News Archives

New 3D integrated semiconductor qubit saves space without sacrificing performance

Posted on September 9, 2021

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.

a three-chip sandwich showing the device architecture. — 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 Eriksson

Nathan 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).

Balantekin named co-PI on NSF grant to solve cosmic mystery

Posted on September 8, 2021

This story has been modified from one originally published by New York Institute of Technology.

A team of University of Wisconsin–Madison and New York Institute of Technology physicists has secured a grant from the National Science Foundation (NSF) in an attempt to solve one of science’s greatest mysteries: how the universe formed from stardust.

Many of the universe’s elements, including the calcium found in human bones and iron in skyscrapers, originated from ancient stars. However, scientists have long sought to understand the cosmic processes that formed other elements—those with undetermined origins. Now, UW–Madison professor of physics Baha Balantekin and co-principal investigator Eve Armstrong assistant professor of physics at New York Institute of Technology, will perform the first known research project that uses weather prediction techniques to explain these events. Their revolutionary work will be funded by a two-year $299,998 NSF EAGER grant, an award that supports early-stage exploratory projects on untested but potentially transformative ideas that could be considered “high risk/high payoff.”

While the Big Bang created the first and lightest elements (hydrogen and helium), the next and heavier elements (up to iron on the periodic table) formed later inside ancient, massive stars. When these stars exploded, their matter catapulted into space, seeding that space with elements. Eventually, stardust matter from these supernovae formed the sun and planets, and over billions of years, Earth’s matter coalesced into the first life forms. However, the origins of elements heavier than iron, such as gold and copper, remain unknown. While they may have formed during a supernova explosion, current computational techniques render it difficult to comprehensively study the physics of these events. In addition, supernovae are rare, occurring about once every 50 years, and the only existing data is from the last explosion in 1987.

Large information-rich data sets are obtained from increasingly sophisticated experiments and observations on complicated nonlinear systems. The techniques of Statistical Data Assimilation (SDA) have been developed to handle very nonlinear systems with sparsely sampled data. SDA techniques, akin to the path integral methods commonly used in physics, are used in fields ranging from weather prediction to neurobiology. Armstrong and Balantekin will apply the SDA methods to the vast amount of data accumulated so far in neutrino physics and astrophysics.

With simulated data, in preparation for the next supernova event, the team will use data assimilation to predict whether the supernova environment could have given rise to some heavy elements. If successful, these “forecasts” may allow scientists to determine which elements formed from supernova stardust.

This project will provide an opportunity to the Physics graduate students interested in neutrinos to master an interdisciplinary technique with many other applications.

“Physicists have sought for years to understand how, in seconds, giant stars exploded and created the substances that led to our existence. A technique from another scientific field, meteorology, may help to explain an important piece of this puzzle that traditional tools render difficult to access,” says Armstrong.

The NSF is an independent agency of the U.S. government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health. NSF funding accounts for approximately 27 percent of the total federal budget for basic research conducted at U.S. colleges and universities.

This project is funded by NSF EAGER Award ID No. 2139004.

The content is solely the responsibility of the authors and does not necessarily represent the official views of the NSF.

The Wonders of Physics ready to hit the road

Posted on August 19, 2021

The Wonders of Physics traveling show is back! After a five-year hiatus, the department is pleased to announce that we have hired a full-time outreach specialist and restarted the program.

Haddie McLean

Haddie McLean, a former meteorologist with WISC-TV / Channel 3 in Madison, began her new role as the Wonders of Physics outreach coordinator on August 9. After a 21-year TV career, McLean says she was looking for a new challenge — and any job that didn’t require her to wake up at 2am was just a bonus.

“I love talking to people about science. And I love seeing kids’ faces light up when they understand a topic or when they learn something new,” McLean says. “I was able to do a little bit of that in my job in TV. But in this position, I’ll get to do a ton more, and that’s what drew me to it.”

McLean’s primary role with The Wonders of Physics will be to further develop and perform the traveling show. She will also play a lead role in the development and performance of the annual shows in February, as well as participating in science outreach events and connecting with Wisconsin science teachers.

The Wonders of Physics traveling show started in the late 1980s as an offshoot of The Wonders of Physics annual shows, which first ran in 1984. Two graduate students at the time, David Newman and Christopher Watts, approached Prof. Clint Sprott — the creator of The Wonders of Physics — and suggested that they take the show on the road.

Over the years, The Wonders of Physics traveling show has gone from an all-volunteer, graduate student-led effort to one that has employed part- or full-time outreach specialists. Past funding has been provided by NSF and DOE. Now, thanks to the support of generous donors — including a successful Day of the Badger fundraising campaign — the department expects that the traveling show will be run by a full-time staff member for years to come.

“I’d like to make this show accessible to all ages, all walks of life,” McLean says. “I’d like to hit all areas of the state, if possible, and bring the university to the kids that wouldn’t otherwise get a chance to experience all that our campus has to offer.”

McLean is already busy prepping the show and hopes to be in schools by late fall 2021 (if university and school district COVID-19 policies permit it). She expects to have a general show available that is as hands-on and interactive as possible. She also plans to make the show customizable as needed, where she can work with teachers to focus the performance on the specific areas of physics that they are teaching at the time.

“My hope for the traveling show is that it’s fun and engaging, gets kids excited, and helps spark an interest in the next generation of scientists,” McLean says.

Sprott, now an emeritus professor with the department who still stars in the annual shows each February, is enthusiastic that McLean will be involved in those shows and that she is reviving the traveling show.

“After several difficult years, I’m delighted that Haddie McLean has joined us to head the Department’s nearly 40-year tradition of physics outreach and public education for people of all ages throughout Wisconsin and beyond,” Sprott says.

Anyone interested in scheduling The Wonders of Physics traveling show can email wonders@physics.wisc.edu or visit wonders.physics.wisc.edu for more information.

The shows are free of charge, but donations are encouraged.

Related: See the winning entries from The Wonders of Physics 2021 video contest!

Magnetic fields implicated in the mysterious midlife crisis of stars

Posted on July 28, 2021

a brightly colored sun with a cutout showing into the core, with lines suggesting the spinning motion — Artist’s impression of the spinning interior of a star, generating the stellar magnetic field. This image combines a dynamo simulation of the Sun’s interior with observations of the Sun’s outer atmosphere, where storms and plasma winds are generated. | Credit:
CESSI / IISER Kolkata / NASA-SVS / ESA / SOHO-LASCO

This post was originally published by the Royal Astronomical Society. UW–Madison physics graduate student Bindesh Tripathi is the lead author of the scientific publication.

Middle-aged stars can experience their own kind of midlife crisis, experiencing dramatic breaks in their activity and rotation rates at about the same age as our Sun, according to new research published today in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.

Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like our Sun to gradually slow down their rotation over billions of years.

In turn, the slower rotation leads to altered magnetic fields and less stellar activity – the numbers of sunspots, flares, outbursts, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields.

Bindesh Tripathi

This decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.

However recent observations indicate that this intimate relationship breaks down around middle age. The new work, carried out by Bindesh Tripathi at UW–Madison and the Indian Institute of Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment. Prof. Dibyendu Nandy, and Prof. Soumitro Banerjee of IISER are co-authors.

Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. A middle aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.

Prof. Nandy comments: “This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”

The new work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen. The best known of these is perhaps the Maunder Minimum around 1645 to 1715, when very few sunspots were observed.

The team hope that it will also shed light on recent observations indicating that the Sun is comparatively inactive, with crucial implications for the potential long-term future of our own stellar neighbor.

2021 Homi Bhabha Award given to Francis Halzen

Posted on July 13, 2021

This story was originally published by the IceCube collaboration.

Francis Halzen | Image: Zig Hampel-Arias, WIPAC.

The International Union of Pure and Applied Physics (IUPAP) and the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, have awarded the 2021 Homi Bhabha Medal and Prize to Francis Halzen, the Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin–Madison and principal investigator of IceCube, for his “distinguished contributions in the field of high-energy cosmic-ray physics and astroparticle physics over an extended academic career.” Halzen accepted the award at the opening session of the virtual 37^th International Cosmic Ray Conference, on July 12, 2021.

The Bhabha Award was established by IUPAP and TIFR in 2010 to honor Dr. Homi Jehangir Bhabha, a cosmic ray physicist well known for the Bhabha-Heitler cascade theory and relativistic positron-electron scattering, also known as Bhabha scattering. Bhabha founded TIFR in 1945 and initiated the nuclear energy program in India in 1951. He initiated experimental programs for the study of cosmic ray particles and their interactions with instruments either carried aloft to the top of the atmosphere with balloons or placed in laboratories at high altitude or deep underground. The Homi Bhabha Medal and Prize consists of a certificate, a medal, a monetary award, and an invitation to visit the TIFR, Mumbai, and the Cosmic Ray Laboratory, Ooty to give public lectures. It is awarded biennially at the International Cosmic Ray Conference.

Born in Belgium, Halzen received his Master’s and PhD degrees from the University of Louvain, Belgium, and has been on the physics faculty at UW–Madison since 1972. The Bhabha Award is just the latest in Halzen’s long and storied career; previous accolades include a 2014 American Ingenuity Award, the 2015 Balzan Prize, a 2018 Bruno Pontecorvo Prize, the 2019 IUPAP Yodh Prize, and the 2021 Bruno Rossi Prize. Halzen is the third IceCube collaborator to win a Bhabha Award after Tom Gaisser in 2015 and Subir Sarkar in 2017.

During his virtual acceptance remarks, Halzen credited his collaborators, saying, “If I made contributions, it is because I ran into incredible collaborators who were leaders in the field, and still are. My ultimate collaborators, of course, I found within the AMANDA collaboration—and now IceCube—who made high-energy neutrinos part of the high-energy cosmic ray spectrum…

“Thanks to everybody, and thanks to IceCube; this prize is shared with all of you.”

Francis Halzen named Vilas Research Professor

Posted on July 13, 2021

UW–Madison physics professor Francis Halzen has been named a Vilas Research Professor. Created “for the advancement of learning,” Vilas Research Professorships are granted to faculty with proven research ability and unusual qualifications and promise. The recipients of the award have contributed significantly to the research mission of the university and are recognized both nationally and internationally.

Halzen, the Gregory Breit and Hilldale Professor of Physics, joined the UW–Madison faculty in 1972. He has made pioneering contributions to particle physics and neutrino astrophysics, and he continues to be the driving force of the international IceCube Collaboration.

Early in his career, Halzen cofounded the internationally recognized phenomenology research institute in the UW–Madison Department of Physics to promote research at the interface of theory and experiment in particle physics. This institute is recognized for this research and for its leadership in the training of postdocs and graduate students in particle physics phenomenology.

The IceCube Neutrino Observatory is the culmination of an idea first conceived in the 1960s, and one in which Halzen has played an integral role in its design, implementation, and data acquisition and analysis for the past three decades. After initial experiments confirmed that the Antarctic ice was ultratransparent and established the observation of atmospheric neutrinos, IceCube was ready to become a reality. From 2004 to 2011, the South Pole observatory was constructed — the largest project ever assigned to a university and one led by Halzen.

After two years of taking data with the full detector, the IceCube Neutrino Observatory opened a new window onto the universe with its discovery of highly energetic neutrinos of extragalactic origin. This discovery heralded the beginning of the exploration of the universe with neutrino telescopes. The IceCube observation of cosmic neutrinos was named the 2013 Physics World Breakthrough of the Year.

Nationally and internationally renowned for this work, Halzen was awarded a 2014 American Ingenuity Award, a 2015 Balzan Prize, a 2018 Bruno Pontecorvo Prize, a 2019 Yodh Prize, and a 2021 Bruno Rossi Prize.

With the Vilas Research Professorship, Halzen is also recognized for his commitment to education and service in the department, university, and international science communities. He has taught everything from physics for nonscience majors to advanced particle physics and special topics courses at UW–Madison. He has actively participated on several departmental and university committees as well as advisory, review, and funding panels. His input is highly sought by committees and agencies that assess future priorities of particle and astroparticle physics research.

“Francis Halzen has had a prolific, internationally recognized research career, has shown excellence as an educator who is able to effectively communicate cutting-edge science on all levels, and has made tireless and valued contributions in service of the department,” says Sridhara Dasu, Physics Department chair. “He is one of the most creative and influential physicists of the last half century and worthy of the prestigious Vilas Research Professorship.”

Vilas awards are supported by the estate of professor, U.S. senator and UW Regent William F. Vilas (1840-1908). The Vilas Research Professorship provides five years of flexible funding — two-thirds of which is provided by the Office of the Provost through the generosity of the Vilas trustees and one-third provided by the school or college whose dean nominated the winner.

Halzen joins department colleagues Profs. Vernon Barger and Sau Lan Wu as recipients of this prestigious UW–Madison professorship.

Ke Fang receives prestigious Shakti Duggal Award

Posted on July 12, 2021

This article was originally published by WIPAC

Ke Fang, professor at the University of Wisconsin–Madison, has been selected as the recipient of the 2021 Shakti P. Duggal Award presented by the International Union of Pure and Applied Physics (IUPAP).

Ke Fang

The Duggal Award was established after cosmic-ray physicist Shakti Duggal’s untimely death in 1982. In honor of Shakti’s long association with cosmic ray physics and his many contributions to the field during his career, his namesake award is given biennially “to recognize an outstanding young scientist for contributions in any branch of cosmic ray physics.” The first Shakti P. Duggal Award was presented at the 19th International Cosmic Ray Conference at La Jolla in 1985. Previous Duggal Award winners have all achieved recognition and prominence in their careers.

Award winners receive a monetary award and, since 1991, an invitation to visit the Bartol Research Institute of the University of Delaware, where Shakti Duggal worked, to present a colloquium and discuss their work.

Fang’s research focuses on understanding the universe through its energetic messengers, including ultra-high-energy cosmic rays, gamma rays, and high-energy neutrinos. She runs numerical simulations to study theories of astroparticle sources and analyzes data from HAWC, Fermi-LAT, and IceCube. She joined WIPAC and the UW–Madison Physics Department as an assistant professor on January 1, 2021. You can learn more about Fang and her research in this Q&A.

“I am very grateful for this special honor,” said Fang. “As a young researcher, I have received enormous support from my mentors and collaborators, to whom the award truly belongs. I look forward to continuing working on and contributing to cosmic ray physics as a member of the Duggal family.”

Sau Lan Wu featured in AIP History newsletter

Posted on June 23, 2021

A biography of Professor Sau Lan Wu was included as the cover story in a recent issue of the AIP History Newsletter (see pages 14-15). The story covers her early life in Hong Kong, her arrival in the U.S. to attend college at Vassar which she attended on a full scholarship, and into her graduate studies and subsequent research programs at MIT, UW–Madison and CERN. There is also a list of references / further readings. Check out the article to learn more about Prof. Wu’s illustrious career so far!

Physics projects funded in first round of UW’s Research Forward initiative

Posted on June 22, 2021

In its inaugural round of funding, the Office of the Vice Chancellor for Research and Graduate Education’s (OVCRGE) Research Forward initiative selected 11 projects, including two with physics department faculty involvement.

OVCRGE hosts Research Forward to stimulate and support highly innovative and groundbreaking research at the University of Wisconsin–Madison. The initiative is supported by the Wisconsin Alumni Research Foundation (WARF) and will provide funding for 1–2 years, depending on the needs and scope of the project.

The two projects from the department are:

Compact and efficient terahertz optical modulators, Deniz Yavuz (PI)
Integrated stellarator concept study for a new U.S. stellarator experiment at UW– Madison, John Sarff and Paul Terry (co-investigators)

Research Forward seeks to support collaborative, multidisciplinary, multi-investigator research projects that are high-risk, high-impact, and transformative. It seeks to fund research projects that have the potential to fundamentally transform a field of study as well as projects that require significant development prior to the submission of applications for external funding. Collaborative research proposals are welcome from within any of the four divisions (Arts & Humanities, Biological Sciences, Physical Sciences, Social Sciences), as are cross-divisional collaborations.

Correlated errors in quantum computers emphasize need for design changes

Posted on June 16, 2021

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.”

Read the full story at https://news.wisc.edu/correlated-errors-in-quantum-computers-emphasize-need-for-design-changes/