Research, teaching and outreach in Physics at UW–Madison
quantum computing
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.
What hobbies and interests do you have?
I like reading, travelling, and juggling.
Margaret Fortman awarded Google quantum computing fellowship
Two UW–Madison graduate students, including physics grad student Margaret Fortman, have been awarded 2022 Google Fellowships to pursue cutting-edge research. Fortman received the 2022 Google Fellowship in Quantum Computing, one of only four awarded.
Margaret Fortman
Google created the PhD Fellowship Program to recognize outstanding graduate students doing exceptional and innovative research in areas relevant to computer science and related fields. The fellowship attracts highly competitive applicants from around the world.
“These awards have been presented to exemplary PhD students in computer science and related fields,” Google said in its announcement. “We have given these students unique fellowships to acknowledge their contributions to their areas of specialty and provide funding for their education and research. We look forward to working closely with them as they continue to become leaders in their respective fields.”
The program begins in July when students are connected to a mentor from Google Research. The fellowship covers full tuition, fees, and a stipend for the academic year. Fellows are also encouraged to attend Google’s annual Global Fellowship Summit in the summer.
Fortman works to diagnose noise interference in quantum bits
Fortman, whose PhD research in Victor Brar’s group specializes in quantum computing, will use the fellowship support to develop a diagnostic tool to probe the source of noise in superconducting quantum bits, or qubits.
Quantum computing has the potential to solve problems that are difficult for standard computers, Fortman said, but the field has challenges to solve first.
“The leading candidate we have for making a quantum computer right now is superconducting qubits,” Fortman said. “But those are currently facing unavoidable noise that we get in those devices, which can actually come from the qubit material itself.”
Fortman works with a low-temperature ultra-high vacuum scanning tunneling microscope on the UW–Madison campus to develop a microscopic understanding of the origins of noise in qubits. She fabricates superconductors to examine under the microscope to identify the source of the noise, and hopefully be able to develop a solution for that interference.
In her time as a graduate student at UW–Madison, Fortman said she has enjoyed collaborating with colleagues in her lab and across campus.
“It’s pretty cool to be somewhere where world-renowned research is happening and to be involved with that,” she said. “My PI and I work in collaborations with other PIs at the university and they’re all doing very important research, and so it’s really cool to be a part of that.”
Fortman is excited to have a mentor at Google through the PhD Fellowship, having been paired with someone who has a similar disciplinary background and who is a research scientist with Google Quantum AI.
“He can be a resource in debugging some parts of my project, as well as general mentorship and advice on being a PhD student, and advice for future career goals,” Fortman said.
The second UW–Madison student who earned this honor is computer sciences PhD student Shashank Rajput, who received the 2022 Google Fellowship in Machine Learning.
Cross-institutional collaboration leads to new control over quantum dot qubits
Qubits are the building blocks of quantum computers, which have the potential to revolutionize many fields of research by solving problems that classical computers can’t.
But creating qubits that have the perfect quality necessary for quantum computing can be challenging.
Researchers at the University of Wisconsin–Madison, HRL Laboratories LLC, and University of New South Wales (UNSW) collaborated on a project to better control silicon quantum dot qubits, allowing for higher-quality fabrication and use in wider applications.
All three institutions are affiliated with the Chicago Quantum Exchange. The work was published in Physical Review Letters, and the lead author, J. P. Dodson, has recently transitioned from UW–Madison to HRL.
“Consistency is the thing we’re after here,” says Mark Friesen, Distinguished Scientist of Physics at UW–Madison and author on the paper. “Our claim is that there is actually hope to create a very uniform array of dots that can be used as qubits.”
Sensitive quantum states
While classical computer bits use electric circuits to represent two possible values (0 and 1), qubits use two quantum states to represent 0 and 1, which allows them to take advantage of quantum phenomena like superposition to do powerful calculations.
Qubits can be constructed in different ways. One way to build a qubit is by fabricating a quantum dot, or a very, very small cage for electrons, formed within a silicon crystal. Unlike qubits made of single atoms, which are all naturally identical, quantum dot qubits are man-made—allowing researchers to customize them to different applications.
But one common wrench in the metaphorical gears of these silicon qubits is competition between different kinds of quantum states. Most qubits use “spin states” to represent 0 and 1, which rely on a uniquely quantum property called spin. But if the qubit has other kinds of quantum states with similar energies, those other states can interfere, making it difficult for scientists to effectively use the qubit.
In silicon quantum dots, the states that most often compete with the ones needed for computing are “valley states,” named for their locations on an energy graph—they exist in the “valleys” of the graph.
To have the most effective quantum dot qubit, the valley states of the dot must be controlled such that they do not interfere with the quantum information-carrying spin states. But the valley states are extremely sensitive; the quantum dots sit on a flat surface, and if there is even one extra atom on the surface underneath the quantum dot, the energies of the valley states change.
The study’s authors say these kinds of single-atom defects are pretty much “unavoidable,” so they found a way to control the valley states even in the presence of defects. By manipulating the voltage across the dot, the researchers found they could physically move the dot around the surface it sits on.
“The gate voltages allow you to move the dot across the interface it sits on by a few nanometers, and by doing that, you change its position relative to atomic-scale features,” says Mark Eriksson, John Bardeen Professor and chair of the UW–Madison physics department, who worked on the project. “That changes the energies of valley states in a controllable way.
“The take home message of this paper,” he says, “is that the energies of the valley states are not determined forever once you make a quantum dot. We can tune them, and that allows us to make better qubits that are going to make for better quantum computers.”
Building on academic and industry expertise
The host materials for the quantum dots are “grown” with precise layer composition. The process is extremely technical, and Friesen notes that Lisa Edge at HRL Laboratories is a world expert.
“It requires many decades of knowledge to be able to grow these devices properly,” says Friesen. “We have several years of collaborating with HRL, and they’re very good at making really high-quality materials available to us.”
The work also benefitted from the knowledge of Susan Coppersmith, a theorist previously at UW–Madison who moved to UNSW in 2018. Eriksson says the collaborative nature of the research was crucial to its success.
“This work, which gives us a lot of new knowledge about how to precisely control these qubits, could not have been done without our partners at HRL and UNSW,” says Eriksson. “There’s a strong sense of community in quantum science and technology, and that is really pushing the field forward.”
Opening doors to quantum research experiences with the Open Quantum Initiative
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This past winter, Katie Harrison, then a junior physics major at UW–Madison, started thinking about which areas of physics she was interested in studying more in-depth.
“Physics is in general so broad, saying you want to research physics doesn’t really cut it,” Harrison says.
She thought about which classes she enjoyed the most and talked to other students and professors to help figure out what she might focus on. Quantum mechanics was high on her list. During her search for additional learning opportunities, she saw the email about the Open Quantum Initiative (OQI), a new fellowship program run by the Chicago Quantum Exchange (CQE).
“This could be something I’m interested in, right?” Harrison thought. “I’ll apply and see what happens.”
What happened was that Harrison was one of 12 undergraduate students accepted into the inaugural class of OQI Fellows. These students were paired with mentors at CQE member institutions, where they conducted research in quantum science information and engineering. OQI has a goal of connecting students with leaders in academia and industry and increasing their awareness of quantum career opportunities. The ten-week Fellowship ran through August 19.
OQI students attend a wrap-up at the University of Chicago on August 17. Each student presented at a research symposium that day, which also included a career panel from leaders across academia, government, and industry and an opportunity to network. | Photo provided by the Chicago Quantum Exchange
OQI also places an emphasis on establishing diversity, equity, and inclusion as priorities central to the development of the quantum ecosystem. Almost 70% of this year’s fellowship students are Hispanic, Latino, or Black, and half are the first in their family to go to college. In addition, while the field of quantum science and engineering is generally majority-male, the 2022 cohort is half female.
This summer, UW–Madison and the Wisconsin Quantum Institute hosted two students: Harrison with physics professor Baha Balantekin and postdoc Pooja Siwach; and MIT physics and electrical engineering major Kate Arutyunova with engineering physics professor Jennifer Choy, postdoc Maryam Zahedian and graduate student Ricardo Vidrio.
Harrison and Arutyunova met at OQI orientation at IBM’s quantum research lab in New York, and they hit it off immediately. (“We have the most matching energies (of the fellows),” Arutyunova says, with Harrison adding, “The synergy is real.”)
OQI Fellow Kate Arutyunova with her research mentors. (L-R) Engineering Physics professor Jennifer Choy, graduate student Ricardo Vidrio, Kate Arutyunova, and postdoc Maryam Zahedian. | Photo provided by Kate Arutyunova
Despite their very different research projects — Harrison’s was theoretical and strongly focused on physics, whereas Arutyunova’s was experimental and with an engineering focus — they leaned on each other throughout the summer in Madison. They met at Union South nearly every morning at 7am to read and bounce ideas off each other. Then, after a full day with their respective research groups, they’d head back to Union South until it closed.
Modeling neutrino oscillations
Harrison’s research with Balantekin and Siwach investigated the neutrinos that escape collapsing supernovae cores. Neutrinos have a neutral charge and are relatively small particles, they make it out of cores without interacting with much — and therefore without changing much — so studying them helps physicists understand what is happening inside those stars. However, this is a difficult task because neutrinos oscillate between flavors, or different energy levels, and therefore require a lot of time and resources to calculate on a classical computer.
Harrison’s project, then, was to investigate two types of quantum computing methods, pulse vs circuit based, and determine if one might better fit their problem than the other. Previous studies suggest that pulsed based is likely to be better, but circuit based involves less complicated input calculations.
“I’ve been doing calibrations and calculating the frequencies of the pulses we’ll need to send to our qubits in order to get data that’s as accurate as a classical computer,” Harrison says. “I’m working with the circuit space, the mathematical versions of them, and then I’ll send my work to IBM’s quantum computers and they’ll calculate it and give results back.”
While she didn’t fully complete the project, she did make significant progress.
“(Katie) is very enthusiastic and she has gone a lot further than one would have expected an average undergraduate could have,” Balantekin says. “She started an interesting project, she started getting interesting results. But we are nowhere near the completion of the project, so she will continue working with us next academic year, and hopefully we’ll get interesting results.”
Developing better quantum sensors
Over on the engineering side of campus, Arutyunova was studying different ways to introduce nitrogen vacancy (NV) centers in diamonds. These atomic-scale defects are useful in quantum sensing and have applications in magnetometry. Previous work in Choy’s group made the NV centers by a method known as nitrogen ion beam implantation. Arutyunova’s project was to compare how a different method, electron beam irradiation, formed the NV centers under different starting nitrogen concentrations in diamond.
Briefly, she would mark an edge of a very tiny (2 x 2 x 0.5 millimeter), nitrogen-containing diamond, and irradiate the sample with a scanning electron microscope. She used confocal microscopy to record the initial distribution of NV centers, then moved the sample to the annealing step, where the diamond is heated up to 1200 celsius in a vacuum annealing furnace. The diamonds are then acid washed and reexamined with the confocal microscope to see if additional NV centers are formed.
“It’s a challenging process as it requires precise coordinate-by-coordinate calculation for exposed areas and extensive knowledge of how to use the scanning electron microscope,” says Arutyunova, who will go back to MIT after the fellowship wraps. “I think I laid down a good foundation for future steps so that the work can be continued in my group.”
Choy adds:
Kate made significant strides in her project and her work has put us on a great path for our continued investigation into effective ways of generating color centers in diamond. In addition to her research contributions, our group has really enjoyed and benefited from her enthusiasm and collaborative spirit. It’s wonderful to see the relationships that Kate has forged with the rest of the group and in particular her mentors, Maryam and Ricardo. We look forward to keeping in touch with Kate on matters related to the project as well as her academic journey.
Beyond the summer fellowship
Both Harrison and Arutyunova think that this experience has drawn them to the graduate school track, likely with a focus on quantum science. More importantly, it has helped them both to learn what they like about research.
“I would prefer to work on a problem and see the final output rather than a question where I do not have an idea of the application,” Arutyunova says. “And I realized how much I like to collaborate with people, exchange ideas, propose something, and listen to people and what they think about research.”
They also offer similar advice to other undergraduate students who are interested in research: do it, and start early.
“No matter when you start, you’re going to start knowing nothing,” Harrison says. “And if you start sooner, even though it’s scary and you feel like you know even less, you have more time to learn, which is amazing. And get in a research group where they really want you to learn.”
Coherent light production found in very low optical density atomic clouds
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No atom is an island, and scientists have known for decades that groups of atoms form communities that “talk” to each other. But there is still much to learn about how atoms — particularly energetically excited ones — interact in groups.
In a study published in PRX Quantum, physicists from the University of Wisconsin–Madison observed communication between atoms at lower and lower densities. They found that the atoms influence each other at 100 times lower densities than probed before, exhibiting slow decay rates and emitting coherent light.
“It seems that (low-density) groups of excited atoms spontaneously organize to then produce light that is coherent,” says David Gold, a postdoctoral fellow in Deniz Yavuz’s group and lead author of the study. “These findings are pretty interesting from a basic science standpoint, and in terms of quantum computing, the takeaway is that even with very low numbers of atoms, you can see significant amounts of (these effects).”
A well-established property of atoms is found in electron excitation: when a specific wavelength of light hits an atom of a specific element, an electron is excited to a higher orbital level. As that electron decays back to its initial state, a photon of a specific wavelength is emitted. A single atom has a characteristic decay rate for that process. When groups of atoms are studied, their interactions are observed: the initial decay rate is very fast, or superradiant, then transitions to a slower, or subradiant, rate.
A schematic of the experimental setup. (Top) the overall apparatus used. (A) shows the setup for the first part of the experiment, where the researchers were measuring decay rates in lower and lower density clouds. (B) shows the setup for the second part of the paper, with the addition of an interferometer
Though well-established in dense clouds, this group-talk has never been studied in less dense clouds of atoms, which could have impacts on applications such as quantum computing.
In their first set of experiments, Gold and colleagues asked what the decay rate of lower-density clouds looked like. They supercooled the atoms in a cloud, hit them with an excitation laser, and recorded the decay rates as an intensity of emitted light over time. They observed the characteristic subradiance. In this case, they did not always see superradiance, likely due to the reduced number of atoms available to measure.
David Gold
Next, they asked what happened if they let the cloud expand — or decrease in density — for varying periods of time before repeating their experiment. They found that as the cloud become less and less dense, the amount of subradiance decreased, until eventually a density was reached where the atoms stopped behaving like a group and instead displayed single-atom decay rates.
“The most subradiance that we observed was at around a hundred times lower optical density than it had previously been observed before,” Gold says.
Now that the researchers knew that a less dense cloud still decays subradiantly to a point, they asked if the decay was happening in an isolated manner, or if the atoms were really acting as a group. If acting as a group, the emitted light would be coherent, or more laser-like, with some structure between the atoms.
They used the same experimental setup but added an interferometer, where light is split and recombined before the photons are detected. They first set the baseline interference pattern by moving the mirror closer or further away from the splitter — changing the path length of one of the beams — and mapping the interference pattern of the split light waves that were emitted from the same atom.
If there were no relationship between the two atoms and the light they emit, then they would have expected to see no interference pattern. Instead, they saw that for some distance of mirror displacement, the lightwaves did interfere, indicating that different atoms being measured were nonetheless producing coherent light.
“I think this is the more exciting thing we found: that the light that’s being emitted is coherent and it has more of the properties of a laser than you would expect,” Gold says. “The atoms are influenced by each other and not in a way we would have expected.”
Aside from the interesting physics seen in the study, Gold says the work is also applicable to quantum computing, particularly as those computers grow bigger in the future.
“Even if everything in a quantum computer is running perfectly and the system was completely isolated, there’s still this inherent thing of, well, the atoms just might decay down from [the computational] state,” Gold says.
UW–Madison, industry partners run quantum algorithm on neutral atom quantum computer for the first time
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A university-industry collaboration has successfully run a quantum algorithm on a type of quantum computer known as a cold atom quantum computer for the first time. The achievement by the team of scientists from the University of Wisconsin–Madison, ColdQuanta and Riverlane brings quantum computing one step closer to being used in real-world applications. The work out of Mark Saffman’s group was published in Nature on April 20.
The Open Quantum Initiative (OQI), a working group of students, researchers, educators, and leaders across the Chicago Quantum Exchange (CQE), announced the launch of the OQI Undergraduate Fellowship as part of their effort to advocate for and contribute to the development of a diverse and inclusive quantum workforce.
The primary mission of the OQI is to champion the development of a more inclusive quantum community. Science, technology, engineering, and mathematics (STEM) fields remain overwhelmingly white and male—only about 20% of bachelor’s degrees in physics, engineering, and computer science go to women, a mere 6% of all STEM bachelor’s degrees are awarded to African American students, and 12% of all STEM bachelor’s degrees are awarded to Hispanic students. But as the field of quantum science is still relatively new compared to other STEM subjects, groups like the OQI see a chance to make the foundations of the field diverse and accessible to all from the start.
“In many respects, we are building a national workforce from the ground up,” says David Awschalom, the Liew Family Professor in Molecular Engineering and Physics at the University of Chicago, senior scientist at Argonne National Laboratory, director of the Chicago Quantum Exchange, and director of Q-NEXT, a Department of Energy quantum information science center led by Argonne. “There are incredible opportunities here to make the field of quantum engineering as inclusive and equitable as possible from the very beginning, creating a strong ecosystem for the future.”
At the heart of the OQI’s effort is a new fellowship starting in summer 2022. For 10 weeks, fellows will live and work at a CQE member or partner institution, completing a research project in quantum information science and engineering under the guidance of a mentor. Students will have numerous opportunities to interact with the other fellows in their cohort during the summer research period and throughout the following academic year.
Through this fellowship, the students can expand their understanding of quantum science, receive career guidance, and grow their professional networks with leaders in academia and industry. The OQI will also aim to provide future research experiences in subsequent summers, as well as provide opportunities to mentor future fellows, helping to build a larger, diverse quantum community over time.
With the support of CQE’s member and partner institutions, including the University of Chicago, Argonne, Fermilab, University of Illinois Urbana-Champaign, University of Wisconsin-Madison, Northwestern University, and The Ohio State University, along with the NSF Quantum Leap Challenge Institute for Hybrid Quantum Architectures and Networks (HQAN) and Q-NEXT, this fellowship helps to establish diversity, equity, and inclusion as priorities central to the development of the quantum ecosystem.
The OQI launched the fellowship alongside a workshop on September 22 and 23. The OQI workshop, titled “Building a Diverse Quantum Ecosystem,” brought together CQE students, researchers, and professionals from across different institutions, including industry, to discuss the prevailing issues and barriers in quantum information science as the field develops. Institutional changemakers also shared what they have learned from their own efforts to increase representation. A panel on education and workforce development at the upcoming Chicago Quantum Summit on Nov. 4 will continue the discussion on building inclusive onramps for the quantum information science field.
“For quantum science and engineering to achieve its full potential, it must be accessible to all,” says Kayla Lee, Academic Alliance Lead at IBM Quantum and keynote speaker of the OQI workshop. “The OQI Undergraduate Fellowship provides explicit support for historically marginalized communities, which is crucial to increasing quantum engagement in a way that creates a more diverse and equitable field.”
Applications for the OQI Undergraduate Fellowship are open now.
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).
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.
Congratulations to Professor Sue Coppersmith on her retirement!
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With the best of wishes — and some sadness — the Department of Physics says “Happy Retirement” to Professor Sue Coppersmith. Her last day at UW–Madison was February 14.
Coppersmith, the Robert E. Fassnacht Professor of Physics, joined the department in 2001. Prior to coming to UW–Madison, she earned her Ph.D. from Cornell University, conducting her thesis work at Bell Labs. She completed a postdoc at Brookhaven National Lab, then worked at Bell Labs for eight years before joining the faculty at the University of Chicago.
Sue Coppersmith
During her tenure here, she served as Department Chair for one three-year term, and earned recognition as a Fellow of the National Academy of Sciences, the American Academy of Arts and Sciences, the American Association for the Advancement of Science, and the American Physical Society.
Scientific Achievements
At UChicago, Coppersmith’s research focused on soft matter physics and non-linear dynamics, work that she continued at UW–Madison, primarily with Prof. Pupa Gilbert. But her research program largely shifted over the years into quantum computing, an area that was just getting started when she started in Madison..
“At the time, I would tell people what we were doing, and of course nothing was working yet, and people would say, ‘Well, that’s all crap, isn’t it?’” Coppersmith recalls. “So, it was really fun to go from a time where there was nothing working, to now we have qubits, and being a part of the effort and feeling like I was helping.”
Coppersmith describes herself as a theorist who went into the lab every day to better understand the experimental side of quantum computing, And, she says, UW–Madison stands out as one of the universities where theory and experiment are so closely tied together. Here, she frequently collaborated with Prof. Mark Eriksson and Distinguished Scientist Mark Friesen.
“She just comes up with a lot of ideas, and what matters most is how many of them are home runs. She had an unusually large number,” Eriksson says. “She came up with the idea for a brand new qubit, the quantum dot hybrid qubit, and we’re still working on it to this day in my lab. And other people around the world have picked it up.”
Friesen adds:
“As a researcher, Sue is highly intuitive and focused more on the high-level physical picture rather than specific technical details. She typically breaks a problem down to a ‘minimal model’ that captures its basic physics. She has studied a wide variety of problems in her career, for which she is highly respected in many different communities, and she is able to apply lessons learned from one area to another. Her memory is legendary! She is also known for her quickness, both in being able to understand a problem (and how it fits into the big picture) and being able to immediately respond to it. I also say this in a good way: she is not shy about expressing her opinions.”
Legacy as Department Chair
Perhaps equal to her scientific achievements is the mark Coppersmith made on the department during her time as Chair, from 2005-08. The Department was hiring three faculty positions, and she reasoned that if eight offers were made, at worst four people would accept.
“But eight people came! And I was famous for it because I ruined the College’s budget,” Coppersmith says. “I think this is the highlight of my Chair career. I loved recruiting people.”
There are a number of factors that go into faculty candidates accepting or not accepting offers, but Eriksson is certain that Coppersmith‘s ability to recruit was a significant one.
“They came in large part because Sue understood and was able to get them to explain and she was able to hear what they really needed, and then go deliver on it,” Eriksson says. “It’s one thing to have any subset of those skills, but she has the whole package.”
Current Department Chair Sridhara Dasu credits Coppersmith with shaping the direction of the department in all areas of physics, adding, “Her tenure continues to be an inspiration for all chairs of the department who followed her.”
Sue with a group of close collaborators from around the world, at a meeting that she arranged in Rio de Janeiro.
Mentorship of students and colleagues
Coppersmith’s mentorship of junior colleagues and students will also be missed. Both Friesen and Susan Nossal, senior scientist and director of the Physics Learning Center, noted that Coppersmith’s support has been crucial to their success as researchers in the department. They both applauded her as a champion of women and girls in science, citing her participation – with Nossal, Gilbert and several graduate students – in the annual Expanding Your Horizons event at which middle school girls participate in fun, hands-on science activities.
“As a mentor, she is highly dedicated to her students and colleagues,” says Friesen, who co-advised several students with Coppersmith. “For me personally, she has been very supportive of my career path, helping me to obtain promotions and advancements, and providing on-point advice.”
Adds Nossal: “As a scientist, you have your ups and downs, and she helped me through some of the downs. It’s always helpful to have people who believe in you, and she helped me in persisting as a scientist.”
Looking forward
Between Coppersmith and everyone else mentioned in this piece, there were certainly plenty of stories that could be shared. But for now, we’ll let emeritus professor Lou Bruch sum up Coppersmith’s tenacity and well-placed ambition with this anecdote:
“Sue touted the usefulness of the Mathematica package and would at times get into competition on speed of getting to the answer — her using the package and me using ad hoc analyses. I recall only one instance where I won.”
Coppersmith may be retired from UW–Madison, but she is not retiring from science. She is currently Professor and Head of the School of Physics at the University of New South Wales in Australia, where she will continue her research and collaborations with colleagues here and around the world.
“Wisconsin was so good to me. The people are so nice, and we did good work,” Coppersmith says. “I like to feel that I contributed in a positive way. I’ll always be grateful.”
