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Getting Under the (Neutron) Skin

October 20, 2022

A photo of Thomas Papenbrock — Papenbrock

Every atomic nucleus, miniscule or massive, is governed by a basic law of nature: the strong force holding it together. The more we know about the complex workings of nuclei the better we understand different kinds of matter, including elements produced in cosmic collisions (like the gold in our jewelry boxes). In Nature Physics, UT’s physicists and their colleagues at Chalmers University of Technology present a groundbreaking model to calculate the properties of a lead nucleus with a method that can be used across the nuclear landscape—all the way to neutron stars.

The Story of Stars in a Single Nucleus

Scientists use computational models to get the most complete picture of a nucleus. Heavier nuclei typically don’t fit well in those models. They may be unstable and fall apart quickly. They might have so many protons and neutrons that it’s difficult to keep track of all the interactions between them. Nuclei with lots of neutrons present a particular challenge. The new approach turns that obstacle into an asset.

When neutrons outnumber protons, equal numbers of each gather in the nucleus’s center while the surplus neutrons collect on its surface, forming a skin. Lead (Pb-208) is a good example. With 126 neutrons and 82 protons, it’s a heavy (yet stable) nucleus covered in a neutron skin. This outer layer is sensitive to the strong force, which holds its protons and neutrons together.

Professor Thomas Papenbrock described how he and his colleagues connected this single, simple nucleus to massive neutron stars, a teaspoon of which would weigh 4 billion tons on Earth.

Combining advanced methods, statistical tools, and computational modeling, they were able to calculate the neutron skin in lead. This informs scientists about the “nuclear equation of state,” which describes matter under various conditions like pressure, volume, or temperature. That equation in turn links neutron skin thickness to the structure and size of neutron stars. Further, the model can be adapted across the nuclear landscape to predict properties of other heavy nuclei that have eluded models in the past.

The work also shows that a heavy nucleus can be computed with tools theorists routinely use: Hamiltonians (functions that describe a dynamic system) rooted in the theory describing the strong force (quantum chromodynamics, or QCD). These tools have well-understood parameters that help quantify uncertainties, making predictions more precise.

The Physics in Your Jewelry Box

Papenbrock outlined multiple reasons why understanding nuclear structure and properties is important. First, there’s understanding the complexity arising from the strong force, one of nature’s four basic interactions (along with the weak, electromagnetic, and gravitational forces).

“Second,” he said, “is understanding the production of elements in the early universe and in stellar processes such as neutron-star mergers and supernovae explosions.”

(Helium and hydrogen were born in the early seconds of our universe, while gold and platinum trace their beginnings to merging neutron stars.)

“This require us to understand very neutron-rich nuclei that cannot all be produced and studied in laboratories,” Papenbrock said. “So, we have to compute them.”

Understanding the nucleus not only helps us understand what happened eons ago or what goes on in far-away objects. It also helps scientists shape future discoveries.

“Searches for physics beyond the Standard Model of elementary particles almost all involve hypothetical reactions of such particles with or inside atomic nuclei,” he explained.

James Bond and Nuclear Physics

Papenbrock pursues nuclear studies through a joint faculty appointment with Oak Ridge National Laboratory. He worked with ORNL’s Zhonghao Sun (former UT postdoc) and Gaute Hagen (an adjunct professor with UT Physics) to calculate lead’s neutron skin, a feat made possible by ORNL’s Summit supercomputer.

“Computing has been a game changer over the last two-to-three decades,” Papenbrock said. “Moore’s law states that computing power doubles roughly every 16 months. Our phones can do things today that seemed science fiction (or James Bond) just a few decades ago.

“This has forced nuclear physicists to collaborate with computer scientists to really use all these technologies,” he continued. “We had to make so many changes to our codes over the years to adapt to this evolution of computing.”

Using collaborative strengths and computing power for nuclear physics is familiar territory for Papenbrock and Hagen. They’re part of the NUCLEI (the NUclear Computational Low Energy Initiative) collaboration that recently won $13 million in funding through SciDAC, the U.S. Department of Energy’s Scientific Discovery Through Advanced Computing program (part of the Office of Science). Papenbrock became the group’s principal investigator this year.

Comprising five national laboratories and seven universities, NUCLEI aims to advance the computations of atomic nuclei and processes. The Nature Physics paper was a result of the collaboration’s research, earning a News and Views article in the journal for its significance. This work, the review explains, “has allowed practitioners to reach for the stars.”

Andrew W. Steiner Elected APS Fellow

October 19, 2022

When a giant star dies, Andrew W. Steiner gets a little more lab space.

An associate professor with a joint appointment at Oak Ridge National Laboratory, Steiner studies the neutron stars left after massive stars collapse. His outstanding research has earned him election as a Fellow of the American Physical Society (APS), making him the 10th APS Fellow on the current physics faculty.

APS Fellowship recognizes members who have made advances in physics through original research and publication or significant innovative contributions in applying physics to science and technology. They may also have made significant contributions to physics teaching or service. Steiner was cited “for pioneering a data-driven approach to constraining neutron star properties and the dense matter equation of state that combines advanced statistical methods, state-of-the-art nuclear theory, experimental constraints on bulk nuclear properties, and astrophysical data.”

A Stellar Laboratory

Dense, with a mass much larger than that of our sun, an average neutron star could fit in the space between the Tennessee Theatre and McGhee Tyson Airport. Yet these stellar bodies are actually good models for atoms. Steiner uses neutron star observations as a kind of laboratory to learn more about how neutrons and protons interact, especially in terms of the strong force (one of nature’s four basic forces) that holds them together. He was a bit surprised the citation mentioned “advanced statistical methods,” because he’s never actually had any courses in statistics.

“It is, in part, a testament to how much our field has changed over the past decade or so,” he said. “Statistical methods, computational tools, and machine learning continue to transform our ability to use data to elucidate our knowledge of the strong nuclear force.”

Steiner’s work has ties to important progress in nuclear astrophysics. The team that first detected gravitational waves produced by colliding neutron stars cited his group’s predictions about tidal deformability (what he calls “squishiness”) in these massive objects. He’s also director of the Nuclear Physics from Multi-Messenger Mergers (NP3M) research hub. The National Science Foundation (NSF) awarded UT $3.25 million to launch this national effort, bringing together scientists with expertise in nuclear physics and astrophysical simulations to study the properties of matter not available on Earth. Among the hub’s goals is training a diverse new generation of scientists.

“Professor Steiner works at the forefront of theoretical nuclear astrophysics where he uses cutting edge computational tools to probe the properties of hot and dense strongly interacting matter,” said Physics Professor and Department Head Adrian Del Maestro. “His dedication to building open source tools ensures that his research has a broad impact on the field as recognized by his peers through election as a Fellow of the APS. We are proud to add him to the list of 10 APS Fellows forging new knowledge and teaching and mentoring students in the Department of Physics and Astronomy at the University of Tennessee.”

Steiner earned a bachelor’s degree in physics at Carnegie Mellon University, followed by master’s and doctoral degrees in physics at the State University of New York at Stony Brook. He joined the UT Physics faculty in 2014. In 2016 he won an NSF Faculty Early Career Development Program (CAREER) Award, following that up in 2019 with a Senior Career Award for Research and Creative Achievement from the College of Arts and Sciences.

Steiner credits a long list of supporters for his career success: his wife, his daughter, his parents, and colleagues from UT Physics and around the country (including Madappa Prakash, his graduate advisor).

Each year, no more than one half of one percent of the society’s membership (excluding student members) is recognized by their peers for election to the status of Fellow. Steiner was recommended for Fellowship by his colleagues in the APS Division of Nuclear Physics. He will be acknowledged at the Division’s annual meeting later this month.

Elbio Dagotto Receives APS Adler Award in Materials Physics

October 11, 2022

Elbio Dagotto doesn’t necessarily take things at face value, at least not when it comes to materials. He is interested in the complexity often going on below the surface—how electrons move, spin, and interact and what happens as a result, often with competing tendencies leading to unusual patterns and properties. Superconductivity, magnetism, and quantum computing all have ties to the fundamental research Dagotto conducts as both a Distinguished Professor of Physics at UT and a Distinguished Scientist in Oak Ridge National Laboratory’s Materials Science and Technology Division. For his outstanding contributions to materials physics, the American Physical Society (APS) has awarded him the 2023 David Adler Lectureship Award in the Field of Materials Physics.

“Professor Dagotto is a leader in the field of strongly correlated electrons, consistently pushing forward new paradigms and ideas to solve some of the most pressing problems facing the world today,” said Professor and Department Head Adrian Del Maestro. “At the same time, he is well known for his dedication to teaching and mentorship, having trained a large number of successful scientists as well as consistently being a recipient of departmental undergraduate teaching awards. He has a knack for communicating complicated ideas in a pedagogical manner, demonstrated through his popular review papers, and he is always at the top of my list to teach introductory quantum mechanics.”

Dagotto is a condensed matter theorist and uses advanced models and computational tools to predict how correlated electrons behave in a wide variety of materials, as well as nanoscale systems.

“We say electrons are correlated when the properties of one individual electron depend strongly on what the rest of the ensemble of many other electrons is doing, a formidable challenge for calculations,” he explained.

These studies provide the bedrock for understanding at a fundamental level how several properties, such as insulation, magnetism, and superconductivity arise: crucial discoveries for unveiling new exotic materials as well as developing atomic scale devices. Dagotto literally wrote the book on Nanoscale Phase Separation and Colossal Magnetoresistance and co-edited another on Multifunctional Oxide Heterostructures. He has authored or co-authored more than 450 publications that have been cited more than 30,000 times. In 2004 he joined UT and ORNL with a 50-50 percent split appointment; that same year he was listed among the world’s top 250 most Highly Cited Physicists. His research is currently funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

Dagotto’s expertise is so well regarded that he has been invited to weigh in—often as sole author—on the state of condensed matter physics for prestigious journals including Science, Nature, and Reviews of Modern Physics. For four years he was a divisional editor specializing in condensed matter for Physical Review Letters. He has also served on the National Academies’ Solid State Sciences Committee (now the Condensed Matter and Materials Research Committee), a body that helps set the national agenda for materials research.

Dagotto earned a PhD in physics at Instituto Balseiro, Bariloche, in his native Argentina. He has a keen interest in supporting other Hispanic scientists at all stages of their careers. To that end he and Professor Adriana Moreo have organized a series of Hispanics in Physics lunch gatherings for the department, welcoming everyone from undergraduates to senior faculty.

The Adler Lectureship Award will now appear on Dagotto’s CV among a host of other honors, including his election as a Fellow of both the American Physical Society and the American Association for the Advancement of Science. The UT Society of Physics Students has also recently selected him as Teacher of the Year the last two times he taught quantum mechanics for undergraduate students.

Dagotto’s official citation for the Adler Lectureship reads:

“For pioneering work on the theoretical framework of correlated electron systems and describing their importance through elegant written and oral communications.”

The David Adler Lectureship Award in the Field of Materials Physics is awarded annually to a scientist making outstanding contributions to the field of materials physics and who is notable for high quality research, review articles, and lecturing. The honor is named for the late David Adler, a condensed matter physicist and professor at the Massachusetts Institute of Technology. The official award presentation will be next March at the American Institute of Physics meeting, where Dagotto will give an invited talk.

New Quantum Networks Research and Training Program Receives $3M NSF Award

July 15, 2022

Courtesy of Professor George Siopsis

The National Science Foundation Research Traineeship Program (NRT) awarded a $3 million Collaborative Grant to the University of Georgia (UGA) and the University of Tennessee, Knoxville, to develop a Quantum Networks Training and Research Alliance in the Southeast (QuaNTRASE).

The NSF award advances convergent research in quantum information science and engineering, which it has identified as a national priority of utmost importance, via training graduate students through a comprehensive traineeship model. The program supports graduate students, educates the STEM leaders of tomorrow, and strengthens the national research infrastructure.

“NSF continues to invest in the future STEM workforce by preparing trainees to address challenges that increasingly require crossing traditional disciplinary boundaries,” said Sylvia Butterfield, acting assistant director for NSF’s Directorate for Education and Human Resources. “Supporting innovative and evidence-based STEM graduate education with an emphasis on recruiting and retaining a diverse student population is critical to ensuring a robust and well-prepared STEM workforce.”

Quantum networks promise a novel and more secure functionality than the classical networks on which current communication encryption technologies are built. Developments surrounding quantum networks include fundamental discoveries in quantum science as well as key applications in cybersecurity, quantum sensors, and quantum computing.

“To realize the promised advantage of a quantum internet, many fundamental science and engineering challenges must be overcome via a convergent combination of expertise from several science and engineering disciplines and the development of a well-trained, interdisciplinary quantum network workforce.” said Yohannes Abate, Susan Dasher and Charles Dasher MD Professor of Physics at UGA. “The goal of this program is to advance quantum networks research through the design and development of components and applications of quantum networks.”

“The program is one of the first comprehensive, interdisciplinary quantum information science and engineering (QISE) training programs in the Southeast.” said George Siopsis, professor of physics at UT and director of the university’s Quantum Leap Initiative.

This joint UGA-UT effort, in collaboration with Oak Ridge National Laboratory and industry partners, will expand the diversity of students in quantum information science and engineering, including historically underrepresented groups.

“The strength of QuaNTRASE is our capacity to integrate the quantum networking expertise from two major research institutions with a national laboratory to advance research and prepare trainees for the developing quantum economy,” said Tina Salguero, professor of chemistry at UGA.

The program will develop MS and PhD programs via five key elements of the education and training frameworks: (i) the development of a curriculum that integrates interdisciplinary and cross-institutional course offerings; (ii) the incorporation of vibrant cross-institutional and interdisciplinary advising and mentoring; (iii) the introduction of quantum technology concepts into existing science and engineering disciplines; (iv) research rotations, which will enhance students’ experience in quantum networks; and (v) additional professional development through national laboratory and industry-university partnerships, a trainee-led career fair, research retreats, and summer internships. This interdisciplinary collaboration will be a core component of the QuaNTRASE research program.

In addition to the scientific activities, the project will develop and deliver STEM outreach activities for local high school students and teachers focused on quantum concepts, careers, and practices through summer and after-school STEM programming.

“Preparing future generations for jobs in the quantum and AI fields is a national priority,” said Mehmet Aydeniz, professor of STEM education at UT. “By reaching out to high school students and introducing them to quantum concepts, practices and careers early on, we aim to prepare the scientists and engineers of the future, who will be instrumental to the nation’s leadership in science and quantum computing specifically.”

A Front Row Seat to Quantum Behavior

July 6, 2022

Helium may bring the fun to party balloons but we’re actually more familiar with its serious side. As a liquid it’s crucial to cooling magnets used in magnetic resonance imaging and manufacturing semiconductors. Cooled to a critical temperature it can become a superfluid: flowing with no viscosity and losing no kinetic energy.

For Professor Adrian Del Maestro, helium holds even more exotic charms. Since his postdoctoral days he’s wanted to confine this element to one dimension, where theory predicts it will become a fluctuating phase of matter that’s not exactly a solid, a liquid, or a superfluid. The model itself (a Tomonaga-Luttinger liquid) was first proposed in 1950 and until now has never been seen in a system of strongly-interacting atoms. Del Maestro and his colleagues at Indiana University Bloomington have found a way to squeeze helium down to single-atom thickness and give scientists a front-row seat to observe quantum mechanical behavior.

Conceptual image of strongly interacting helium atoms (top)
and helium atoms moving through a 1-D pipe coated with argon. — *Conceptual image of strongly interacting helium atoms (top) and helium atoms moving through a 1-D pipe coated with argon.*

Building an Atomic Scale Pipe

The promise (and challenge) of quantum science lies in understanding how things work in lower dimensions. Take carbon, for example. In 3-D it’s graphite, the soft stuff that lets a pencil glide across paper. In 2-D it’s graphene, an ultra-light and ultra-strong system.

“A sheet of graphene weighing less than the whisker of a cat could support the cat’s weight,” Del Maestro explained.

Helium has similar differences.

“In 3-D, the same helium atoms that fill balloons can whiz around each other to form a superfluid phase of matter,” he said. “In 1-D, the atoms are forced to interact strongly as they are all made to stand in a line, and they can’t easily exchange places.”

Though its properties make helium an ideal system for getting a glimpse into one-dimensional behavior, confining its atoms to this scale is no trivial task.

“You literally need to make a pipe that is only a few atoms wide,” Del Maestro said. “No normal liquid would ever flow through such a narrow pipe as friction would prevent it.”

Fortunately, in 2015 he met IU’s Paul Sokol at a conference and they merged their theoretical and experimental expertise to build this atomic structure.

“Paul had worked on confining superfluids for years, but just couldn’t get them small enough,” Del Maestro said. “I had done numerical simulations that found the ‘sweet’ spot on the pipe size we needed.”

Del Maestro suggested they paint the inside of a pipe to make it smaller. Sokol came up with the idea to pre-plate it with a rare gas. They took a nanoporous material, whose structure is like a sponge with ordered pores, and coated the inside with a perfect layer of argon to make it angstrom scale (a hundred-millionth of a centimeter). Now they had their 1-D pipe. They filled it with liquid helium, which adsorbed inside the pre-plated nanopores, and then bombarded the helium with neutrons. The resulting excitations told them what phase of matter they had.

“You can think of this as testing whether something is a liquid or solid by asking what happens when you throw something at it,” Del Maestro said. “Does it bounce off, or travel through? The results of the experiments can be modeled with theoretical calculations and simulations to confirm the existence of the Luttinger liquid state of matter.”

The Power of Positive Thinking

Del Maestro explained that helium confined in one dimension holds possibilities that other systems exhibiting 1-D do not.

“It can be adjusted with pressure, all the way from a gas to a solid … and provide the possibility for tuning and optimizing devices exploiting quantum phenomena,” he said.

A one-dimensional pipe filled with liquid helium attached to a device can also sense tiny rotations, pointing to future applications in geo-sensing, gyroscopes, and autonomous navigation in extreme environments where GPS isn’t feasible, such as drones on other planets.

The isotopes helium-3 and helium-4 offer the chance to further test the Luttinger liquid theory that a 1-D liquid of bosons and fermions—particles that make up matter and carry forces—should have similar behavior at low temperatures. Del Maestro and Sokol have opened an avenue for this research with the experimental realization of 1-D helium. The findings, published in Nature Communications, were the result of work from their respective groups, as well as patience, persistence, and positive thinking.

“It’s something I’ve been thinking about for 15 years … and was only possible because of the tight integration of theory and experiment,” Del Maestro said. “I’d ask Paul if something was possible, and he would say ‘No, but I’ll think about it.’ Paul would show me some crazy result and say ‘Do you know what is going on at the atomic level?’ and I would reply with ‘No, but I’ll think about it.’”

Eventually, he said, all the pieces of this 1-D puzzle fit together.

Finding Hidden Physics

June 7, 2022

Tova Holmes awarded DOE Early Career Research award to break the Standard Model

Tova Holmes has a challenging but welcome task: looking for hidden physics with particles no human can see. She’ll pursue this aim with an Early Career Research award from the US Department of Energy Office of Science. The grant begins July 1 and includes $750,000 of support over the next five years.

Breaking the Standard Model

Particle physics helps drive not only discoveries about our universe but also innovative tools that improve our lives here on earth. As a field it describes the stuff of matter but it’s also the foundation for practical spinoffs like Wi-Fi and magnetic resonance imaging (MRIs). Holmes joined the department as an assistant professor in 2020 and is part of the CMS group, which uses the Compact Muon Solenoid detector to study high-energy particle collisions in the search for new particles, and new physics, at the Large Hadron Collider (LHC) in Geneva.

The LHC is where scientists discovered the Higgs boson, a feat that led to a Nobel Prize and completed the Standard Model of physics. All matter in the universe is made of fundamental particles and forces and the Standard Model explains how they relate to one another. The Higgs has a crucial role in this framework.

“When we designed the LHC, we wanted to find the Higgs,” Holmes explained. “There’s a Higgs field everywhere and it contains energy everywhere. All of our other particles are basically continuously interacting with this Higgs field as they move through space. That mechanism is exactly what gives most of the particles in the Standard Model mass.”

The Higgs may be a giver, but it’s also a taker.

“There’s a key problem that only gets worse when you find the Higgs, which is that the Higgs mass itself doesn’t make any sense,” Holmes said. “(It) not only gives mass to those other particles, it’s going to collect additional mass from the interactions. That means that its mass should blow up. You’re solving one mass problem by introducing the Higgs, but you’re creating a new one.”

This problem fuels the idea that there’s something else that so far has escaped detection.

“We can tell that we’re missing things,” Holmes said. “There’s direct evidence that we have missed something that could be as big as the Standard Model itself. We might be looking at a tiny corner of what actually exists out there.”

Finding something new would in effect break the Standard Model, and that’s where Holmes’ research comes in.

“I am studying things that would couple to the Higgs and solve this mass problem,” she said.

Her proposal will upgrade detector capabilities and take new data that can identify phenomena the LHC might have previously missed, including long-lived particles.

Chasing the Unconventional, and Why It Matters

These particles Holmes hopes to find have unconventional signatures, which she describes as “things that our detectors weren’t designed to look for.”

The CMS detector sits on one of four collision points on the LHC ring, the most powerful particle accelerator ever built. Though CMS is described as a giant, high-speed camera, “we never directly see any of the particles that we study,” Holmes said. “That’s the inherent challenge.”

The detector does the looking instead.

The CMS detector resembles a giant cylinder with several layers wrapped around it. Collisions occur in the center. Exotic, unstable particles like the Higgs decay almost immediately, while more commonplace particles like electrons and muons have longer trajectories and filter through more layers. As they travel, the detector measures what they leave behind (electric charge, energy, etc.) as a way to identify them. The detector’s design limits what it can measure, but it’s not the only restriction.

“Not only our detector, but also our trigger system,” Holmes said, “which is how we decide while we’re running our experiment which of the data to keep. We collide 40 million times a second and we keep about a thousand of those collisions per second. If our trigger isn’t designed for a signature, then it’s something we would have missed.”

CMS Detector diagram — CMS Detector, courtesy of CERN

Longer-lived particles—the ones that make it through more layers before they decay—leave the unconventional signatures she wants to find. They can show up in supersymmetry (SUSY), which Holmes described as “a whole zoo of particles that exist, one for each of the Standard Model particles.” Every particle would have a partner that’s its opposite, balancing things out. Finding SUSY, she said, “magically solves all your problems.”

With the DOE grant her group will expand the detector’s triggering capabilities to record new data that she hopes will reveal a wider variety of long-lived particles, including those that might show up in SUSY. Students and postdocs will play a key role, Holmes said, as “every major physics output will be driven by a graduate student.”

Beyond the research and educational aspects, why does this research matter? It has to do both with the fate of our planet and how well we live on it.

“We might find as we understand the Higgs better that we’re actually in an unstable point of the Higgs vacuum, which is to say at any point our universe could tunnel to a new state and explode, or collapse into a tiny nothing,” Holmes explained. “We don’t fundamentally understand where our universe is going. Understanding the Higgs is really key to that.”

(She emphasizes she doesn’t expect this to happen anytime soon.)

“From a much more practical point of view, we have a long history of finding fundamental particles and not knowing why anyone would care, and then finding out there are really good reasons,” she said.

When electromagnetic radiation was discovered people thought it was a nice, if useless, idea. Now we know it’s the foundation of Wi-Fi, infrared and X-rays, among other applications. The same goes for MRIs. To build the former Tevatron particle accelerator at Fermilab required bending beams with magnets.

“In developing that magnet technology, they made the first large-scale production of these really high-field magnets that are now the magnets that are used in MRIs,” Holmes explained. “It was particle physics that created that technology. That’s the nature of trying to do technical accomplishments that have never been achieved before: you build technology that gets used by everybody else. Basically everything that we have technology-wise today was built on fundamental discoveries.”

“That’s the nature of trying to do technical accomplishments that have never been achieved before: you build technology that gets used by everybody else. Basically everything that we have technology-wise today was built on fundamental discoveries.”
Tova Holmes

Challenges, Not Impossibilities

Pursuing these fundamental discoveries is a good fit for the Early Career Research Program, which made 83 awards this cycle (Livia Casali of the UT Department of Nuclear Engineering was also among the awardees). The initiative supports outstanding scientists early in their careers whose research reinforces the DOE Office of Science mission to deliver scientific discoveries and tools to transform our understanding of nature and advance the energy, economic, and national security of the United States.

For Holmes that means keeping an eye on where particle physics should go next. While the LHC has been wildly successful, she believes future particle accelerators will require some new thinking.

“We went from tiny, tabletop rings to bigger ones that are the size of a building,” she said. “Then we went to where it fits, just barely, in the laboratory site. Now we’re at the LHC where it literally goes across countries’ borders.”

Building bigger rings can only take you so far, she said, adding that “if we want our field to continue, we need alternative paths.”

Her team has been working on a muon collider to solve some of these problems. It’s a circular collider where the ring size is related to the mass of the particle you’re accelerating. There are some hurdles, like trying to get a beam out of unstable particles, but Holmes is undeterred by those obstacles.

“That creates some interesting experimental challenges,” she said. “But those are problems you can solve—not impossibilities.”

Building Bridges for the Quantum Era

May 31, 2022

Speed.

Adrian Del Maestro and Hatem Barghathi are talking about speed, and memory, and how many bytes you need to type a single letter. In classical computers those workings are neatly spelled out: things are either on or off. But Del Maestro and Barghathi think in quantum scales, the tiny systems where classical mechanics doesn’t hold up and the multitude of particle configurations is so large, it seems impossible to count and store.

With colleagues from the Tickle College of Engineering, they’ve devised a shorthand to describe these configurations, dramatically speeding up calculations and with them scientists’ ability to predict how quantum systems behave. This kind of innovative thinking sets the stage for quantum materials to take the baton from silicon as new technologies emerge. With investment in the Quantum Materials for Future Technologies cluster, UT is well situated to be a scientific leader in this new era.

The Foundation for a Post-Silicon World

Del Maestro, a professor in both the Department of Physics and Astronomy and the Min H. Kao Department of Electrical Engineering and Computer Science, has been intrigued by quantum science since graduate school. The cluster opportunity brought him to UT from the University of Vermont. Barghathi, a postdoctoral researcher in his group, came along. Counting six new hires and current faculty in physics and engineering, by next year the cluster will have 15 or so active researchers mining the depths of quantum materials. Daunting as they may seem with their rebellion against classical physics and their maddeningly small architecture, leaving that territory unexplored would be both a scientific and economic mistake.

“Fundamental advances in materials led to our current ability to harness technology and that has given us the modern age,” Del Maestro said. “The fact that we have billions of transistors in our pockets in our iPhones is because of that foundational work.

“At the time, people weren’t trying to build iPhones,” he continued. “They were trying to understand the properties of materials. And so what we’re trying to do right now is understand and harness the properties of a new class of quantum materials that can exhibit dizzying non-classical behavior.”

Technology often follows a long path that’s not always obvious. A physicist came up with the principle for a transistor in 1925, engineers developed it in 1947, and in 1961 we had the first silicon chip. Quantum materials could be the next chapter.

“We’re really at the start of that,” Del Maestro said. “The hope is that if we can learn to understand how to first describe, quantify, characterize, and then harness these things we don’t have classical analogs for, then almost by necessity they’ll lead to what we call post-silicon quantum technologies. That’s really the purpose of the cluster. UT was already a leader in this area. We want to move from being a regional leader to being a national or global leader.”

Twenty Times Faster

Leading means solving problems, and quantum materials present plenty. One is that it’s hard to predict what atoms (and their electrons) are going to do in a quantum system. Scientists and engineers know how to control electronic current in an iPhone’s transistors, but that’s classical, not quantum physics.

A diagram illustration Adrian Del Maestro's and Hatem Barghathi's counting tool "Balls and Walls."

“Say you have 10 blue balls,” Del Maestro explained. “Classically, you can always tell the difference between macroscopic objects, like two baseballs or billiard balls. If you look hard enough you can tell them apart. One of the absolutely amazing things about quantum mechanics is that if I give you two electrons or two rubidium atoms, there is no experiment that you can do to distinguish them.”

Those atoms are situated on lattice sites—corners, if you will, of the jungle gym-like structure of quantum materials. Complicating matters is that they’re made up of particles that can be fermions or bosons. Fermions sit one to a seat. Bosons can pile on top of each other, causing another headache, especially when you can’t tell them apart. It’s like putting together a football playbook when all the players in front of you have different abilities but look exactly alike, wear identical jerseys, and in some cases hide behind each other.

“I have these atoms and I ask how many different ways can I distribute them. But the atoms are truly indistinguishable. That’s a very hard problem to solve,” Del Maestro said.

To find the possible configurations in this scenario, he and Barghathi adapted a counting tool called “balls and walls.” At its most basic, it’s a mathematical formula to quickly figure out problems like how many combinations are possible if you want to buy a dozen donuts in four available flavors.

In this research, Del Maestro and Barghathi wanted to find how many different particle combinations were possible on lattice sites. They found the trick was to consider the setup as two objects: one as an object itself and the second as the edges of buckets you’re putting those objects into. Combining the objects and edges using a special mathematical formula gives you the number of slots available for the objects you want to count.

“Balls and walls allows me to use a simple counting argument to enumerate all possible configurations,” Del Maestro said. “Why is it important? If you want to know what the most likely configuration is, you need to know how many times it appears. Configurations are the language in which we solve a problem. We need to know the words in that language before we can write a sentence.”

He said the fastest computers in the world have only been able to study 21 fermions on 42 lattice sites, and far fewer bosons. The coding he and Barghathi developed can study 20 bosons on 20 sites: 20 times faster than current methods. Beyond that, it takes much less memory, reducing the needed storage by a factor of the number of sites.

A graph showing the "Balls and Walls" counting tool time scaling.

“Think about how much memory you need for each letter that you store in a computer,” Barghathi said. “It will basically take eight bits. A number is at least going to take the memory of a letter.”

When facing huge numbers of possible configurations, he said, “you need to know how much memory you need to store them and how fast you can deal with them.”

Del Maestro explained that “the number of total configurations you need to store is gigantic, so if you can get a factor of 20 smaller, that’s the difference between being able to solve the problem or not. If something used to take a month, now it can take a day. Modern quantum experiments can start to use these larger system sizes and now we can actually predict what they’re going to measure in the lab.”

Spontaneous Advances

The findings are published in the first paper to come out of two departments from UT’s quantum materials cluster. While Del Maestro and Barghathi brought physics expertise in quantum mechanics, Del Maestro said “to understand this representation at the level of the bits, we needed computer science. This is where our colleagues like Micah Beck, a co-author on the paper, come in.”

The collaboration was a little slow at the beginning, with physicists and engineers speaking different languages about caches and buffers; bosons and symmetries.

“Interdisciplinarity is hard,” Del Maestro said. “It takes time and effort and thoughtfulness to build that bridge.”

The cluster plays a crucial role in that construction.

“Without that, it’s easy to be very fractured, even if we’re all working in quantum materials,” he said.

Del Maestro explained that because the cluster embedded him in engineering and computer science he wasn’t an outsider: he knew who to ask about this problem because he knew Beck was interested in it as well.

“That, I think, is the strength of the cluster,” he said. “It was very much spontaneous. The cluster will enable new types of spontaneous advances. We can’t predict where they’re going to come from because they really come at the interface where different expertise all of a sudden fits together and produces something profoundly new.”