This award supports theoretical research on quantum materials where the strong electron-electron interaction leads to unique transport, thermodynamic and magnetic properties. The research agenda addresses both fundamental physics of electronic interactions in complex materials and practical physics of mesoscopic devices relevant for applications in the domain of quantum science with micro and nanostructures.
The conversion of heat into electricity in solid state systems is governed by thermoelectric effects. The thermoelectric transport in quantum materials and devices is at the heart of various modern electronics applications. Over the last decade, transport measurements in atomically thin two-dimensional materials, such as graphene composed of a single layer of carbon atoms, provided overwhelming evidence that the flow of electrons in such systems exhibits hydrodynamic behavior that resembles the flow of a viscous fluid. These advances pushed the limits of hydrodynamics, providing new perspectives on old fundamental problems and opening doors for completely new discoveries of emergent physics phenomena. This project is, in part, devoted to new research on thermoelectric resistance of such systems as they are subjected to magnetic fields. The PI will also extend these studies to other forms of low-temperature electronic behavior in solids such as superconductivity, where electrons flow without any resistance, and magnetism, as well as their coexistence.
This award also supports the PI’s educational and outreach activities. The project places significant emphasis on training graduate and undergraduate students by engaging them in research in a highly collaborative environment with a postdoctoral scholar and colleagues from other groups. The PI will reach out to the public and high-school student audiences through (i) collaboration with the USA Physics Olympiad team to foster new generation of physicists and train high-school students for international scholastic competition and (ii) public education via entertaining Wonders of Physics shows. The PI will also be involved in the scientific coordination of a physics summer school as well as organization of international conferences and workshops.
Fringe benefits: new technique makes x-rays more laser-like
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Detecting a chemical change is often easy: colors may change, heat may be released, or something may smell different. Seeing reactions at the molecular level is not quite so easy, but knowing exactly when and how chemical bonds form or atoms move around is crucial to understanding chemical processes.
X-ray pulses allow researchers to create real-time movies of chemical reactions at the molecular scale (click on image to see animation)
In a new study published March 15 in Proceedings of the National Academy of Sciences, University of Wisconsin–Madison physics professor Uwe Bergmann and his collaborators have turned ultrafast x-ray pulses into something more akin to an optical laser, with cleaner, directional pulses. Their work may lead to visualizing chemical reactions faster than ever at the atomic scale.
“This work is the first step to do with x-rays the same kind of [techniques] which you do with regular lasers,” says Bergmann, the study’s senior author. “We have opened a time window for looking at chemical processes with attosecond [one billionth of one billionth of a second] precision. It’s a new frontier.”
Uwe Bergmann
Around a decade ago, researchers began using powerful x-ray free-electron lasers, which allow them to make ultrafast movies of molecular changes in real time on the femtosecond scale (one thousand times slower than an attosecond). Compared to visible lasers, which provide clean, single-wavelength beams of light, x-ray lasers are somewhat dirty: they contain multiple wavelengths of light of randomly varying intensity.
“What all scientists have done, and are still doing, is that you just adapt to what you get and then you design your experiments around them,” Bergmann says. “That also means that certain experiments, which in the optical laser regime are now standard, have not been possible.”
Bergmann and his colleagues somewhat accidentally discovered a way to make x-rays more like an optical laser. In their experimental setup, they shine intense but dirty x-ray pulses at a manganese sample. When these pulses hit a manganese atom, a lower-level electron is ejected and the hole it leaves is rapidly filled by a higher-level electron. The energy difference is emitted as a photon of a characteristic color. Very intense pulses can create enough of these holes. An emitted photon can effectively stimulate the emission of another one, leading to an avalanche of stimulated X-ray emission, mostly in the forward direction towards their detector.
Every so often, the detector that captures these stimulated emissions showed something they were not expecting: strong fringes, the characteristic pattern that results from constructively and destructively interfering signals. The fringes suggested that they had observed two x-ray emission pulses, separated by only a few femtoseconds.
“We were confused,” Bergmann says. “How did such a pair of x-ray pulses come about?”
After many discussions, calculations, and simulations, the team ruled out many possible explanations, until they finally realized what had happened: occasionally, two of the many spikes in the dirty pulses were much stronger than the rest of them. When these strong spikes occurred a few femtoseconds apart, and each had enough intensity, a clean pair of stimulated x-ray emission pulses emerged.
Two strong x-ray pulses hit the manganese sample, are stretched through the monochromator, and overlap, leading to the characteristic fringes.
Before these two pulses reach the detector, they first travel through a monochromator — essentially a prism that stretches light, much like how white light passing through a clear prism is stretched into a rainbow. These stretched pulses then overlap timewise, and those frequencies that are in phase with each other can add up to become more intense or cancel each other out into dark troughs. Hence, the fringes.
“At times, each signal is rather clean and of similar strength, and one obtains very strong interference fringes,” Bergmann says. “We know that the fringe spacings are directly related to the time difference of the two pulses, and because we can measure them very precisely, we can obtain their time difference with extreme, attosecond precision.”
Currently, these pulse pairs are generated very rarely, Bergmann and his collaborators will work with the accelerator scientists to find ways to manipulate the ‘dirty’ pulses and enhance the chance of producing the pairs. They are optimistic that their work opens the door to new applications of such x-ray pulse pairs — the types of techniques that are used commonly with visible laser light.
“We’re trying to move nonlinear laser optics into the x-ray regime,” Bergmann says. “In the x-ray regime, you can probe certain phenomena that you just cannot optically access. X-ray wavelengths are comparable to the distances between atoms, and we can knock out lower-level electrons to get element specificity with them.”
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Bergmann’s contribution to this research was funded in part by the U.S. Department of Energy, Laboratory Directed Research and Development program at SLAC National Accelerator Laboratory (DE-AC02-76SF00515). Other authors were supported by various funding as described in the study.
Ultraprecise atomic clock poised for new physics discoveries
Their instrument, known as an optical lattice atomic clock, can measure differences in time to a precision equivalent to losing just one second every 300 billion years and is the first example of a “multiplexed” optical clock, where six separate clocks can exist in the same environment. Its design allows the team to test ways to search for gravitational waves, attempt to detect dark matter, and discover new physics with clocks.
From one sphere of supercooled strontium atoms, Kolkowitz’s group multiplexes them into six separate spheres, each of which can be used as an atomic clock.
“Optical lattice clocks are already the best clocks in the world, and here we get this level of performance that no one has seen before,” says Shimon Kolkowitz, a UW–Madison physics professor and senior author of the study. “We’re working to both improve their performance and to develop emerging applications that are enabled by this improved performance.”
Atomic clocks are so precise because they take advantage of a fundamental property of atoms: when an electron changes energy levels, it absorbs or emits light with a frequency that is identical for all atoms of a particular element. Optical atomic clocks keep time by using a laser that is tuned to precisely match this frequency, and they require some of the world’s most sophisticated lasers to keep accurate time.
By comparison, Kolkowitz’s group has “a relatively lousy laser,” he says, so they knew that any clock they built would not be the most accurate or precise on its own. But they also knew that many downstream applications of optical clocks will require portable, commercially available lasers like theirs. Designing a clock that could use average lasers would be a boon.
Shimon Kolkowitz one of four UW professors awarded Sloan Fellowship
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Four University of Wisconsin–Madison professors, including assistant professor of physics Shimon Kolkowitz, have been named to Sloan Research Fellowships — competitive, prestigious awards given to promising researchers in the early stages of their careers.
“Today’s Sloan Research Fellows represent the scientific leaders of tomorrow,” says Adam F. Falk, president of the Alfred P. Sloan Foundation, which has awarded the fellowships since 1955. “As formidable young scholars, they are already shaping the research agenda within their respective fields—and their trailblazing won’t end here.”
Kolkowitz, an assistant professor of physics, builds some of the most precise clocks in the world by trapping ultracold atoms of strontium — clocks so accurate they could be used to test fundamental theories of physics and search for dark matter.
UW–Madison’s other 2022 Sloan Fellows are Tatyana Shcherbina (math), Zachary K. Wickens (chemistry) and Andrew Zimmer (math).
The UW–Madison professors are among 118 researchers from the United States and Canada honored by the New York-based philanthropic foundation. The four new fellows join 110 UW–Madison researchers honored in the past.
Each fellow receives $75,000 in research funding from the foundation, which awards Sloan Research Fellowships in eight scientific and technical fields: chemistry, computer science, economics, mathematics, computational and evolutionary molecular biology, neuroscience, ocean sciences and physics.
Shimon Kolkowitz earns NSF CAREER award
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Shimon Kolkowitz
Shimon Kolkowitz has already developed one of the most precise atomic clocks ever. Now, the UW–Madison physics professor has been awarded a Faculty Early Career Development (CAREER) award from the National Science Foundation (NSF) to use his atomic clocks to potentially answer some big questions about the physics of our universe.
The five-year, $800,000 in total award will cover research expenses, graduate student support, and outreach projects based on the research.
“I am honored and proud to receive an NSF CAREER award, which will help my research group expand our experimental efforts and build upon our recent results,” Kolkowitz says. “This award will support research into new ways to harness the remarkable precision of optical atomic clocks for exciting physics applications such as searching for dark matter and detecting gravitational waves.”
From one sphere of supercooled strontium atoms, Kolkowitz’s group multiplexes them into six separate spheres, each of which can be used as an atomic clock.
Atomic clocks are so precise because they take advantage of the natural vibration frequencies of atoms, which are identical for all atoms of a particular element. Kolkowitz and his research group have developed atomic clocks that can detect the difference in these frequencies between two clocks that would only disagree with each other by one second after 300 billion years, the tiniest detectable frequency changes to date. These clocks, then, can measure effects that shifts their frequency by only 0.00000000000000001%, opening the possibility of using them in the search for new physics.
A significant advancement in Kolkowitz’s clocks is that they are multiplexed, with six or more separate clocks in one
vacuum chamber, effectively placing each clock in the same environment. Mutliplexing means that comparisons between the clocks, and not their individual accuracy, is what matters — and allows the group to use commercially available, robust and portable lasers in their measurements. Though the clocks are not yet ready to be used to detect gravitational waves, Kolkowitz says the current setup “looks a bit like how you would eventually do that,” and will allow him to test out and demonstrate the concept.
In the spirit of the Wisconsin Idea and the NSF’s “broader impacts” to benefit society beyond scientific merit, with this award, Kolkowitz will focus efforts on quantum science outreach with pre-college students.
“We’ll be developing new demos and hands-on activities designed to introduce K-12 students to modern physics concepts,” Kolkowitz says. “We’ll use these activities to engage students at live shows and interactive events as part of The Wonders of Physics outreach program, with an emphasis on reaching rural and Native American communities in Wisconsin.”
NSF established these awards to help scientists and engineers develop simultaneously their contributions to research and education early in their careers. CAREER funds are awarded by the federal agency to junior-level faculty at colleges and universities.
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).
Magnetic fields implicated in the mysterious midlife crisis of stars
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Artist’s impression of the spinning interior of a star, generating the stellar magnetic field. This image combines a dynamo simulation of the Sun’s interior with observations of the Sun’s outer atmosphere, where storms and plasma winds are generated. | Credit: CESSI / IISER Kolkata / NASA-SVS / ESA / SOHO-LASCO
This post was originally published by the Royal Astronomical Society. UW–Madison physics graduate student Bindesh Tripathi is the lead author of the scientific publication.
Middle-aged stars can experience their own kind of midlife crisis, experiencing dramatic breaks in their activity and rotation rates at about the same age as our Sun, according to new research published today in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.
Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like our Sun to gradually slow down their rotation over billions of years.
In turn, the slower rotation leads to altered magnetic fields and less stellar activity – the numbers of sunspots, flares, outbursts, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields.
Bindesh Tripathi
This decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.
However recent observations indicate that this intimate relationship breaks down around middle age. The new work, carried out by Bindesh Tripathi at UW–Madison and the Indian Institute of Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment. Prof. Dibyendu Nandy, and Prof. Soumitro Banerjee of IISER are co-authors.
Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. A middle aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.
Prof. Nandy comments: “This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”
The new work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen. The best known of these is perhaps the Maunder Minimum around 1645 to 1715, when very few sunspots were observed.
The team hope that it will also shed light on recent observations indicating that the Sun is comparatively inactive, with crucial implications for the potential long-term future of our own stellar neighbor.
Francis Halzen named Vilas Research Professor
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Francis Halzen
UW–Madison physics professor Francis Halzen has been named a Vilas Research Professor. Created “for the advancement of learning,” Vilas Research Professorships are granted to faculty with proven research ability and unusual qualifications and promise. The recipients of the award have contributed significantly to the research mission of the university and are recognized both nationally and internationally.
Halzen, the Gregory Breit and Hilldale Professor of Physics, joined the UW–Madison faculty in 1972. He has made pioneering contributions to particle physics and neutrino astrophysics, and he continues to be the driving force of the international IceCube Collaboration.
Early in his career, Halzen cofounded the internationally recognized phenomenology research institute in the UW–Madison Department of Physics to promote research at the interface of theory and experiment in particle physics. This institute is recognized for this research and for its leadership in the training of postdocs and graduate students in particle physics phenomenology.
The IceCube Neutrino Observatory is the culmination of an idea first conceived in the 1960s, and one in which Halzen has played an integral role in its design, implementation, and data acquisition and analysis for the past three decades. After initial experiments confirmed that the Antarctic ice was ultratransparent and established the observation of atmospheric neutrinos, IceCube was ready to become a reality. From 2004 to 2011, the South Pole observatory was constructed — the largest project ever assigned to a university and one led by Halzen.
After two years of taking data with the full detector, the IceCube Neutrino Observatory opened a new window onto the universe with its discovery of highly energetic neutrinos of extragalactic origin. This discovery heralded the beginning of the exploration of the universe with neutrino telescopes. The IceCube observation of cosmic neutrinos was named the 2013 Physics World Breakthrough of the Year.
Nationally and internationally renowned for this work, Halzen was awarded a 2014 American Ingenuity Award, a 2015 Balzan Prize, a 2018 Bruno Pontecorvo Prize, a 2019 Yodh Prize, and a 2021 Bruno Rossi Prize.
With the Vilas Research Professorship, Halzen is also recognized for his commitment to education and service in the department, university, and international science communities. He has taught everything from physics for nonscience majors to advanced particle physics and special topics courses at UW–Madison. He has actively participated on several departmental and university committees as well as advisory, review, and funding panels. His input is highly sought by committees and agencies that assess future priorities of particle and astroparticle physics research.
“Francis Halzen has had a prolific, internationally recognized research career, has shown excellence as an educator who is able to effectively communicate cutting-edge science on all levels, and has made tireless and valued contributions in service of the department,” says Sridhara Dasu, Physics Department chair. “He is one of the most creative and influential physicists of the last half century and worthy of the prestigious Vilas Research Professorship.”
Vilas awards are supported by the estate of professor, U.S. senator and UW Regent William F. Vilas (1840-1908). The Vilas Research Professorship provides five years of flexible funding — two-thirds of which is provided by the Office of the Provost through the generosity of the Vilas trustees and one-third provided by the school or college whose dean nominated the winner.
Halzen joins department colleagues Profs. Vernon Barger and Sau Lan Wu as recipients of this prestigious UW–Madison professorship.
Physics projects funded in first round of UW’s Research Forward initiative
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In its inaugural round of funding, the Office of the Vice Chancellor for Research and Graduate Education’s (OVCRGE) Research Forward initiative selected 11 projects, including two with physics department faculty involvement.
OVCRGE hosts Research Forward to stimulate and support highly innovative and groundbreaking research at the University of Wisconsin–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.
Research Forward seeks to support collaborative, multidisciplinary, multi-investigator research projects that are high-risk, high-impact, and transformative. It seeks to fund research projects that have the potential to fundamentally transform a field of study as well as projects that require significant development prior to the submission of applications for external funding. Collaborative research proposals are welcome from within any of the four divisions (Arts & Humanities, Biological Sciences, Physical Sciences, Social Sciences), as are cross-divisional collaborations.
Correlated errors in quantum computers emphasize need for design changes
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Quantum computers could outperform classical computers at many tasks, but only if the errors that are an inevitable part of computational tasks are isolated rather than widespread events.
Now, researchers at the University of Wisconsin–Madison have found evidence that errors are correlated across an entire superconducting quantum computing chip — highlighting a problem that must be acknowledged and addressed in the quest for fault-tolerant quantum computers.
The researchers report their findings in a study published June 16 in the journal Nature, Importantly, their work also points to mitigation strategies.
“I think people have been approaching the problem of error correction in an overly optimistic way, blindly making the assumption that errors are not correlated,” says UW–Madison physics Professor Robert McDermott, senior author of the study. “Our experiments show absolutely that errors are correlated, but as we identify problems and develop a deep physical understanding, we’re going to find ways to work around them.”
In this artistic rendering, a high-energy cosmic ray hits the qubit chip, freeing up charge in the chip substrate that disrupts the state of neighboring qubits.
