New 3D integrated semiconductor qubit saves space without sacrificing performance

a three-chip sandwich showing the device architecture.

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.”

profile photo of Mark Eriksson
Mark Eriksson
profile photo of Nathan Holman
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).

 

Correlated errors in quantum computers emphasize need for design changes

artist rendition of a 4-qubit chip with a dotted-line-like cosmic ray hitting it from out of the image frame, lighting up two neighboring qubits "red" to indicate they are affected by the cosmic ray's energy

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/

artist rendition of a 4-qubit chip with a dotted-line-like cosmic ray hitting it from out of the image frame, lighting up two neighboring qubits "red" to indicate they are affected by the cosmic ray's energy
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. 

Victor Brar awarded prestigious Sloan Fellowship

University of Wisconsin–Madison physics professor Victor Brar has been named a 2021 Sloan Research Fellow, a competitive award given to researchers in the early stages of their careers.

Victor Brar

“A Sloan Research Fellow is a rising star, plain and simple,” says Adam F. Falk, president of the Alfred P. Sloan Foundation. “To receive a Fellowship is to be told by the scientific community that your achievements as a young scholar are already driving the research frontier.”

Brar’s research focuses on developing new microscopy techniques to look at quantum systems in ways that current microscopes cannot. Applying these techniques to study defects in materials — where a perfect crystal lattice is disrupted by one or more anomalous atoms — could lead to improvements in quantum computer performance or the discovery of new Physics.

“Everyone in the world is trying to make a quantum computer, but we don’t really have good diagnostics for what all the quantum systems are inside of a material,” Brar says. “One goal with this microscope is to figure out what’s in a material that could interfere with a quantum computer.”

Additionally, Brar hopes that by applying this technique to complex materials, new particles may be identified and studied. For example, many particle physics discoveries, such as the Higgs boson and the positron, have been first theorized based on materials science research and repurposed into high energy physics experiments.

“At CERN, for example, they try to get to higher and higher energies to see particles, and at some point CERN just can’t get high enough,” Brar explains. “But in a material, you can get analogous particles for what CERN scientists are looking for but at much lower energies. There are particles that we’ve never seen outside of a material, but we can see them in a material, and those are the kinds of things that we’d ideally like to study.”

Images of quantum defects embedded in the atomic lattice of tungsten diselenide (credit: Victor Brar)

The technique that Brar is developing combines optical and electron microscopy, two methods he worked on as a graduate student and post-doc. By bringing them together now, he hopes that his unique method will bring significant advances to his field — and that the Sloan Fellowship indicates that other scientists agree.

“The Sloan award has a history behind it, and they have a track record of funding good science,” Brar says. “So, it means a lot to be recognized by Sloan and I hope it will help when we start to try to make our case for why this method is important.”

The Sloan Research Fellowship is open to early-career scientists in one of eight fields, including physics. More than 1000 researchers are nominated each year for 128 fellowship slots. Winners receive a two-year, $75,000 fellowship which can be spent to advance the fellow’s research.

“Prof. Victor Brar winning the Sloan Foundation Fellowship is a very welcome recognition,” says Sridhara Dasu, chair of the UW–Madison physics department. “For decades now, the Sloan Fellowship is a highly sought-after honor amongst young scientists, and it is wonderful to note that our enthusiasm and confidence in Prof. Brar’s research prowess is recognized by an international panel selecting the Sloan Fellows.”

Shimon Kolkowitz awarded two grants to push optical atomic clocks past the standard quantum limit

a metalilc chamber with a blue glowing orb of illuminated atoms in the center

Optical atomic clocks are already the gold standard for precision timekeeping, keeping time so accurately that they would only lose one second every 14 billion years. Still, they could be made to be even more precise if they could be pushed past the current limits imposed on them by quantum mechanics.

With two new grants from the U.S. Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, UW–Madison physics professor Shimon Kolkowitz proposes to introduce quantum entanglement — where atoms interact with each other even when physically distant — to optical atomic clocks. The improved clocks would allow researchers to ask questions about fundamental physics, and they have applications in improving quantum computing and GPS.

Atomic clocks are so precise because they take advantage of the natural vibration frequencies of atoms, which are identical for all atoms of a particular element. These clocks operate at or near the standard quantum limit, a fundamental limit on performance imposed on clocks where the atoms are all independent of each other. The only way to push the clocks past that limit is to achieve entangled states, strange quantum states where the atoms are no longer independent and they become intertwined.

a cartoon showing the atoms in their pancakes as described in the text“That turns out to be hard for a number of reasons. Entanglement requires these atoms to interact with each other, but a good clock requires them not to interact with each other or anything else,” Kolkowitz says. “So, you need to engineer a situation where you can make the atoms interact strongly, but you can also switch those interactions off. And those are some of the same requirements that are necessary for quantum computing.”

Kolkowitz is already building an optical atomic clock in his lab, albeit one that is not yet using entangled states. To make the clock, they first laser-cool strontium atoms to one millionth of one degree Celsius above absolute zero, then load the atoms into an optical lattice. In the lattice, the atoms are separated into what is effectively a tiny stack of pancakes — each atom can move around within their own flat disk, but they cannot jump into another pancake.

Though the atoms’ are stuck in their own pancake, they can interact with each other if their electrons are highly excited. This type of atom, known as Rydberg atoms, becomes close to one million times larger than an unexcited counterpart because the excited electron can be microns away from the nucleus.

“It’s kind of crazy that a single atom can be that big, and when you make them that much bigger, they interact much more strongly with each other than they do in their ground states,” Kolkowitz says. “Basically it means you can go from the atoms not interacting at all to interacting very strongly. That’s exactly what you want for quantum computing, and it’s what you want for this atomic clock.”

With the two ARO grants, Kolkowitz expects to generate Rydberg atoms in his lab’s atomic clock. One of the grants, a Defense University Research Instrumentation Program (DURIP), will fund the specialized UV laser that generates the high energy photons needed to excite the atoms into the highly excited Rydberg states. The second grant will fund personnel and other supplies. Kolkowitz will collaborate with UW–Madison physics professor Mark Saffman, who, along with physics professor Thad Walker, pioneered the use of Rydberg atoms for quantum computing.

In addition to being useful for developing new approaches to ask questions about fundamental physics in his research lab, these ultraprecise atomic clocks are of interest to the Department of Defense for atomic clock-based technologies such as GPS, and because they can be used to precisely map Earth’s gravity.

Researchers awarded Department of Energy Quantum Information Science Grant

Three UW–Madison physics professors and their colleagues have been awarded a U.S. Department of Energy (DOE) High Energy Physics Quantum Information Science award for an interdisciplinary collaboration between theoretical and experimental physicists and experts on quantum algorithms.

The grant, entitled “Detection of dark matter and neutrinos enhanced through quantum information,” will bring a total of $2.3 million directly to UW-Madison. Physics faculty include principal investigator Baha Balantekin as well as Mark Saffman, and Sue Coppersmith. Collaborators on the grant include Kim Palladino at the University of Oxford, Peter Love at Tufts University, and Calvin Johnson at San Diego State University.

With the funding, the researchers plan to use a quantum simulator to calculate the detector response to dark matter particles and neutrinos. The simulator to be used is an array of 121 neutral atom qubits currently being developed by Saffman’s group. Much of the research plan is to understand and mitigate the behavior of the neutral atom array so that high accuracy and precision calculations can be performed. The primary goal of this project is to apply lessons from the quantum information theory in high energy physics, while a secondary goal is to contribute to the development of quantum information theory itself.

Mark Friesen promoted to Distinguished Scientist

profile photo of Mark Friesen
profile photo of Mark Friesen
Mark Friesen

Congratulations to Mark Friesen on his promotion to Distinguished Scientist! The distinguished title is the highest title available to an academic staff member at UW–Madison.

Friesen joined the physics department in 2004 as an associate scientist, and has been with UW–Madison since 1998, when he began a postdoc in the Materials Sciences and Engineering department. His main research effort at UW–Madison has been related to silicon quantum dot quantum computing, in collaboration with physics professors Mark Eriksson, Sue Coppersmith, Bob Joynt, Maxim Vavilov, and others.

Friesen says his most important achievement in the department is serving as a research advisor: In 16 years with UW–Madison physics, he has advised or co-advised six postdocs, 11 Ph.D. theses, four current Ph.D. students, two M.S. theses, and several undergraduate research projects. He also has 123 peer-reviewed publications and five U.S. patents, and serves as a consultant for ColdQuanta, a quantum computing company.

“Mark is known around the world for his expertise in semiconductor-based quantum computing,” Mark Eriksson says. “He is especially well known for his calculations on how the band structure in silicon interacts with interfaces to determine the quantum states for electrons in silicon-based quantum devices.”

Congrats, Mark Friesen, on this well-deserved honor!

Q-NEXT collaboration awarded National Quantum Initiative funding

the tip of a scanning electron microscope is poised over a setup with metal clips pointing out

The University of Wisconsin–Madison solidified its standing as a leader in the field of quantum information science when the U.S. Department of Energy (DOE) and the White House announced the Q-NEXT collaboration as a funded Quantum Information Science Research Center through the National Quantum Initiative Act. The five-year, $115 million collaboration was one of five Centers announced today.

Q-NEXT, a next-generation quantum science and engineering collaboration led by the DOE’s Argonne National Laboratory, brings together nearly 100 world-class researchers from three national laboratories, 10 universities including UW–Madison, and 10 leading U.S. technology companies to develop the science and technology to control and distribute quantum information.

“The main goals for Q-NEXT are first to deliver quantum interconnects — to find ways to quantum mechanically connect distant objects,” says Mark Eriksson, the John Bardeen Professor of Physics at UW–Madison and a Q-NEXT thrust lead. “And next, to establish a national resource to both develop and provide pristine materials for quantum science and technology.”

profile photo of Mark Eriksson
Mark Eriksson

Q-NEXT will focus on three core quantum technologies:

  • Communication for the transmission of quantum information across long distances using quantum repeaters, enabling the establishment of “unhackable” networks for information transfer
  • Sensors that achieve unprecedented sensitivities with transformational applications in physics, materials, and life sciences
  • Processing and utilizing “test beds” both for quantum simulators and future full-stack universal quantum computers with applications in quantum simulations, cryptanalysis, and logistics optimization.

Eriksson is leading the Materials and Integration thrust, one of six Q-NEXT focus areas that features researchers from across the collaboration. This thrust aims to: develop high-coherence materials, including for silicon and superconducting qubits, which is an essential component of preserving entanglement; develop a silicon-based optical quantum memory, which is important in developing a quantum repeater; and improve color-center quantum bits, which are used in both communication and sensing.

“One of the key goals in Materials and Integration is to not just improve the materials but also to improve how you integrate those materials together so that in the end, quantum devices maintain coherence and preserve entanglement,” Eriksson says. “The integration part of the name is really important. You may have a material that on its own is really good at preserving coherence, yet you only make something useful when you integrate materials together.”

Six other UW­–Madison and Wisconsin Quantum Institute faculty members are Q-NEXT investigators: physics professors Victor Brar, Shimon Kolkowitz, Robert McDermott, and Mark Saffman, electrical and computer engineering professor Mikhail Kats, and chemistry professor Randall Goldsmith. UW–Madison researchers are involved in five of the six research thrusts.

“I’m excited about Q-NEXT because of the connections and collaborations it provides to national labs, other universities, and industry partners,” Eriksson says. “When you’re talking about research, it’s those connections that often lead to the breakthroughs.

The potential impacts of Q-NEXT research include the creation of a first-ever National Quantum Devices Database that will promote the development and fabrication of next generation quantum devices as well as the development of the components and systems that enable quantum communications across distances ranging from microns to kilometers.

“This funding helps ensure that the Q-NEXT collaboration will lead the way in future developments in quantum science and engineering,” says Steve Ackerman, UW–Madison vice chancellor for research and graduate education. “Q-NEXT is the epitome of the Wisconsin Idea as we work together to transfer new quantum technologies to the marketplace and support U.S. economic competitiveness in this growing field.”

infographic of all q-next partner national labs, universities, and industry
The Q-NEXT partners

New study expands types of physics, engineering problems that can be solved by quantum computers

A well-known quantum algorithm that is useful in studying and solving problems in quantum physics can be applied to problems in classical physics, according to a new study in the journal Physical Review A from University of Wisconsin–Madison assistant professor of physics Jeff Parker.

Quantum algorithms – a set of calculations that are run on a quantum computer as opposed to a classical computer – used for solving problems in physics have mainly focused on questions in quantum physics. The new applications include a range of problems common to physics and engineering, and expands on the types of questions that can be asked in those fields.

profile photo of Jeff Parker
Jeff Parker

“The reason we like quantum computers is that we think there are quantum algorithms that can solve certain kinds of problems very efficiently in ways that classical computers cannot,” Parker says. “This paper presents a new idea for a type of problem that has not been addressed directly in the literature before, but it can be solved efficiently using these same quantum computer types of algorithms.”

The type of problem Parker was investigating is known as generalized eigenvalue problems, which broadly describe trying to find the fundamental frequencies or modes of a system. Solving them is crucial to understanding common physics and engineering questions, such as the stability of a bridge’s design or, more in line with Parker’s research interests, the stability and efficiency of nuclear fusion reactors.

As the system being studied becomes more and more complex — more components moving throughout three-dimensional space — so does the numerical matrix that describes the problem. A simple eigenvalue problem can be solved with a pencil and paper, but researchers have developed computer algorithms to tackle increasingly complex ones. With the supercomputers available today, more and more difficult physics problems are finding solutions.

“If you want to solve a three-dimensional problem, it can be very complex, with a very complicated geometry,” Parker says. “You can do a lot on today’s supercomputers, but there tends to be a limit. Quantum algorithms may be able to break that limit.”

The specific quantum algorithm that Parker studied in this paper, known as quantum phase estimation, had been previously applied to so-called standard eigenvalue problems. However, no one had shown that they could be applied to the generalized eigenvalue problems that are also common in physics. Generalized eigenvalue problems introduce a second matrix that ups the mathematical complexity.

Parker took the quantum algorithm and extended it to generalized eigenvalue problems. He then looked to see what types of matrices could be used in this problem. If the matrix is sparse ­— meaning, if most of the numerical components that make it up are zero — it means this problem could be solved efficiently on a quantum computer.

The study shows that quantum algorithms could be applied to classical physics problems, such as nuclear fusion mirror machines. | Credit: Cary Forest

“What I showed is that there are certain types of generalized eigenvalue problems that do lead to a sparse matrix and therefore could be efficiently solved on a quantum computer,” Parker says. “This type includes the very natural problems that often occur in physics and engineering, so this study provides motivation for applying these quantum algorithms more to generalized eigenvalue problems, because it hasn’t been a big focus so far.”

Parker emphasizes that quantum computers are in their infancy, and these classical physics problems are still best approached through classical computer algorithms.

“This study provides a step in showing that the application of a quantum algorithm to classical physics problems can be useful in the future, and the main advance here is it shows very clearly another type of problem to which quantum algorithms can be applied,” Parker says.

The study was completed in collaboration with Ilon Joseph at Lawrence Livermore National Laboratory. Funding support was provided by the U.S. Department of Energy to Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 and U.S. DOE Office of Fusion Energy Sciences “Quantum Leap for Fusion Energy Sciences” (FWP SCW1680).

UW–Madison named member of new $25 million Midwest quantum science institute

cartoon showing a quantum hardware network

As joint members of a Midwest quantum science collaboration, the University of Wisconsin–Madison, the University of Illinois at Urbana–Champaign and the University of Chicago have been named partners in a National Science Foundation Quantum Leap Challenge Institute, NSF announced Tuesday.

The five-year, $25 million NSF Quantum Leap Challenge Institute for Hybrid Quantum Architectures and Networks (HQAN) was one of three in this first round of NSF Quantum Leap funding and helps establish the region as a major hub of quantum science. HQAN’s principal investigator, Brian DeMarco, is a professor of physics at UIUC. UW–Madison professor of physics Mark Saffman and University of Chicago engineering professor Hannes Bernien are co-principal investigators.

“HQAN is very much a regional institute that will allow us to accelerate in directions in which we’ve already been headed and to start new collaborative projects between departments at UW–Madison as well as between us, the University of Illinois, and the University of Chicago.” says Saffman, who is also director of the Wisconsin Quantum Institute. “These flagship institutes are being established as part of the National Quantum Initiative Act that was funded by Congress, and it is a recognition of the strength of quantum information research at UW–Madison that we are among the first.”

Read the full story at https://news.wisc.edu/uw-madison-named-member-of-new-25-million-midwest-quantum-science-institute/

cartoon showing a quantum hardware network
In a hybrid quantum network, hardware for storing and processing quantum information is linked together. This design could be beneficial for applications that rely on distributed quantum computing resources. | Credit: E. Edwards, IQUIST