performance.
“Even though there’s all this added complexity, the devices didn’t perform any worse than devices that are easier to make,” Holman says. “I think this work makes it conceivable to go to the next step with this technology, whereas before it was very tricky to imagine past a certain number of qubits.”
Holman emphasizes that this work does not solve all the design and functionality issues currently hampering the success of fully functional quantum computers.
“Even with all the resources and large industry teams working on this problem, it is non-trivial,” Holman says. “It’s exciting, but it’s a long-haul excitement. This work is one more piece of the puzzle.”
The article reports that this work was sponsored in part by the Army Research Office (ARO) under Grant Number W911NF-17-1-0274 (at UW–Madison) and by the Assistant Secretary of Defense for Research & Engineering under Air Force Contract No. FA8721-05-C-0002 (at MIT Lincoln Laboratory).
