“Sandwich” structure found to reduce errors caused by quasiparticles in superconducting qubits

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.

profile photo of Robert McDermott
Robert McDermott
profile photo of Vincent Liu
Vincent 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.

four panels showing the new chip architecture. The two on the left just show the two computer chips, and then the top right panel shows them "sandwiched" on top of each other. The bottom right panel is a circuit diagram of the whole setup.
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).

Physics has three winners in the Cool Science Image contest!

The winners of the UW–Madison 13th annual Cool Science Image contest were announced, and Physics has three winners! Our winners include graduate student Jacob Scott, the graduate student-professor pairing of Jimena González and Keith Bechtol, and alum Aedan Gardill, PhD ’23. Their winning images are below.

A panel of experienced artists, scientists and science communicators chose 12 winning images based on the aesthetic, creative and scientific qualities that distinguished them from scores of entries. The winning entries showcase the research, innovation, scholarship and curiosity of the UW–Madison community through visual representations of socioeconomic strata, brain cells snuffed out in Parkinson’s disease, the tangle of technology required to equip a quantum computing lab and a bug-eyed frog that opened students’ eyes to the world.

The winning images go on display this week in an exhibit at the McPherson Eye Research Institute’s Mandelbaum and Albert Family Vision Gallery on the ninth floor of the Wisconsin Institutes for Medical Research, 111 Highland Ave. The exhibit, which runs through the end of 2023, opens with a public reception at the gallery Thursday, Sept. 28, from 4:30 to 6:30 p.m. The exhibit also includes historical images of UW science, in celebration of the 175th anniversary of the University of Wisconsin’s founding.

The Cool Science Image Contest recognizes the technical and creative skills required to capture and create images, videos and other media that reveal something about science or nature while also leaving an impression with their beauty or ability to induce wonder. The contest is sponsored by Madison’s Promega Corp., with additional support from UW–Madison’s Office of University Communications.

a photograph of a room with the lights off, but the bulk of the image is taken up by a large piece of complicated equipment with many different colored laser lights visible, illuminating the shape of the equipment
The glow of red and green lasers and an array of supporting electronics fill a UW–Madison lab where physicists study the behavior of cesium atoms cooled within a fraction of a degree of absolute zero. The atoms could be used to store information in quantum computing systems. | Jacob Scott
an oddly-shaded portrait of physicist Marie Curie, which can only be viewed when a light polarizer is held in front of the portrait
Like the radiation she studied, this portrait of physicist Marie Curie is invisible until revealed by the proper equipment — in this case, a polarizer, a filter that blocks all light waves except those oscillating in a certain direction. One polarizing filter on the back layer of the portrait organizes the light shining through to the viewer. That light passes through layers of colorless cellophane, which rotate the waves a little or a lot depending on the layer’s thickness. A second polarizing filter, held by the viewer, filters the light again, selecting light at the wavelengths that correspond to the intended colors of the portrait. The image above is as the portrait appears viewed through a polarizer. | Aedan Gardill PhD ’23
an array of red-glowing images on a dark black background
Each image in this collage is of an astronomical phenomenon known as a strong gravitational lens, in which the light from a galaxy or cluster of galaxies is curved by a massive object in the foreground. The light is distorted into bright arcs, exhibiting physics theorized by Albert Einstein. Strong gravitational lenses offer a way to study dark matter, difficult to detect but considered a crucial factor in the structure, evolution and fate of the cosmos. | Jimena González and Keith Bechtol

Jimena González wins 2023 OSG David Swanson Award

Early in her thesis research, Jimena González was waiting. A lot.

To better understand the nature of dark energy, she uses machine learning to search Dark Energy Survey cosmology data for evidence of strong gravitational lensing — where a heavy foreground galaxy bends the light of another galaxy, producing multiple images of it that can get so distorted that they appear as long arcs of light around the large galaxy in telescope images. She also focuses on finding very rare cases of strong gravitational lensing in which two galaxies are lensed by the same foreground galaxy, systems known as double-source-plane lenses.

First, she had to create simulations of the galaxy systems. Next, she used those simulations to train the machine learning model to identify the systems in the heaps and heaps of DES data. Lastly, she would apply the trained model to the real DES data. All told, she expected to find hundreds of “simple” strong gravitational lenses and only a few double-source-plane lenses out of 230 million images.

“But, for example, when I did the search the first time, I mostly only got spiral galaxies, so then I had to include spiral galaxies in my training,” says González, a physics graduate student in Keith Bechtol’s group.

The initial steps took around two weeks (hence the waiting) before she could even know what needed to be changed to better train the model. Once she had the model trained and would be ready to apply it to the entire dataset, she estimated it would take five to six years just to find the images of interest — and then she would finally be able to study the systems found.

a woman stands in front of a screen with a research slide on the screen, she faces the audience and is gesturing with her hands.
Jimena González presents an award lecture at the 2023 Throughput Computing Conference. (provided by Jimena González)

Then, the email from the Open Science Grid (OSG) Consortium came. The OSG Consortium operates a fabric of distributed High Throughput Computing (dHTC) services, allowing users to take advantage of massive amounts of computing power. Researchers can apply to the OSG User School, an annual workshop for scientists who want to learn and use dHTC methods.

“[dHTC] is parallelizing things. It’s like if you had 500 exams to grade, you can distribute them among different people and it would take less time,” González says. “It sounded perfect for me.”

González applied and was accepted into the 2021 program, which was run virtually that year. At the OSG User School, she learned methods that would allow her to take advantage of dHTC and apply them to her work. Her multi-year processing time was cut down to mere days.

“Because it was so fast, there were many new things that I could implement in my research,” González says. “A lot of the methodology I implemented would not have been possible without OSG.”

This summer, González was selected as one of two recipients of the OSG David Swanson Award.

David Swanson was a longtime champion of and contributor to OSG, who passed away in 2016. In his memory, the award is bestowed annually upon one or more former students of the OSG User School who have subsequently achieved significant dHTC-enabled research outcomes.

She accepted the award at the Throughput Computing 2023 conference, where she presented her research and discussed how she used her training from the OSG User School to successfully comb through the DES data and find the systems of interest.

“When I got the award, I didn’t know anything about [Swanson],” González says. “But once I attended this event, I heard so many people talking about him, and I understood why it was created. It is such an honor to receive this award in his name.”

Zain Abhari selected for SACLA graduate internship

a woman stands in front of a tree with a Japanese castle in the near background
Zain Abhari on a tour at Himeji Castle, which is one of the last few fully intact castles in Japan and is located near the SACLA facility | Photo provided by Zain Abhari

Congrats to Physics PhD student Zain Abhari for being selected to the SACLA Research Support Program for Graduate Students. The one-year internship run by SACLA (the SPring-8 Angstrom Compact free electron LAser) accepts graduate students with a demonstrated interest in using X-ray free electron lasers (XFELs) for their research and provides them with training and beam time at the facility in Japan.

Prior to her acceptance in the program, Abhari had already spent time at SACLA as part of her research in Uwe Bergmann’s group. While there, her collaborator told her about the program and recommended she apply.

“My goal after the PhD is to work at one of these large-scale facilities, specifically the X-ray free electron lasers and there’s only six of them right now in the world,” Abhari says. “If I can get my foot in the door in Japan, or get the experience to then help me with any of the other ones, that would be pretty awesome.”

Abhari was also interested in the program because SACLA’s laser aligns well with the goals of her thesis research, which is to obtain intense, stable XFEL pulses to apply to different spectroscopy techniques. For about the past decade, XFEL has allowed researchers to make ultrafast movies of molecular changes, essentially helping them to see chemical reactions take place. But x-ray lasers are “dirty,” and they contain multiple wavelengths of light of varying intensity. Last year, Bergmann and his colleagues somewhat accidentally discovered a way to make the pulses cleaner through two intense, femtosecond-spaced pulses.

Even though the researchers think they know how the useful pulses work, producing and controlling them are a completely different story — and one that Abhari hopes to unravel in her research.

“Right now, they’re just random,” Abhari says. “So the goal is to understand them. And then if we understand them, can we control them? If we can control them, can we apply them?”

Abhari will travel to Japan for three months beginning in September, where the program provides on-campus housing and time on the laser. Without the access she is now granted by this internship, her research would have been much more focused on short rounds of data collection followed by off-site data analysis.

“Now, I can get my hands on the laser and collect data to try to understand parameters that allow us to get the specific output we’re looking for,” Abhari says. “I have data that allude me to what those parameters will be, but now in real time, I can be like, ‘If I do this, I see this; if I do that, I see that.’”

In addition to her thesis research, Abhari will be working with her SACLA collaborators on a machine learning project to optimize beam focusing, and helping learn about and improve a portable beam nanofocusing apparatus.


Physics students earn 2023 NSF graduate fellowships

Congrats to current Physics PhD students Samuel Hori and Alysa Rogers and undergraduate Emil Pellett on earning 2023 National Science Foundation Graduate Research Fellowships! Congrats also to PhD student Spencer Weeden for earning an honorable mention!

The Graduate Research Fellowship Program (GRFP) supports high-potential scientists and engineers in the early stages of their careers. Each year, more than 12,000 applicants compete for ~2,000 fellowship awards. NSF GRFP awards are highly sought and competitive. The fellowship is awarded to individuals in the early stages of their graduate study, who intend to pursue research-based graduate studies in science, technology, engineering, and mathematics (STEM).

The program provides awardees with three years of financial support consisting of a $37,000 annual stipend and a $12,000 education allowance. UW–Madison contributes toward fringe benefits.

UW–Madison researchers key in search for neutrino emission from the brightest gamma-ray burst ever detected

This story was originally published by WIPAC

On October 9th, 2022, an unusually bright pulse of high-energy radiation whizzed past Earth, captivating astronomers around the world. The luminous emission came from a gamma-ray burst (GRB), one of the most powerful classes of explosions in the universe. Named GRB 221009A, it triggered detectors at NASA’s Gamma-ray Burst Monitor and Large Area Telescope (both on board the Fermi Gamma-ray Space Telescope), the Neil Gehrels Swift Observatory, and the Wind spacecraft as well as other telescopes that quickly turned to the GRB site to study its aftermath.

profile photo of Jessie Thwaites
Jessie Thwaites

This record-shattering GRB is one of the closest and the brightest GRB ever spotted, earning it the nickname BOAT (“brightest of all time”). This GRB is believed to come from an exploding star and likely signals the birth of a black hole.

In a new study by the IceCube Collaboration, published today in The Astrophysical Journal Letters, UW–Madison researchers presented results of one of five searches for neutrino emission from GRB 221009A that leveraged the full detector range, covering nine orders of magnitude in energy. Because no significant emission was found across samples spanning 10 MeV to 10 PeV, the results are the most stringent constraints on neutrino emission from GRBs.

As some of the most energetic sources in the universe, GRBs have long been considered a possible astrophysical source of neutrinos—tiny “ghostlike” particles that travel through space and large amounts of matter unhindered. These high-energy neutrinos are of particular interest to the National Science Foundation-supported IceCube Neutrino Observatory, a gigaton-scale neutrino detector at the South Pole.

IceCube is run by the international IceCube Collaboration, which comprises over 350 scientists from 58 institutions around the world. The Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center at UW–Madison, is the lead institution for the IceCube project.

Previously, IceCube has performed searches for neutrino emission from GRBs, but thus far, a correlation has not been found between high-energy neutrinos and GRBs. The recent observation of GRB 221009A presented IceCube with the best opportunity yet to search for neutrino emission by GRBs.

profile photo of Justin Vandenbroucke
Justin Vandenbroucke

“Not only was this GRB the brightest ever detected in gamma rays, it also occurred in a region of the sky where IceCube is very sensitive,” says UW–Madison physics professor Justin Vandenbroucke, who helped lead the analysis.

For the study, collaborators carried out five complementary IceCube analyses that encompassed the full energy range of the detector. Each analysis targeted a specific energy range, with the idea of covering as wide an energy range as possible. UW–Madison physics PhD student Jessie Thwaites was one of the main analyzers.

Thwaites performed a “fast response” analysis based on real-time data from the South Pole to search for high-energy (0.10 teraelectronvolts to 10 petaelectronvolts) neutrinos from the direction of the GRB. They chose two time windows: one three-hour window covering all of the triggers reported in real time, and one covering two days. Their analysis, which set strong constraints on neutrino emission from GRBs, was quickly reported to the community, within hours of the GRB being detected by the gamma-ray satellites.

“In the high energies, our upper limits are very constraining—they are below the observations from gamma-ray telescopes,” says Thwaites. “These upper limits, combined with the observations from many electromagnetic telescopes, give us more information about GRBs as potential particle accelerators.”

Because this GRB is so bright, and because it has been so well studied, IceCube is able to place constraining upper limits on neutrino emission models proposed for this specific GRB. These constraints will enable better understanding of how GRBs work.

The collaborators are already developing new methods to improve searches for neutrinos from GRBs and other transient astrophysical sources. In addition, future upgrades and proposed extensions of IceCube, including the IceCube Upgrade project and IceCube-Gen2, could be the key to finding high-energy neutrino emission from GRBs or other transients.

According to Vandenbroucke, “This GRB illustrates the capabilities of IceCube for real-time follow-up of astrophysical transients. IceCube views the entire sky, all the time, over a factor of a billion in energy range. There is likely a burst of neutrinos already flying towards us from some other cosmic source, and we are ready for it.”

+ info “Limits on Neutrino Emission from GRB 221009A from MeV to PeV using the IceCube Neutrino Observatory,” The IceCube Collaboration: R. Abbasi et al. Published in The Astrophysical Journal Letters. arxiv.org/abs/2302.05459

IceCube performs the first search for neutrinos from novae

an oval map of the galaxy with symbols indicating where the novae analyzed are located

White dwarfs are very dense, compact objects that are one of the possibilities for the final evolutionary state of stars. If they happen to be in a binary system with another companion star, the white dwarf may pull material from the companion star onto its surface. In this case, if enough material is accumulated, a nuclear reaction may occur on the surface of the white dwarf, causing a luminous burst of photons called a nova. Historically, astronomers believed they were seeing stars being born, hence the name, although we now know that is not the case. In the past decade, GeV and even TeV gamma rays were discovered from novae, suggesting that neutrinos—neutral, nearly massless cosmic messengers—could originate from novae as well.

In a paper recently submitted to The Astrophysical Journal, the IceCube Collaboration presents its first search for neutrinos from novae using a subarray of the IceCube Neutrino Observatory, a gigaton-scale detector operating at the South Pole. Although significant emission from novae was not found, IceCube set the first observational upper limits on neutrino emission from novae.

According to Justin Vandenbroucke, professor of physics at the University of Wisconsin–Madison and one of the study leads, “Novae, the little cousins of supernovae, are one of the longest known types of astrophysical transient. The discovery that they produce gamma rays was a huge surprise. Our neutrino analyses are starting to add to the modern understanding of these historical phenomena.”

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Justin Marquez and Sam Kramer named L&S Teaching Mentors

Justin Marquez
profile photo of Sam Kramer
Sam Kramer

Congrats to physics PhD students Justin Marquez and Sam Kramer on being named 2023-24 L&S Teaching Mentors!

The L&S TA Training & Support Team is responsible for welcoming and training hundreds of new TAs each year. Teaching Mentors are the heart of this crucial undertaking: they serve as facilitators at the annual L&S Fall TA Training event and provide mentorship throughout the semester. Those selected to be Teaching Mentors have not only a proven track record of excellence as educators, but also a strong desire to share their experience and mentor new TAs navigating their first year.

Soren Ormseth earns campus-wide teaching award

This post is adapted from one originally published by the Graduate School

profile photo of Soren Ormseth
Soren Ormseth

Twenty-one outstanding graduate students — including physics PhD student Soren Ormseth — have been selected as recipients of the 2022-23 UW–Madison Campus-Wide Teaching Assistant Awards, recognizing their excellence in teaching. Ormseth earned a Dorothy Powelson Teaching Assistant Award.

UW–Madison employs over 2,300 teaching assistants (TAs) across a wide range of disciplines. Their contributions to the classroom, lab, and field are essential to the university’s educational mission. To recognize the excellence of TAs across campus, the Graduate School, the College of Letters & Science (L&S), and the Morgridge Center sponsor these annual awards.

Ormseth is a graduate student in the Department of Physics specializing in detector physics. He has taught intermediate physics lab and intermediate electronics lab.

“The best teachers hone their communication skills to make subject material and lessons interesting, relevant, well organized, and right at that difficulty-sweet-spot. At the end of the day though, every student has their own unique way of looking at the world and engaging with a particular topic,” Ormseth said. “When it comes time to deliver a lecture, write a textbook, or create a presentation, a teacher needs to work on those communication skills. But when it comes time to engage with an individual student, the best thing that a teacher can do is be approachable, flexible, and willing to listen with the intent to understand the student’s perspective. Mastering these two teaching modes is a lifelong journey which never stops!”

Celebrating International Day of Women and Girls in Science!

a collage of women, some profile pictures and some with their research equipment

February 11 is the International Day of Women and Girls in Science, and we’re more than happy to showcase some of our women physicists! We collected photos from women in the department, which you can see in a collage above. Some women also chose to share a bit about their research and/or what being a woman in science and woman in physics means to them. Those quotes are below.

Abby Warden, graduate student

My name is Abby Warden, a 5th year graduate student working in experimental high energy physics. My current work includes assembling Gas Electron Multiplier (GEM) chambers for electronics testing. GEMs are the newest muon sub detector that will be installed in the general particle detector, the Compact Muon Solenoid (CMS).

Dr. Camilla Galloni, Post-doc 

Seen some muons? Working on the muon detector of the CMS experiment: GE11 installed in 2020 and successfully operated is the precursor of the GE21 and ME0 detectors now being constructed for the high luminosity LHC upgrade. This big “camera” takes “snapshots” of particles produced in high energy proton collisions and helps understand the fundamental interactions of nature.

Elise Chavez, graduate student

I’ve always been drawn to figuring out how the world works and it led me to my research and passion of learning how the universe works fundamentally at the subatomic level. I work with the Compact Muon Solenoid (CMS) that lies along the Large Hadron Colider (LHC) at CERN in Geneva. Being a woman in physics is a strange duality. There are times when I feel empowered and times I feel very small. It is strength, confidence, and understanding, but it is also alienating, discouraging, and conflicting. It has taught me a lot about people and myself. It gave me a passion to help and support women and minorities in physics because it is for everyone. Diversity is what helps discovery thrive and I hope one day that it can be solely an uplifting experience.

Dr. Charis Koraka, post-doc

Curiosity, along with kindness and compassion are some of the greatest human qualities and those that make societies prosper. The quest of understanding the laws and properties of the universe, has always been a driving source and what made me turn to physics. With perseverance, nothing is impossible!

Wren Vetens, graduate student

My experience as a woman in physics has been marked by perseverance, community, and solidarity. There is still much to be done to achieve equality within the field of physics but we can do our part by standing up for and supporting each other, especially supporting our juniors and those who are disabled, LGBTQ+, and/or POC. I chose physics because I am compelled to always look deeper when I have questions about the nature of life, the universe, and everything. The very same drive that led me to study Physics also led me to coming to terms with my own identity as a queer person, nonbinary person, and transgender woman. Suffice to say, I would not be who I am today without physics or without my gender, and really the two are simply manifestations of that drive. I am currently wrapping up my PhD in experimental particle physics as a part of the CMS collaboration and hoping to graduate this year. My research topic is a search for the unique signature of a long-lived composite particle made of six quarks, which could in principle be produced at the LHC and detected with the CMS detector.

Prof. Tulika Bose

I am an experimental particle physicist working on the CMS experiment at the Large Hadron Collider. I love being part of a large international physics collaboration looking to answer some of the most fundamental questions in physics today – what is responsible for dark matter ? What is the matter-antimatter asymmetry in our universe due to ? Are there new exotic particles out there ? We try to answer these questions using our detector, cutting-edge instrumentation, modern software (incorporating Artificial Intelligence/Machine Learning) and high-performance computing!

(for a video describing Prof. Bose’s work, please see: https://www.youtube.com/watch?v=E7Kzx2xZFdc)

Prof. Ellen Zweibel

I study plasma astrophysics: how electric and magnetic fields interact with charged particles in astrophysical systems. This is an incredibly broad field and I enjoy all of it  –  how sunspots and solar flares work, how a single proton can acquire the energy of a hard hit tennis ball, and what the blotchy rings imaged around supermassive black holes are really telling us, to give just a few examples.

Having the time and capacity to study these things has been an incredible privilege. I’m grateful to my parents, who thought my mind was worth developing, and to  my many wonderful teachers, colleagues, and students – I hope I do as well by them as they did and do by me. I’m grateful to the social infrastructure that gave me food, water, and shelter, cured my illnesses, and allowed me reproductive freedom of choice so I could become a person who lives her dreams.

Haddie McLean, outreach specialist

I love that my job allows me to bring physics to children, our next generation of scientists. I want to show them that physics is fun and it’s for everyone. I hope to inspire them to pursue a career in science.