Researchers Play a Microscopic Game of Darts with Melted Gold

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Published: Friday, February 28 2025 01:00

Sometimes, what seems like a fantastical or improbable chain of events is just another day at the office for a physicist.

In a recent experiment by University of Maryland researchers at the Laboratory for Physical Sciences, a scene played out that would be right at home in a science fiction movie. A tiny speck glinted faintly as it hovered far above a barren, glassy plain. Suddenly, an intense green light shone toward the ground and enveloped the speck, now a growing dark spot like a meteorite or UFO descending in the emerald beam. Once the object crashed into the ground, the light abruptly disappeared, and the flat landscape was left with a new landmark and treasure for physicists to find: a chunk of gold rapidly cooling from a molten state.

This scene, which played out at a minuscule scale in repeated runs of the experiment, was part of a research project on nanoparticles—objects made of no more than a few thousand atoms. Each piece of gold was a bead hundreds of times smaller than the width of a human hair. In each run, the golden projectile was melted by a green laser and traveled almost a million times its own length to land on a glass slide.

Nanoparticles interest scientists and engineers because they often have exotic and adaptable properties. Unlike larger samples of a material, a nanoparticle can undergo dramatic changes with only small tweaks to its environment or size. For instance, a tiny gold nugget in a California stream has the same melting point, reflectivity and thermal conductivity as a 400-pound block of gold in Central Park, but two gold nanoparticles that differ in diameter by mere billionths of a meter have significantly different properties from the large pieces and, more importantly, each other.

The broad range of properties that nanoparticles have makes them a versatile toolbox for researchers and engineers to draw from. For example, people have used gold-based nanoparticles to detect the influenza virus, deliver medications in the body, and achieve a variety of vibrant colors in stained glass. However, since nanoparticles are so small and easily influenced, researchers must use a variety of specialized tools to study them.

When examining nanoparticles, some properties are best measured by tools—like a scanning electron microscope (SEM)—that get up close and personal with the sample. An SEM can get phenomenal detail on the size and shape of a nanoparticle if it is attached to a larger material that is easy to move and handle. However, the small size of nanoparticles can make other properties, like how they conduct heat, almost impossible to measure if they are touching anything. The mere presence of larger objects can often alter a nanoparticle’s properties or drown out its interaction with the measurement device. Fortunately, many nanoparticles can be isolated from the influence of other materials by using electric fields to levitate them, allowing researchers to use lasers to study certain properties, like heat conduction, from a distance.

JQI Fellow Bruce Kane and UMD researcher Joyce Coppock perform levitation experiments to study tiny pieces of graphene, which are sheets of carbon atoms. And in their quest to develop new tools, they have also turned their attention to tiny gold beads.

However, Kane and Coppock aren’t satisfied with the insights available from levitation experiments alone. They want the best of both worlds: to measure a sample levitated in isolation and then recover it for direct inspection. So, the pair are developing a method to recover tiny samples after they are released from the fields levitating them. In a paper published in Applied Physics Letters, the pair described how they were able to deposit gold nanoparticles on a slide after levitation and how they refined the technique to hone their aim. They hope mastering the process with gold will be useful in future experiments depositing more finicky graphene samples.

Before experimenting with depositing gold, Kane and Coppock had initially tried depositing graphene nanoparticles. Levitation is important for studying graphene on its own because its thickness—just a single atom—makes it challenging to study certain properties when it’s sitting on top of another material. For instance, a bulky material under a piece of graphene generally retains or moves heat around much more dramatically than the graphene, overwhelming any attempts to measure the heat conduction of the graphene itself. Additionally, simply sitting atop another material is often enough to stretch or squeeze a graphene sample in ways that change important properties, like its electrical resistance.

To avoid these issues, Kane and Coppock typically levitate their graphene samples in a vacuum. But the properties best measured directly without levitation are required to get a complete picture of a nanoparticle.

Ideally, Kane and Coppock would like to do both styles of measurement on individual nanoparticles. However, the existing levitation procedure makes it impractical either to perform direct probes on a sample before levitating it or to recover a sample once they remove the electric fields. That’s because there isn’t a convenient way to select a single tiny particle and reliably drop it into the field or recover it from the field.

In their experiments, Kane and Coppock first create an electric field designed to capture charged particles inside a vacuum chamber. To levitate a sample, they fill the chamber with many charged nanoparticles and watch to see if one of them falls into the field. After they make their measurements of that lucky particle, it gets released and becomes just another anonymous, invisible nanoparticle scattered about the vacuum chamber.

But Kane and Coppock had an idea for how to recover samples. Instead of just dropping the electric field and letting the particle fly in a random direction, they realized they could adjust the field to give it a shove in a particular direction as they released it. Then they just had to see if they could get the tiny projectile to land in an area they could easily search.

The pair placed a removable glass slide coated with a thin, conductive layer in the chamber as their target. Connecting a charge sensor to the conducting film allowed them to detect if an electrical charge landed on the slide. They also pointed a camera at the slide. The camera couldn’t watch the nanoparticles as they traveled, but each nanoparticle is just large enough that it will normally show up as a change of a single pixel in the camera image.

The pair’s calculations suggested that if a graphene sheet lands flat on the prepared slide it should stick. However, when they tried out the experiment, they kept measuring a spike in charge at the target—suggesting it hit—but almost never spotted where the sample landed. They suspected that most samples were bouncing off the slide or landing outside the area their camera covered.

So, they simplified the experiment by switching their projectile. Instead of using sheets of graphene that need to land perfectly flat, they tried spherical gold nanoparticles, which can be more uniformly produced and don’t have a preferential orientation for making contact. Kane and Coppock were already familiar with working with gold nanoparticles from previous experiments in which they levitated them and melted them with laser light.

Similar to the graphene sheets, the gold spheres were detected by the charge sensor but then couldn’t be found in the camera image. So, Kane and Coppock applied their melting technique to allow each particle to squish a little when it lands, greatly increasing the chance of sticking. All that was required to melt the gold was to turn up the power on the laser they already had installed for studying samples.

“Lo and behold, the minute we started doing that, we started seeing images on the camera,” says Coppock. “So basically, what was needed was to increase the adhesion by melting the particle.”

After that, they could reliably find the particles. However, repeated tries revealed that a sequence of deposited samples tended to spread far apart on the slide. Being able to place a sample in a consistent area would make the technique more useful and increase their chances of finding deposited graphene samples down the road.

“It's like the problem that people have going to the moon, right?” says Coppock. “You're a tiny person on Earth, and you have to get yourself a long distance to the moon. If you just launched yourself off the Earth, there's no way you would hit the moon. If we just launched the particle out of the trap, there's no way it would both hit the substrate and we would know where it was on the substrate. Finding a 200-nanometer particle on a one-inch sized substrate is like finding a needle in a haystack.”

So, they started working on the consistency with which they launched their tiny samples. The same electrical charge that allows Kane and Coppock to levitate the particles, also allows them to guide particles on the way to the slide. They surrounded the path they wanted the nanoparticles to follow with metal rings and then applied a voltage to the rings during the journey. The applied voltage creates an electric field that nudges a nanoparticle back onto a narrower path if it starts to stray. The way the electric fields bend charged particles back to a central focal point resembles a glass lens focusing light, so researchers call the setup an electrostatic lens.

By experimenting with the voltages that they used to launch the sample and guide it along its path, they were able to change where the particles tended to end up. They adjusted the voltages from a low setting where the samples spread over an area roughly 3,000 micrometers wide to a higher setting where all the particles clustered in an area about 120 micrometers across.

Plots of where gold particles from repeated runs of the experiment landed. The colors of the dots reflect the voltages applied to achieve electrostatic lenses of various strengths. The weakest lens (light blue dots) spread the samples across an area that is about 3,000 micrometers wide, and the strongest lens (red dots) focused all the particles into a cluster just 120 micrometers across. The lower right frame has increased magnification to show the distribution of particles within the cluster created by the strongest lens. (Credit: Laboratory for Physical Sciences)

If the initial scatter area were scaled up to the size of a dartboard, then their improved aim was like clustering their golden darts well within the outer bullseye. This is even more impressive since the scaled-up version of each gold bead is a dart only as wide as a human hair and is being thrown from the equivalent of about 35.5 meters away—about 15 times the normal distance between a dartboard and the throw line.

Moving forward, Kane and Coppock hope to further improve their ability to focus samples into a particular area and to use their refined aim in attempts to recover deposited graphene samples.

Original story by Bailey Bedford: https://jqi.umd.edu/news/researchers-play-microscopic-game-darts-melted-gold

IceCube Search for Extremely High-energy Neutrinos Contributes to Understanding of Cosmic Rays

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Published: Wednesday, February 12 2025 02:18

Twisted Light Gives Electrons a Spinning Kick

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Published: Saturday, November 30 2024 01:00

It’s hard to tell when you’re catching some rays at the beach, but light packs a punch. Not only does a beam of light carry energy, it can also carry momentum. This includes linear momentum, which is what makes a speeding train hard to stop, and orbital angular momentum, which is what the earth carries as it revolves around the sun.

In a new paper, scientists seeking better methods for controlling the quantum interactions between light and matter demonstrated a novel way to use light to give electrons a spinning kick. They reported the results of their experiment, which shows that a light beam can reliably transfer orbital angular momentum to itinerant electrons in graphene, on Nov. 26, 2024, in the journal Nature Photonics.

Having tight control over the way that light and matter interact is an essential requirement for applications like quantum computing or quantum sensing. In particular, scientists have been interested in coaxing electrons to respond to some of the more exotic shapes that light beams can assume. For example, light carrying orbital angular momentum swirls around its axis as it travels. When viewed head-on, a light beam with orbital angular momentum contains a dark spot in the middle, a vortex opened up by the beam’s corkscrew character.In a new experiment, light beams carrying orbital angular momentum caused electrons in graphene to gain (blue beam) and lose (red beam) angular momentum, transporting them across the sample and generating a current that researchers measured. (Credit: Mahmoud Jalali Mehrabad/JQI)

“The interaction of light that has orbital angular momentum with matter has been thought about since the 90s or so,” says Deric Session, a postdoctoral researcher at JQI and the University of Maryland (UMD) who is the lead author of the new paper. “But there have been very few experiments actually demonstrating the transfer.”

Part of the challenge has been a size mismatch. In order for electrons to feel a tug from a light beam carrying momentum, they have to experience the way that the beam changes as it passes by. In many cases, the length over which a light beam changes dwarfs the size of the matter that researchers are interested in manipulating, making it especially challenging to pick out electrons as targets.

For instance, atoms and their orbiting electrons—mainstays of quantum physics experiments and favorite targets for precise manipulation—are roughly 1,000 times smaller than the light beams that researchers use to interact with them. Light travels as repeating waves of electric and magnetic fields, and the length that a light beam travels before it repeats is called the wavelength. In addition to being an important characteristic size, the wavelength of light also determines the amount of energy carried by individual particles of light called photons. Only photons that carry particular amounts of energy can interact with atoms, and those photons tend to have wavelengths much bigger than the atoms themselves. So while atoms as a whole will happily absorb energy and momentum from these photons, the wavelength is too big for the internal pieces of the atom—the nucleus and the electrons—to notice any relative difference. This makes it very difficult to transfer orbital angular momentum solely to an atom’s electrons.

One way to overcome this difficulty is to shrink the wavelength of light. But that increases the energy carried by each photon, ruling out atoms as reliable targets. In the new experiment, the researchers, who included JQI Fellows Mohammad Hafezi and Nathan Schine, JQI Co-Director Jay Sau and JQI Adjunct Fellow Glenn Solomon, pursued an alternate approach: Instead of shrinking the wavelength of the light, they puffed up electrons to make them occupy more space.

Electrons bound to the nucleus of an atom can only roam so far before they are liberated from the atom and useless for experiments. But in conductive materials, electrons have more latitude to travel far and wide while remaining under control. The researchers turned to graphene, a flat material that is one of the best known electrical conductors, in search of a way to make electrons take up more space.

By cooling a sample of graphene down to just 4 degrees above absolute zero and subjecting it to a strong magnetic field, electrons that are ordinarily free to move around become trapped in loops called cyclotron orbits. As the field gets stronger, the orbits become tighter and tighter until many circulating electrons are packed in so tightly that no more can fit. Although the orbits are tight, they are still much larger than the electron orbitals in atoms—the perfect recipe for getting them to notice light carrying orbital angular momentum.

The researchers used a sample of graphene wired up with electrodes for their experiments. One electrode was in the middle of the sample, and another made a ring around the outer edges. Earlier theoretical work, developed in 2021 by former JQI and UMD graduate student Bin Cao and three other authors of the new paper, suggested that electrons circulating in such a sample could gain angular momentum in chunks from incoming light, increase the size of their orbits and eventually get absorbed by the electrodes.

“The idea is that you can change the size of the cyclotron orbits by adding or subtracting orbital angular momentum from the electrons, thus effectively moving them across the sample and creating a current,” Session says.

In the new paper, the research team reported observing a robust current that survived under a wide range of experimental conditions. They hit their graphene sample with light carrying orbital angular momentum that circulated clockwise and observed the current flowing in one direction. Then they hit it with light carrying counterclockwise orbital angular momentum and found that the direction of the current flipped. They flipped the direction of the applied magnetic field and observed the current flip directions, too—an expected finding since changing the magnetic field direction also swaps the direction electrons flow in their cyclotron orbits. They changed the voltage across the inner and outer electrodes and continued to see the same difference between currents generated by clockwise and counterclockwise vortex light. They also tested sending circularly polarized light, which carries an intrinsic angular momentum, at the sample, and they found that it barely generated any current. In all cases, the signal was clear: The current only appeared in the presence of light carrying orbital angular momentum, and the direction of the current was correlated with whether the light carried momentum that spun clockwise or counterclockwise.

The result was the culmination of several years of work, which included some false starts with sample fabrication and difficulties collecting enough good data from the experiment.

“I spent over a year just trying to make graphene samples with this kind of geometry,” Session says. Ultimately, Session and the team reached out to a group they had worked with before, led by Roman Sordan, a physicist at the Polytechnic University of Milan in Italy and an expert at preparing graphene samples. “They were able to come through and make the samples that we used,” says Session.

Once they had samples that worked well, they still had trouble aligning their twisted light with the sample to observe the current.

“The signal we were looking at was not quite consistent,” says Mahmoud Jalali Mehrabad, a postdoctoral researcher at JQI and UMD and a co-author of the paper. “Then one day, with Deric, we started to do this spatial sweep. And we kind of mapped the sample with really high accuracy. Once we did that—once we nailed down the very peak, optimized position for the beam—everything started to make sense.” Within a week or so, they had collected all the data they needed and could pick out all the signals of the current’s dependence on the orbital angular momentum of the beam.

Mehrabad says that, in addition to demonstrating a new method for controlling matter with light, the technique might also enable fundamentally new measurements of electrons in quantum materials. Specially prepared light beams, combined with interference measurements, could be used as a microscope that can image the spatial extent of electrons—a direct measurement of the quantum nature of electrons in a material.

“Being able to measure these spatial degrees of freedom of free electrons is an important part of measuring the coherence properties of electrons in a controllable manner—and manipulating them,” Mehrabad says. “Not only do you detect, but you also control. That’s like the holy grail of all this.”

Original story: https://jqi.umd.edu/news/twisted-light-gives-electrons-spinning-kick

In addition to Session, Hafezi, Schine, Sau, Solomon, Cao, Sordan and Mehrabad, the paper had several other authors: Nikil Paithankar, a graduate student at the Polytechnic University of Milan in Italy; Tobias Grass, a former JQI postdoctoral researcher who is now a research fellow at the Donostia International Physics Center in Donostia, Spain; Christian Eckhardt, a graduate student at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany; Daniel Gustavo Suárez Forero, a former JQI postdoctoral researcher who will be starting as an assistant professor of physics at the University of Maryland, Baltimore County in 2025; Kevin Li, a former JQI and UMD undergraduate student; Mohammad Alam, a former JQI and UMD undergraduate student who now works at IonQ; and Kenji Watanabe and Takashi Taniguchi, both researchers at the National Institute for Materials Science in Tsukuba, Japan.

This work was supported by ONR N00014-20-1-2325, AFOSR FA95502010223, ARO W911NF1920181, MURI FA9550-19-1-0399, FA9550-22-1-0339, NSF IMOD DMR-2019444, ARL W911NF1920181, Simons and Minta Martin foundations, and EU Horizon 2020 project Graphene Flagship Core 3 (grant agreement ID 881603). Tobias Grass acknowledges funding by BBVA Foundation (Beca Leonardo a Investigadores en Fisica 2023) and Gipuzkoa Provincial Council (QUAN-000021-01).

Repurposing Qubit Tech to Explore Exotic Superconductivity

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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.

New Design Packs Two Qubits into One Superconducting Junction

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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