Research, teaching and outreach in Physics at UW–Madison
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Mark Friesen and team named finalists in 2023 WARF Innovation Awards
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Each fall the WARF Innovation Awards recognize some of the best inventions at UW–Madison. WARF receives hundreds of new invention disclosures each year. Of these disclosures, the WARF Innovation Award finalists are considered exceptional in the following criteria:
Has potential for high long-term impact
Presents an exciting solution to a known important problem
Could produce broad benefits for humankind
One of the six finalists comes from Physics. Distinguished Scientist Mark Friesen led a team that included John Bardeen Prof. of Physics Mark Eriksson, Honorary Associate Robert Joynt, Research Associate Benjamin Woods, and Graduate Student Emily Joseph in the development of a quantum device that shows a significant increase in valley splitting, a key property needed for error-free quantum computing. The device features a novel structural composition that turns conventional wisdom on its head.
Two winners, selected from the six finalists, will be announced in WARF’s annual holiday greeting; sign up to receive the greeting here. Each of the two Innovation Award winners receive $10,000, split among UW inventors.
“Sandwich” structure found to reduce errors caused by quasiparticles in superconducting qubits
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Qubits are notoriously more prone to error than their classical counterparts. While superconducting quantum computers currently use on the order of 100 to 1000 qubits, an estimated one million qubits will be needed to track and correct errors in a quantum computer designed for real-world applications. At present, it is not known how to scale superconducting qubit circuits to this size.
In a new study published in PRX Quantum, UW–Madison physicists from Robert McDermott’s group developed and tested a new superconducting qubit architecture that is potentially more scalable than the current state of the art. Control of the qubits is achieved via “Single Flux Quantum” (SFQ) pulses that can be generated close to the qubit chip. They found that SFQ-based control fidelity improved ten-fold over their previous versions, providing a promising platform for scaling up the number of qubits in a quantum array.
Robert McDermottVincent Liu
The architecture involves a sandwich of two chips: one chip houses the qubits, while the other contains the SFQ control unit. The new approach suppresses the generation of quasiparticles, which are disruptions in the superconducting ground state that degrade qubit performance.
“This structure physically separates the two units, and quasiparticles on the SFQ chip cannot diffuse to the quantum chip and generate errors,” explains Chuan-Hong Liu, PhD ’23, a former UW–Madison physics graduate student and lead author of the study. “This design is totally new, and it greatly improves our gate fidelities.”
Liu and his colleagues assessed the fidelity of SFQ-based gates through randomized benchmarking. In this approach, the team established operating parameters to maximize the overall fidelity of complex control sequences. For instance, for a qubit that begins in the ground state, they performed long sequences incorporating many gates that should be equivalent to an identity operation; in the end, they measured the fraction of the population remaining in the ground state. A higher measured ground state population indicated higher gate fidelity.
Inevitably, there are residual errors, but the reduced quasiparticle poisoning was expected to lower the error rate and improve gate fidelities — and it did.
The quantum-classical multichip module (MCM). (a) A micrograph of the qubit chip. (b) A micrograph of the SFQ driver chip. (c) A photograph showing the assembled MCM stack; the qubit chip is outlined in red and the SFQ chip is outlined in blue. (d) The circuit diagram for one qubit-SFQ pair. | From Liu et al, PRX Quantum.
“Most of the gates had 99% fidelity,” Liu says. “That’s a one order of magnitude reduction in infidelity compared to the last generation.”
Importantly, they showed the stability of the SFQ-based gates over the course of a six-hour experimental run.
Later in the study, the researchers investigated the source of the remaining errors. They found that the SFQ unit was emitting photons with sufficient energy to create quasiparticles on the qubit chip. With the unique source of the error identified, Liu and his colleagues can develop ways to improve the design.
“We realized this quasiparticle generation is due to spurious antenna coupling between the SFQ units and the qubit units,” Liu says. “This is really interesting because we usually talk about qubits in the range of one to ten gigahertz, but this error is in the 100 to 1000 gigahertz range. This is an area people have never explored, and we provide a straightforward way to make improvements.”
This study is a collaboration between the National Institute of Standards and Technology, Syracuse University, Lawrence Livermore National Laboratory, and UW–Madison.
This work was funded in part by the National Science Foundation (DMR-1747426); the Wisconsin Alumni Research Foundation (WARF) Accelerator; Office of the Director of National Intelligence, Intelligence Advanced Research Projects Activity (IARPA-20001-D2022-2203120004); and the NIST Program on Scalable Superconducting Computing and the National Nuclear Security Administration Advanced Simulation and Computing Beyond Moore’s Law program (LLNL-ABS-795437).
Choy leads team awarded National Science Foundation Quantum Sensing Challenge Grant
The National Science Foundation has selected a proposal “Compact and robust quantum atomic sensors for timekeeping and inertial sensing” by an interdisciplinary team led by University of Wisconsin-Madison researchers for...
37 years after joining the faculty of the department of physics at the University of Wisconsin–Madison, Prof. Bob Joynt has announced his retirement at the end of July.
Joynt is a condensed matter theorist who began as an assistant professor in 1986. His early work focused largely on superconductivity, including high temperature superconductors. He also played an important role in better understanding the Quantum Hall effect, dating back to his graduate work and continuing here. After a decade and a half, his career took a fortuitous turn when he wrote a quantum computing grant proposal with physics professor Mark Eriksson and other researchers in engineering.
Prof. Bob Joynt
“That was really a pivotal point in my career, and I’ve been doing quantum computing mostly ever since,” Joynt recalls. “Change is good, I found. I enjoyed that change and I’m glad I did it.”
His work for the past 20 years has mainly focused on understanding the origins of noise and decoherence in quantum systems and in the design of semiconductor structures for quantum computing. Joynt is a fellow of the American Physical Society and a UW–Madison Romnes Faculty Fellow. He has co-authored over 175 peer-reviewed publications and trained 26 doctoral students, in addition to numerous postdocs and MS Physics–Quantum Computing students.
Joynt’s academic and research achievements alone comprise an illustrious career that any retiring professor would likely be happy with. Still, his contributions to the department span so much more.
Joynt served as department chair from 2011-2014, for which he focused his efforts on department fundraising. He was responsible for starting the Board of Visitors, a group of people, mostly in industry, with strong ties to the department. The BoV advises and assists on department priorities, plays a leading role in fundraising, and provides a professional network for current students and alumni. From 2017-2022, Joynt additionally served as the department’s Associate Chair for Alumni Relations and the Board of Visitors.
Prof. Joynt lectures in this undated photo from earlier in his career
Around 2016, Joynt noted that doctoral students with quantum computing research experience were in such high demand that employers were often entering bidding wars for them. Was there a way to meet the demands of the quantum computing workforce by training students in a year or two? And so, thanks to Joynt’s vision and persistence, the MS in Physics–Quantum Computing program — the first MS in quantum computing in the U.S. — enrolled its first cohort in Fall 2019.
“We take about 25-30 PhD students each year, and now we take about the same number of MSQPC students,” Joynt says. “It’s become a big part of the department’s educational program.”
Adds Mark Eriksson, Department Chair and John Bardeen Professor of Physics: “Our department’s MSPQC program was the first in the nation and remains a model for others, thanks to Professor Joynt’s vision and energy.”
The department boasts the oldest hands-on science museum in the country — a claim we now feel confident making thanks to Joynt’s extensive research on the history of the Ingersoll Physics Museum for its 100th anniversary in 2018. The museum and physics outreach in general have always been important to Joynt. He has served in an informal capacity as faculty lead for the museum for several years now, helping to raise funds and ensure the museum fulfills its mission of providing free, hands-on, inquiry-based exhibits.
When asked what he wanted to be remembered for in the department, Joynt reflected on lessons from his career and then looked forward: “My advice to the department is: do new things. Don’t be afraid of change. Science changes, education changes, all these things are changing, and you need to change with them.”
Joynt’s retirement is official as of July 31, but he emphasizes that he is only retiring from administrative and teaching duties. He plans to continue his research efforts, sometimes in Madison and often abroad.
Mark Friesen, a senior scientist and long-time collaborator of Joynt’s, says he looks forward to continuing to work with Joynt in this new stage of his career, adding:
“When I joined the department, I knew Bob through reputation as one of the bright condensed matter physicists of his generation. I feel very fortunate to have worked with him, first as a mentor, and later as a colleague. Bob has a tremendous intuition for condensed matter that spans far beyond his immediate research efforts. He also has an easy-going and gracious style that draws in collaborators, and he is just fun to interact with, both inside and outside the department.”
Partnerships bring together UW–Madison quantum computing research, industry leaders
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Two leading companies in semiconductor quantum computing are partnering with researchers at the University of Wisconsin–Madison, itself a long-time academic leader in quantum computing.
UW–Madison’s separate partnerships with Intel and HRL Laboratories are part of a first round of collaborations announced June 14 by the LPS Qubit Collaboratory (LQC), a national Quantum Information Science Research Center hosted at the Laboratory for Physical Sciences (LPS). Established in support of the National Quantum Initiative Act, LQC is facilitating partnerships between industry and academic and national labs to advance research in quantum information science.
“These collaborations are great examples of UW–Madison partnering with industry on the development of important technologies, in this case semiconductor quantum computers,” says physics professor Mark Eriksson, the UW–Madison lead on the partnerships.
NASA’s Fundamental Physics Program has selected seven proposals, including one from UW–Madison physics professor Shimon Kolkowitz, submitted in response to the Research Opportunities in Space and Earth Sciences – 2022 Fundamental Physics call for proposal.
The selected proposals are from seven institutions in seven states, with the total combined award amount of approximately $9.6 million over a five-year period. Kolkowitz’s proposal is ““Developing new techniques for ultra-high-precision space-based optical lattice clock comparisons.”
Three of the selected projects will involve performing experiments using the Cold Atom Laboratory (CAL) aboard the International Space Station (ISS). Four of the selected proposals call for ground-based research to help NASA identify and develop the foundation for future space-based experiments.
The Fundamental Physics Program is managed by the Biological and Physical Sciences Division in NASA’s Science Mission Directorate. This program performs carefully designed research in space that advances our understanding of physical laws, nature’s organizing principles, and how these laws and principles can be manipulated by scientists and technologies to benefit humanity on Earth and in space.
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.
Resolving very small objects that are close together is a frequent goal of scientists, making the microscope a crucial tool for research in many different fields from biology to materials science.
The resolution of even the best modern confocal microscopes — a common optical microscope popular in biology, medicine, and crystallography — is limited by an optical bound on how narrow a laser beam can be focused, known as the diffraction limit.
In a study recently published in the journal ACS Photonics, UW–Madison physics professor Shimon Kolkowitz and his group developed a method to image atomic-level defects in diamonds with super-resolution, reaching a spatial resolution fourteen times better than the diffraction limit achievable with their optics. And, because the technique uses a standard confocal microscope, this super-resolution should be available to any researchers that already have access to this common equipment.
Aedan Gardill
While methods to achieve super-resolution already exist, such as stimulated emission depletion microscopy (STED), nearly all of these methods either require the addition of special optics, which can be expensive and difficult to install, or specialized samples and extensive post processing of the data. The UW–Madison technique, which they call “super-resolution Airy disk microscopy” (SAM), avoids such barriers to entry.
“You can get this all for free with the existing setup that a lot of labs already have, and it performs almost just as well,” says Aedan Gardill, a graduate student in Kolkowitz’s group and lead author of the paper. “We were able to get resolution down to twenty nanometers, which is comparable with standard techniques using [STED].”
The ‘Airy disk’ in SAM refers to a key feature of light beams that gives rise to the diffraction limit but which the researchers turned to their advantage.
Confocal microscopes use laser beams of specific wavelengths to excite matter in a sample, causing that matter to emit light. On the microscopic scale, the laser beam does not create a solid circle of light on the sample in the same way a flashlight would. Rather, light hits the object in a series of light and dark rings called an Airy pattern. Within the dark rings, the matter receives no light, which means it cannot be detected by the microscope’s light sensors.
The novelty of the SAM technique is in its two laser beam pulses, one spatially offset from the other such that the overlapping Airy patterns can distinguish between two closely spaced objects.
In their paper, the research team studied nitrogen-vacancy (NV) centers in diamond crystal, which are regions in the crystal lattice where one of two neighboring carbon atoms is replaced by a nitrogen atom, and the other is left empty. NV centers are known to have two different charge states based on how many electrons are in the defect, one that fluoresces and one that remains dark when yellow light is applied to them.
To resolve two NV centers separated by a distance less than the diffraction limit of the microscope, the SAM procedure first shines green light on them, preparing both centers into their fluorescent charge state. Then, a red laser is applied, offset such that only one of the two NV centers is in the dark ring of the Airy pattern and thus is not affected by the beam. The NV center that does see the red light is switched to the dark state.
Super-resolution Airy disk Microscopy uses the Airy disk (red pattern) generated by diffraction from an objective lens aperture (gray cylinder) to localize and control an emitter (here a nitrogen vacancy center in diamond) below the diffraction limit. Emitter fluorescence is suppressed everywhere except in a very narrow ring (blue donut).
“It goes to another dark charge state where it does not interact with yellow light,” Gardill explains. “But the initial bright charge state does interact with yellow light and will emit light.”
Finally, when the yellow laser is applied, one NV center emits light while the other does not, effectively differentiating between the two neighboring sites. By repeating these steps iteratively over a grid, the researchers could reconstruct a full image of the two nearby NVs with spectacular resolution.
The idea for this technique came as a bit of a surprise while the team was studying charge properties of NV centers in 2020.
“We tried the combinations of red-green, green-red, red-red, green-green with those first two [laser] pulses, and the one that was green then red, we ended up seeing this ring,” Gardill recounts. “And Shimon was like, ‘The width of the ring is smaller than the size of [the confocal image of] the NV. That is super-resolution.’”
This method could find wide use in many different fields, including biology and chemistry where NV centers are used as nanoscale sensors of magnetic and electric fields and of temperature in compounds and organic material. NV centers have also been studied as candidates for quantum repeaters in quantum networks, and the research team has considered the feasibility of using the SAM technique to aid in this application. Currently, the SAM method has only been applied to NV centers in diamond crystal, and more research is needed to extend its use to different systems.
That all of this can be done with hardware that many labs across the world already have access to cannot be overstated. Gardill reiterates, “If they have a basic confocal microscope and don’t want to buy another super-resolution microscope, they can utilize this technique.”
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0020313.
Daniel Heimsoth is a second-year PhD student in Physics. This was his first news story for the department.
Welcome, assistant professor Ilya Esterlis
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Ilya Esterlis
When Lake Mendota freezes over in the winter and thaws in the spring, those water/ice phase transitions might seem mundane. But, says new assistant professor of physics Ilya Esterlis, interesting things happen during phase transitions, and commonalities exist between phase transitions of any matter.
“That’s very surprising and strange sounding, but it turns out that there’s a very general framework in which to understand [these commonalities],” Esterlis says. “It’s this notion of universality, and by studying phase transitions you’re simultaneously studying a very broad class of materials.”
Esterlis, a condensed matter theorist whose research focuses on materials and phase transitions, joins the department January 1, 2023. He is currently a postdoctoral fellow at Harvard, and joined us for a virtual interview earlier this fall.
Can you please give an overview of your research?
I am a condensed matter theorist, so I study materials, and in particular I try to classify different phases of matter and the phase transitions between those phases of matter. I’m mostly interested in electronic systems, where you have a large macroscopic number of interacting electrons and are trying to understand the kind of phenomena that can emerge when you have that large number of degrees of freedom interacting with one another. And a lot of these things are motivated by experiments — not all of them. There are some more academic questions that I’m interested in investigating and they’re a bit more formal. But I’m also motivated by interesting things that are happening in the lab. Part of my work is not only trying to characterize and understand phases of matter, but also trying to propose ways that different phases could be detected experimentally, how they would manifest themselves in different experimental signatures.
I’m also interested in superconductivity. My PhD work focused a lot on trying to understand the optimal conditions for making superconductors — if you could have every knob at your disposal, what would you do to optimize them? Optimize in this case means: make superconductors that exist at as high of a temperature as possible. Superconductivity is typically a low temperature phenomenon, so there’s a holy grail in condensed matter physics trying to make higher temperature superconductors. Part of my work has been organized around trying to understand what would be even in principle the optimal route towards achieving higher temperature superconductors.
Once you’re in Madison, what are one or two research projects you and your group will focus on?
I will focus a good amount of my research efforts on studying superconductivity, continuing this line of investigation into what the optimal conditions for superconductors are. If you had all the freedom in the world, how would you build the best superconductor that exists to high temperatures and under normal laboratory conditions? Not under extreme, unrealistic conditions but in an everyday parameter regime. And that involves understanding the superconducting state itself. Superconductors are a phase of matter that is distinct from, say, a metal, which is also a good conductor but not a superconductor. But oftentimes to understand superconductors better, one has to understand the state from which they came. That is to say, you take a metal and you cool it down to low temperatures and it goes from being a good conductor to a superconductor. To understand that superconductor, it’s often helpful to understand the metal from which it came at higher temperature. And sometimes those metals can be conventional, like copper wires, but sometimes they can be very unconventional metals and strange for various reasons. One open question is: what is the interplay between superconductivity and unusual metals? If you take a high temperature unusual metal, what is the kind of superconductor that it turns into at lower temperature? And unusual in this context means that it has some properties that are not typical to conventional metals. For instance, there’s predictions for how resistance changes with temperature in a conventional metal but unusual metals have rather different resistance behaviors.
What is your favorite element and/or elementary particle?
Helium is remarkable in that it has a number of unusual properties. For instance, if you cool it down to zero temperature it does not crystallize, it remains a liquid. That’s solely due to quantum mechanics, which is kind of an incredible thing. If you do make it crystallize by applying pressure, then that solid itself also has very interesting properties.
And my favorite elementary particle is the anyon. It’s not elementary, say, in the sense of electrons or quarks. But it’s this really remarkable thing that happens in condensed matter systems where if you take a macroscopic number of electrons and you subject them to a very large magnetic field, then a remarkable thing happens where the behavior of the system, as viewed kind of on macroscopic scales, does not look like the behavior of electrons, it really looks like the behavior of particles called anyons that have fractional electric charge. So they are elementary in condensed matter physics.
What hobbies and interests do you have?
I really love to play music, guitar specifically. And I have two small kids, two daughters, and I just like hanging out with them.
Welcome, Roman Kuzmin, the Dunson Cheng Assistant Professor of Physics
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Roman Kuzmin
In the modern, cutting-edge field of quantum computing, it can be a bit puzzling to hear a researcher relate their work to low-tech slide rules. Yet that is exactly the analogy that Roman Kuzmin uses to describe one of his research goals, creating quantum simulators to model various materials. He also studies superconducting qubits and ways to increase coherence in this class of quantum computer.
Kuzmin, a quantum information and condensed matter scientist, will join the department as an the Dunson Cheng Assistant Professor of Physics on January 1. He is currently a research scientist at the University of Maryland’s Joint Quantum Institute in College Park, Md, and recently joined us for an interview.
Can you please give an overview of your research?
My main fields are quantum information and condensed matter physics. For example, one of my interests is to solve complicated condensed matter problems using new techniques and materials which quantum information science developed. Also, it works in the other direction. I am also trying to improve materials which are used in quantum information. I work in the subfield of superconducting circuits. There are several different directions in quantum information, and the physics department at Wisconsin has many of them already, so I will complement work in the department.
Once you’re in Madison and your lab is up and running, what are the first big one or two big thingsyou want to really focus your energy on?
One is in quantum information and quantum computing. So, qubits are artificial atoms or building blocks of a quantum computer. I’m simplifying it, of course, but there are environments which try to destroy coherence. In order to scale up those qubits and make quantum computers larger and larger — because that’s what you need eventually to solve anything, to do something useful with it — you need to mitigate decoherence processes which basically prevent qubits from working long enough. So, I will look at the sources of those decoherence processes and try to make qubits live longer and be longer coherent.
A second project is more on the condensed matter part. I will build very large circuits out of Josephson junctions, inductors and capacitors, and such large circuits behave like some many-body objects. It creates a problem which is very hard to solve because it contains many parts, and these parts interact with each other such that the problem is much more complicated than just the sum of those parts.
What are some applications of your work?
Of course this work is interesting for developing theory and understanding our world. But the application, for example for the many-body system I just described, it’s called the quantum impurity. One of my goals is to use this to create a simulator which can potentially model some useful material. It’s like if you have a quantum computer, you can write a program and it will solve something for you. A slide rule is a physical device that allows you to do complicated, logarithmic calculations, but it’s designed to do only this one calculation. I’m creating kind of a quantum slide rule.
What is your favorite element and/or elementary particle?
So, I have my favorite circuit element: Josephson junction. (editor’s note: the question did not specify atomic element, so we appreciate this clever answer!). And for elementary particle, the photon, especially microwave photons, because that’s what I use in these circuits to do simulations. They’re very versatile and they’re just cool.
