Repurposing Qubit Tech to Explore Exotic Superconductivity

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Category: Research News

Published: Wednesday, November 20 2024 01:00

Decades of quantum research are now being transformed into practical technologies, including the superconducting circuits that are being used in physics research and built into small quantum computers by companies like IBM and Google. The established knowledge and technical infrastructure are allowing researchers to harness quantum technologies in unexpected, innovative ways and creating new research opportunities.

For superconducting circuits to be used as qubits—the basic building blocks of a quantum computer—the circuits need to reliably interact with delicate quantum states while keeping them carefully isolated from other influences. Superconducting circuits and the quantum states that occur in them are both sensitive to external influences, like shifting temperatures, stray electric fields or a passing particle, so manipulating qubits without external disruption is a delicate art. Instead of fashioning qubits in their superconducting circuits, researchers incorporated a sample with a magnetic layer atop a superconductor. They were able to use the sensitivity of the circuit to explore the quantum world hidden in the sample. The above image shows the changes in the circuit’s behavior as an applied magnetic field and the circuit’s properties were varied, revealing a strong interaction between the superconducting and ferromagnetic properties of the sample. (Credit: Harvard University)

The sensitivity of superconducting circuits means that minor blemishes in fabrication and installation can ruin a qubit, so much work has gone into establishing precise fabrication techniques to produce usable tech. All that work now means that the underlying sensitivity that makes superconducting qubits a pain to work with can be exploited as part of a quantum sensor. 

In a paper published in the journal Nature Physics earlier this year, a collaboration between theorists at JQI and experimentalists at Harvard University presented a technique that repurposes the technology of superconducting circuits to study samples with exotic forms of superconductivity. The collaboration demonstrated that by building samples of interest into a superconducting circuit they could spy on exotic superconducting behaviors that have eluded existing measurement techniques.

“Essentially, we turned bugs of superconducting qubits into features,” says JQI postdoctoral researcher Andrey Grankin, who is an author of the paper.

The team’s experiments provided a new way to distinguish exotic forms of superconductivity from the conventional, well-understood variety. The technique also allowed them to study superconducting currents that occur in such a thin section of a sample that most existing techniques for researching superconductivity aren’t reliable. Their results also suggest that the approach might be beneficial in other areas of condensed matter research—the field that studies how the interactions of the multitude of particles in a material produce its properties.

“By capitalizing on this technology from the quantum computing side of things, it turns out that we were able to show that this is actually a very nice sensor for looking at fundamental condensed matter physics problems, in particular for understanding superconductivity in very small systems,” says Harvard physics graduate student Nick Poniatowski, who is a co-lead author of the paper.

Superconducting circuits commonly rely on resonators in which electromagnetic waves bounce around for extended periods and interact with quantum states. Researchers can use the bouncing waves to both manipulate a nearby qubit and determine what state it is in.

The project started when researchers at Harvard began experimenting with placing a sample near the resonator instead of building a qubit there. Incorporating a sample into the superconducting circuits provided the researchers with a convenient way to send currents through the sample as well as a built-in sensor, in the form of the resonator, to detect subtle changes in its electronic states.

In particular, the team studied samples where a magnetic layer lies on a superconductor, which research indicated might be a promising environment for hunting down exotic forms of superconductivity.

Ordinary superconductivity doesn’t mix well with magnetism, but researchers have discovered signs of interesting interactions that occur when magnetic and superconducting materials are brought together. Superconductivity and magnetism are connected because electrons must be paired up to create a superconducting current and magnetism influences how electrons can partner up.

Every electron has a quantum property called spin, which makes it behave similarly to a tiny magnet. Like a bar magnet will flip around when it is forced toward a magnet with the same orientation, electrons want the magnetic fields of their spins to point in opposite directions. As a result, the best-understood superconductors have electrons that pair with counterparts with the opposite spin. But magnetic materials—ferromagnets—have a magnetic field that works to twist all the electrons so that their fields point the same way, preventing such pairings.

However, experiments have indicated that there are likely more exotic forms of superconductivity with alternative ways that electrons form pairs. Some of the exotic pairings—called spin-triplet pairings—have spins that point in the same direction. Evidence suggests that spin-triplet pairs might be found in certain seemingly conventional superconductors, just in small numbers that are obscured by the large crowd of traditional superconducting pairs.

Prior results have indicated that at the interface of a ferromagnet and a superconductor, some spin-triplet electron pairs from the superconductor might be able to bleed over into the magnetic territory. Researchers hope that studying exotic superconductors and their pairing mechanisms will open new opportunities in research and quantum computing and possibly even identify a superconductor that works under more convenient conditions than the frigid temperatures or high pressures required for known superconductors.

In the new work, the experimentalists chose samples composed of a magnetic layer of a material called permalloy attached to a superconducting layer of niobium. As they performed experiments, they weren’t getting a clear picture of what was happening at the interface or identifying a smoking gun to indicate unconventional superconductivity, so they turned to theorists. Poniatowski previously studied physics as an undergraduate at UMD, and he knew that Grankin and Professor and JQI Fellow Victor Galitski had an interest in the theories that describe superconducting qubits. Poniatowski reached out, and Grankin and Galitski, who is also a Chesapeake Chair Professor of Theoretical Physics in the Department of Physics, joined the project.

“Eventually, Andrey came up with these models that then guided us a little bit more in the experiment to perform certain experimental tests,” says Yale postdoctoral associate Charlotte Bøttcher, a co-lead author of the paper who was a Harvard graduate student when she performed this research. “Before that, it was like shooting a little bit in the dark and just trying to play with whatever we had available. But then the interaction with theory was really what made things come together.”

The experiment needed a clear way of distinguishing exotic electron pairs from their conventional counterparts. Normally, measuring the electrical resistance—a material’s opposition to currents, which make electric circuits lose energy—is a cornerstone of superconductor research since the defining feature of a superconducting current is that it experiences no resistance. But all superconductors having zero resistance means resistance measurements can’t distinguish between exotic and traditional superconducting paired electrons.

However, a superconductor can have a distinctive inductance—a material’s resistance to changes in the electrical current. The inductance depends on many things, like the size and shape of the material. In superconductors, the inductance depends on the number of superconducting electron pairs that are present, with the dependence getting more dramatic the fewer electron pairs there are. The team focused on measuring the inductance to tease out what was occurring near the interface.

“Really, the advantage of our probe is we're measuring the badness of the superconductor not the goodness,” says Poniatowski. “So if a superconducting current is very weak, we can see a very strong response.”

Using this technique allowed the team not only to measure how much superconductivity occurred at the interfaces but also to observe how that amount changed as they varied the temperature. This was crucial to distinguishing what type of superconductivity they were observing. Since the exotic spin-triplet pairs are more delicate than their traditional counterparts, their population should fall off much more quickly as the temperature increases.

The team obtained even more information by performing the experiments with the sample placed in magnetic fields that pointed in various directions relative to the flow of the electrical current. Grankin’s contributions included determining how the experimental signatures should differ if the magnetic field points along the direction of the current or perpendicular to it.

The population changes they observed matched the signature of the exotic spin-triplet states described by the theory.

“Using this new technique borrowing from quantum information technologies we were able to actually see evidence for this very subtle, hard-to-detect exotic pairing state in this simple system,” says Poniatowski.

While the results indicated the presence of exotic superconductivity, they also revealed further mysteries to be investigated about how those states can exist in the material. The theories predict that in addition to being disrupted easily by heat, the superconducting states should also be easily disrupted by any mess in the material’s structure.

“Typically, this state is so fragile that any disorder should kill the state,” says Bøttcher. “And our systems are, for sure, not perfect, so it's a bit of a puzzle why this state is even there in the first place. And when there's something you don't understand, I think that that means we're not done yet, that more work needs to be done.”

The researchers hope to unravel the lingering mysteries and explore other research avenues their experiments have opened. They are working on additional research to further explore the behaviors of spins in the ferromagnet layer. And, based on their results, they are also optimistic that with further work similar devices may provide a window into the interactions of light and the magnetic excitations in a material—an emerging research field called magnonics.

“I actually think that the most important thing and our most important job as researchers is to look for what you didn't expect because I think that's where the new research and the new physics lies,” says Bøttcher. “So, I am always very excited when I see something I didn't expect, and I think this project was exactly that.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/repurposing-qubit-tech-explore-exotic-superconductivity

In addition to Grankin, Galitski, Bøttcher and Poniatowski, co-authors of the paper include Harvard physics professor Amir Yacoby, Harvard postdoctoral fellow Uri Vool, and Harvard graduate students Zihan Yan and Marie Wesson.

This research was supported by the Quantum Science Center, a National Quantum Information Science Research Center of the US Department of Energy; the NSF NNIN award ECS-00335765; the Department of Defense through the NDSEG fellowship program; the National Science Foundation Grant No. DMR-2037158 and DMR-1708688; the US Army Research Office Contract No. W911NF1310172; the Simons Foundation; and the Gordon and Betty Moore Foundation Grant No. GBMF 9468.

UMD Awarded $2 Million to Build a Quantum Biosensing Test Bed

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Category: Department News

Published: Monday, November 18 2024 10:07

Physics Professor Wolfgang Losert, Cell Biology and Molecular Genetics Professor Kan Cao, Chemistry and Biochemistry Professor John Fourkas, and Electrical and Computer Engineering Associate Professor Cheng Gong were awarded $2 million by the U.S. Air Force Office of Scientific Research to build a test bed to study how neural networks process information and develop new approaches to quantum computing and sensing inspired by the living brain. As principal investigator of this multidisciplinary and cross-institutional project, Losert will collaborate with both UMD faculty members as well as other academic and industry partners to better understand and recreate the brain’s unique capacity for learning and adapting quickly—abilities that far surpass traditional computer systems.Wolfgang Losert

“The human brain is remarkable in how efficiently it can learn and process information. For example, we only need to touch a hot stove once to learn not to do it again,” Losert explained. “But current artificial intelligence systems need more than just that. Typically, they require enormous amounts of data and computing power to learn new tasks through numerous rounds of trial and error.”

While traditional computers process information through individual components working in sequence, the brain distributes information across many networks of cells working in parallel. This fundamentally different approach allows for faster learning and adaptivity but with far less energy consumption than a computer. Losert and his team hope to identify the biological mechanisms behind this efficient method of learning in the brain.

For this research, a key focus is on astrocytes, a type of brain cell that makes up more than half of the cells in the human brain. Long considered mere support cells for neurons, astrocytes are now recognized as crucial to how the brain processes information. By engineering laboratory-based systems that incorporate both neurons and astrocytes, Losert’s team will closely observe how the two types of cells form living neural networks and react when exposed to various types of stress like ultrasound or electrical fields.

Recent discoveries by the neuroscientist on Losert’s team, assistant research scientist Kate O’Neill, and other researchers have already shown that astrocytes actively participate in brain signaling and may be essential to the brain’s ability to both learn and adapt to new situations quickly. Further observations could provide insights into how the brain maintains its performance under different conditions and may lead to more resilient forms of artificial intelligence (AI).

“Interestingly, one aspect that makes biocomputing so unique—the multitude of different signals in living neural networks, such as electro-magnetic, chemical and mechanical signals—also opens up another exciting aspect of our work. We can use living neural networks to test and improve quantum sensors for a range of biomedical applications,” said Losert, who is also an MPower Professor and interim associate dean for research in the College of Computer, Mathematical, and Natural Sciences with a joint appointment in the Institute for Physical Science and Technology.

Quantum sensors have the potential to measure minute physical changes like the presence of magnetic fields or electrochemical activity in cells in minimally invasive ways. Novel non-invasive biosensors could allow scientists and health care professionals to observe brain processes in patients that they couldn’t see before, potentially leading to better medical treatments and a more nuanced understanding of brain performance.

With this award, Losert’s team aims to bridge the gap between artificial and biological computing systems and help create new technologies that combine the best features of both.

“By understanding and replicating how brain cells work together, we hope to create more efficient and adaptable computing systems,” Losert said. “This project represents the start of a new paradigm in biocomputing that may help shape the future of both AI and neuroscience.”

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The grant will also facilitate collaborations with researchers from the U.S. Air Force Research Library, the National Quantum Laboratory (QLab), Lockheed Martin, the National Research Council of Italy (CNR) and the University of Bari Aldo Moro.

New Design Packs Two Qubits into One Superconducting Junction

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Category: Research News

Published: Monday, October 21 2024 00:01

Quantum computers are potentially revolutionary devices and the basis of a growing industry. However, their technology isn’t standardized yet, and researchers are still studying the physics behind the diverse ways to build these quantum devices. Even the most basic building blocks of a quantum computer—qubits—are still an active research topic.A superconducting circuit studied in Alicia Kollár’s lab. The middle of the three rectangles along the bottom are junctions that hold quantum states that may each be used as a qubit. A proposal to adjust the dimensions of the junctions would allow chips like this to host twice as many qubits.

In an article published September 23, 2024 in the journal Physical Review A, JQI researchers proposed a way to use the physics of superconducting junctions to let each function as more than one qubit. They also outlined a method to use the new qubit design in quantum simulations. While these proposed qubits might not immediately replace their more established peers, they illustrate the rich variety of quantum physics that remains to be explored and harnessed in the field.

Superconducting junctions are part of many diverse qubit designs, including those in the prototype quantum computers of IBM and Google. All the designs feature an island made of a superconductor joined to the rest of a superconducting circuit by a thin layer of insulator that forms the junction between the two sections. To cross the barrier, electrons in the circuit must quantum tunnel through the junction, influencing which quantum states the circuit can naturally hold.

JQI Fellow Mohammad Hafezi and JQI postdoctoral researcher Andrey Grankin, who is the first author of the paper, reviewed the research on junctions in superconducting circuits, and what they found left them wondering if the existing qubit designs were taking advantage of the full breadth of physics that can be realized in superconducting junctions. The design of a junction—the geography of the superconducting island—impacts which states it can host, and current designs have focused on small junctions and the simplest states.

In some qubit designs, the quantum states depend on the geography of the island because of how electrons in a superconductor are free to move around like a fluid. Like water in a small pool, the electrons can slosh back and forth and form waves that are influenced by their surroundings.

Certain electron waves isolated onto a superconducting island can be very stable and long lasting, which makes them useful for storing quantum information in a qubit. The waves that are stable are examples of a more general phenomenon, called standing waves, that occur when a wave isn’t interrupted during the slope of one of its hills or valleys; instead, its oscillations are perfectly completed at the edges (the walls of a pool, the points where a string is being held, etc.). A guitar string’s harmonics are also examples of standing waves. 

But just having a stable standing wave in the superconducting electrons isn’t enough to be useful for quantum calculations. To use a standing wave as a qubit, a quantum computer must be able to distinguish it from all other standing waves and individually target it. Current superconducting qubit designs circumvent this issue by using short junctions that host just a single standing wave; as long as a junction is sufficiently short, the physics governing the superconducting electrons effectively only allows a single standing wave on the superconducting island. Researchers have also studied junctions that meet along very long interfaces and found that they can easily host a vast array of standing waves. Unfortunately, the abundance of standing waves comes with a downside: The more standing waves there are, the more similar the waves become, which makes them difficult to tell apart and inconvenient for quantum computing. 

“Historically short and long junctions were researched quite extensively,” says Grankin, who is the first author of the paper. “But the intermediate junction lengths have not been studied.”

Hafezi—who is also a Minta Martin professor of electrical and computer engineering and physics at the University of Maryland (UMD) and a Senior Investigator at the National Science Foundation Quantum Leap Challenge Institute for Robust Quantum Simulation (RQS)—and Grankin became interested in this intermediate regime. The pair consulted with JQI Fellow Alicia Kollár, another author of the paper who works with superconducting qubits in her research.

“Andrey and Mohammad came to me with a creative new idea for how to make a junction host multiple qubit excitations,” says Kollár, who is also a Chesapeake Assistant Professor of Physics and a Co-Associate Director of Research for RQS. “Our main challenge was coming up with a design that would yield practical device parameters and a device that is actually within reach of current state-of-the-art fabrication techniques.”

Together, the group explored the behaviors of electrons in the intermediate case and if it is practical to produce multiple excitations that can be easily distinguished and separately manipulated. Since the pool of electrons is within a solid superconductor, setting them into motion isn’t as simple as plucking a string. To push and pull on electrons you need an electric field. One way that physicists like to push electrons around is using a special reflective chamber—or resonator—that is full of electric and magnetic fields in the form of light. 

Light in a resonator can form its own standing waves that act on the electrons. Similar to a guitar string vibrating due to the sound waves from another string—or more dramatically the sound of a singer’s voice shaking a glass until it shatters—the right light waves inside a resonator can excite electrons in a superconducting junction into a standing wave, which physicists call a mode of the junction. 

The team analyzed how medium-sized junctions should behave inside a resonator and found promising results. The various modes of a junction each respond more or less strongly to particular frequencies of light, so light can be selected to target a specific mode. The response of a mode to a standing wave of light in a resonator also depends on whether the symmetries of the mode and the light match. If the waves of light in the resonator are symmetrical across the center of a junction, they naturally push electrons into waves with a similar symmetry. For instance, if the light waves crossing the junction form a hill on one side and a valley on the other, they can’t push the electrons into a simple hill reflected across the center of the device, but they might be able to excite a similarly lopsided mode of the junction. 

So, light that creates one mode in the superconducting electrons may be ignored by another mode. In the paper, the team described a method of exploiting these two effects to excite or manipulate only a targeted mode. The researchers proposed a design where two distinct modes are targeted so that a single junction functions as two independent qubits. They also described a method to use a one-dimensional line of junctions to simulate interactions between two-dimensional grids of quantum particles. However, they haven’t yet tackled fabricating the junctions and demonstrating the feasibility of their proposal.

“This project started from a fundamental interest in the electrodynamics of extended junctions,” Grankin says. “Then it turned out to be also useful from the quantum information and simulation perspective.”

Original story by Bailey Bedford: https://jqi.umd.edu/news/new-design-packs-two-qubits-one-superconducting-junction

HAWC Finds High-Energy Gamma-Ray Emissions from Microquasar V4641 Sagittarii

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Category: Research News

Published: Wednesday, October 16 2024 00:01

A new study in Nature, “Ultra-high-energy gamma-ray bubble around microquasar V4641 Sgr,"   has  revealed a groundbreaking discovery by researchers from the High Altitude Water Cherenkov (HAWC) observatory:  TeV gamma-ray emissions from V4641 Sagittarii (V4641 Sgr), a binary system composed of a black hole and a main sequence B-type companion star. This discovery provides fresh insights into particle acceleration in large-scale jets emitted from microquasars, which serve as natural laboratories for studying high-speed jets produced by matter falling onto spinning black holes. The findings show that V4641 Sgr's gamma-ray emissions occur at similar distances from the black hole as those observed in another well-known microquasar, SS 433. This makes V4641 Sgr stand out for its super-Eddington accretion and one of the fastest superluminal jets in the Milky Way. With a gamma-ray spectrum that ranks among the hardest of any known TeV sources, the emissions are detected at energies exceeding 200 TeV. Schematic illustration of the V4641 Sgr region.

The study's results indicate that the gamma rays are likely produced by extremely high-energy protons. While studies of SS 433 indicate that the emission from this object is likely electrons, the high energy emission at large distances from V4641 argue against electrons due to their rapid energy loss at high energies. The implications are profound: the environment around the microquasar may play a critical role in determining whether the emission from the large-scale jets comes from electrons or protons, and they could play a significant role as a source of Galactic cosmic rays. These observations open new avenues for understanding particle acceleration in extreme environments and contribute to the broader study of high-energy astrophysics. 

 "At a zenith angle of 46° from HAWC's field of view, the microquasar V4641 Sgr has accelerated particles to the knee of the cosmic-ray spectrum, pushing the boundaries of our understanding of particle acceleration and transport in such extreme environments," said Dr. Dezhi Huang, a Post-Doctoral researcher at the University of Maryland. "HAWC observatory continues to deliver exceptional performance, providing valuable insights into these high-energy processes that were previously beyond our reach. "

“The HAWC survey has discovered for the first time very high energy gamma rays from the extended 100 pc jet of the microquasar V4641 Sgr. This proves that jets launched in accreting systems can accelerate particles up to PeV energies, and therefore that microquasars are potentially significant contributors to the Galactic cosmic ray population at high energies," saidDr. Sabrina Casanova, Professor from Institute of Nuclear Physics of the Polish Academy of Sciences. "Furthermore, although very high energy protons are strongly suspected to exist in the kpc jets of active galactic nuclei (AGN), the associated hundred TeV emission has never been observed from an extended region from an AGN jet due to strong gamma-ray absorption over the long distances to Earth.”Differential spectrum weighted by E2 for the northern and southern sources in a model with two point sources and for the asymmetric extended source in a model with a single asymmetric extended source. The shaded regions indicate the best-fit spectra and 1σ statistical uncertainties when fitting a single-power-law model to the data from 10 to >200 TeV. The markers correspond to the best-fit values and their 1σ statistical uncertainties obtained when fitting a single-power-law model to data in individual energy bins. The chosen energy range for plotting the spectrum is specified in the Methods.

This study was supported by the collaborative efforts of multiple institutions, with major contributions from the University of Maryland. Distinguished University Professor Jordan Goodman, a member of the collaboration's internal editorial board, played a key role in guiding the publication. Dr. Dezhi Huang, one of the corresponding authors, led significant aspects of the analysis, while Dr. Kristi Engel helped refine the paper. Additional support came from UMD HAWC group members Research Scientist Dr. Andrew Smith, Project Engineer Michael Schneider, Dr. Zhen Wang, Dr. Jason Fan and graduate students Sohyoun Yun-Cárcamo.

More on the finding: https://www.nature.com/articles/d41586-024-03191-x