Cross-institutional collaboration leads to new control over quantum dot qubits

a greyscale image makes up the border of this square image, with a full-color square in the exact center. the image shows tiny tunnel-like features, all congregating in the middle

This story was originally published by the Chicago Quantum Exchange

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

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.

11 students pose on a rock wall, all students are wearing the same Chicago Quantum Exchange hooded sweatshirt
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.”)

Four people stand in a lab in front of electronics equipment
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.”

Search for neutrino emission associated with LIGO/Virgo gravitational waves

Gravitational waves (GWs) are a signature for some of the most energetic phenomena in the universe, which cause ripples in space-time that travel at the speed of light. These events, spurred by massive accelerating objects, act as cosmic messengers that carry with them clues to their origins. They are also probable sources for highly energetic neutrinos, nearly massless cosmic messengers hurtling through space unimpeded. Because neutrinos rarely interact with surrounding matter, they can reveal phenomena that are otherwise unobserved with electromagnetic waves. These high-energy neutrinos are detected by the IceCube Neutrino Observatory, a cubic-kilometer detector enveloped in Antarctic ice at the South Pole.

Both GWs and neutrinos are recently introduced messengers in astronomy and have yet to be detected by the same source. Such a major discovery would not only shed light on the sources of cosmic rays but would also help in understanding the most energetic processes in the universe. By coordinating traditional observations (from radio to gamma rays) with these new messengers, researchers can gain deeper insights into astrophysical sources that were unobtainable before.

Previously, the IceCube Collaboration looked for joint emission of GWs and high-energy neutrinos with data collected by IceCube, the Laser Interferometer Gravitational-Wave Observatory (LIGO), and the Virgo gravitational wave detector. These results were from GWs observed during the first two observing runs (O1 and O2) of LIGO and Virgo. IceCube researchers from the University of Wisconsin–Madison and Columbia University conducted an updated analysis of GWs from the third observing run (O3) of the LIGO/Virgo detectors. The increased number of GWs improved the researchers’ overall analysis. Their findings were recently submitted to The Astrophysical Journal.

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X(ray) marks the spot in elemental analysis of 15th century printing press methods

a woman (left, bending down) and a man (right, crouched) position an ancient manuscript into a machine

This story was originally published by University Communications

In 15th century Germany, Johannes Gutenberg developed a printing press, a machine that allowed for mass production of texts. It is considered by many to be one of the most significant technological advancements of the last millennium.

Though Gutenberg often receives credit as the inventor of the printing press, sometime earlier, roughly 5,000 miles away, Koreans had already developed a movable-type printing press.

There is no question that East Asians were first. There is also no question that Gutenberg’s invention in Europe had a far greater impact.

“What is not known is whether Gutenberg knew about the Korean printing or not. And if we could shed light on that question, that would be earth shattering,” says Uwe Bergmann, a professor of physics at the University of Wisconsin–Madison who, with UW–Madison physics graduate student Minhal Gardezi, is part of a large, interdisciplinary team that is analyzing historical texts.

He adds: “But even if we don’t, we can learn a lot about early printing methods, and that will already be a big insight.”

These texts include pages from a Gutenberg bible and Confucian texts, and they’re helping investigate these questions. The team includes 15th century Korean texts experts, Gutenberg experts, paper experts, ink experts and many more.

a person, with essentialy just their hands visible, holds a wooden box that is wrapped with a leather tie and has Korean text on the side
One of the leaves scanned was printed by a Korean movable type printing press in 1442. One of the team members from UNESCO, Angelica Noh, traveled with the preserved documents from Korea to SLAC. IMAGE PROVIDED BY MINHAL GARDEZI

How did two physicists end up participating in a seemingly very non-physics cultural heritage project? Bergmann had previously worked on other historical text analyses, where he pioneered the application of a technique known as X-ray fluorescence (XRF) imaging.

In XRF imaging, a powerful machine called a synchrotron sends an intense and very small X-ray beam — about the diameter of a human hair — at a page of text at a 45-degree angle. The beam excites electrons in the atoms that make up the text, requiring another electron to fill in the space left by the first (all matter is made up of atoms, which contain even smaller components called electrons).

The second electron loses energy in the process, and that energy is released as a small flash of light. A detector placed strategically nearby picks up that light, or its X-ray fluorescence, and measures both its intensity and the part of the light spectrum to which it belongs.

“Every single element on the periodic table emits an X-ray fluorescence spectrum that is unique to that atom when hit with a high-energy X-ray. Based on its ‘color,’ we know exactly which element is present,” says Gardezi. “It’s a very high-precision instrument that tells you all the elements that are at every location in a sample.”

With this information, researchers can effectively create an elemental map of the document. By rapidly scanning a page across the X-ray beam, they can create a record of the XRF spectrum at each pixel. One page can produce several million XRF spectra.

This summer, Bergmann and Gardezi were part of a team that used XRF scanning at the SLAC National Accelerator Laboratory in California to produce elemental maps of several large areas from original pages of a first-edition, 42-line Gutenberg Bible (dating back to 1450 to 1455 A.D.) and from Korean texts dating back to the early part of that. century.

They scanned the texts at a rate of around one pixel every 10 milliseconds, then filtered the data by elemental signature, providing high-resolution maps of which elements are present and in what relative quantities.

a three panel image. The top shows a regular photograph of the Korean text, with a dotted white line around two lines of 6 characters. The second panel shows the XRF of those characters, with a blue backround, a yellow-green hue aorund the characters, and red in the characters themmselves. The bottom panel shows a second XRF scan, but here almost the entire panel is blue, except for the circles of the characters, which are red, indicating the element being filtered was only present in the circles of the text.
A photograph of a scanned Korean text. The white dotted box indicates the areas shown in the middle and bottom panels. Each element produces a unique X-ray fluorescence. After scanning the text, the researchers applied filters for the known XRF patterns of different elements and created a color-coded heat-map of their abundance, from lowest (blue) to highest (red). An element found in only small quantities is in the red circles in the bottom part of the image. IMAGE PROVIDED BY MINHAL GARDEZI

In a way, the work is like digging for treasure from an old map — Gardezi says the researchers do not know exactly what they are looking for, but they are most interested in the unexpected.

For example, she recently presented early results of scans to the team, to demonstrate the approach had worked and that the researchers could separate out different elements. It turns out this isn’t what the team found most interesting.

“Instead, these scholars spent 15-to-20 minutes talking about, ‘Why is (this element) present?’ and coming up with hypotheses,” Gardezi says. “As physicists, we wouldn’t even recognize if something is surprising or not. It’s really this interdisciplinary aspect that tells us what to look for, what the smoking gun is.”

As more questions arise based on the elemental analyses, Bergmann and Gardezi will help guide the team to address those questions quantitatively. They are already planning to recreate some early printings in the lab — with known types, papers and inks — then compare these XRF scans with the originals.

The research may never definitively determine if Gutenberg knew about the Korean presses or if he developed his press independently. But without access to the original presses themselves, these texts hold the only clues to understanding the nature of these transformative machines.

“The more you read about it, the more you learn that there is less certainty about several things related to early printing presses,” Bergmann says. “Maybe this technique will allow us to view these prints as a time capsule and gain invaluable insight into this watershed moment in human history.”

Watch Minhal Gardezi show off XRF at SLAC National Accelerator Laboratory.

The UW–Madison efforts in the project are supported by the Overseas Korean Cultural Heritage Foundation.

Study led by UW–Madison researcher confirms star wreck as source of extreme cosmic particles

Astronomers have long sought the launch sites for some of the highest energy protons in our galaxy. Now, a study using 12 years of data from NASA’s Fermi Gamma-ray Space Telescope (Fermi) confirms that a remnant of a supernova, or star explosion, is just such a place, solving a decade-long cosmic mystery.

a mostly black image of space, with some small white-ish out-of-focus stars, and a large fuzzy pink blob partially overlapping a green-hued amorphous apparition
The newly discovered PeVatron (in pink) is hosted by a supernova remnant (in green) called G106.3+2.7. The supernova remnant is believed to have formed together with the pulsar (in magenta) about 10,000 years ago. Particles accelerated by the shock waves of the supernova remnant interact with the gas in the interstellar medium, producing high-energy gamma-ray emission. Credit: Jayanne English, University of Manitoba, NASA/Fermi/Fang et al. 2022, and Canadian Galactic Plane Survey/DRAO.

Previously, Fermi has shown that the shock waves of exploded stars boost particles to speeds comparable to that of light. Called cosmic rays, these particles mostly take the form of protons, but can include atomic nuclei and electrons. Because they all carry an electric charge, their paths become scrambled as they whisk through our galaxy’s magnetic field, which masks their origins. But when these particles collide with interstellar gas near the supernova remnant (SNR), they produce a telltale glow in gamma rays—the highest-energy light there is.

“Theorists think the highest energy cosmic ray protons in the Milky Way reach a million billion electron volts, or PeV energies,” said Ke Fang, an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC), a research center at the University of Wisconsin–Madison. “The precise nature of their sources, which we call PeVatrons, has been difficult to pin down.”

Fang, who led the study, performed the data analysis and developed the theory models. The research team identified a few suspected PeVatrons, including one at the center of our galaxy. Naturally, SNR top the list of candidates. Yet out of about 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.

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