Fast-flowing electrons may mimic astrophysical dynamos

dynamo galitski1 blue galleryCertain materials may host an electron fluid that flows fast enough to generate turbulence and bootstrap a dynamo. (Credit: E. Edwards/JQI)

A powerful engine roils deep beneath our feet, converting energy in the Earth’s core into magnetic fields that shield us from the solar wind. Similar engines drive the magnetic activity of the sun, other stars and even other planets—all of which create magnetic fields that reinforce themselves and feed back into the engines to keep them running.

Much about these engines, which scientists refer to as dynamos, remains unknown. That’s partly because the math behind them is doubly difficult, combining the complex equations of fluid motion with the equations that govern how electric and magnetic fields bend, twist, interact and propagate. But it’s also because lab-bound dynamos, which attempt to mimic the astrophysical versions, are expensive, dangerous and do not yet reliably produce the signature self-sustaining magnetic fields of real dynamos.

Now, Victor Galitski, a Fellow of the Joint Quantum Institute (JQI), in collaboration with two other scientists, has proposed a radical new approach to studying dynamos, one that could be simpler and safer. The proposal, which was published Oct. 25 in Physical Review Letters, suggests harnessing the electrons in a centimeter-sized chunk of solid matter to emulate the fluid flows in ordinary dynamos.

If such an experiment is successful, it might be possible for researchers in the future to study the Earth’s dynamo more closely—and maybe even learn more about the magnetic field flips that happen every 100,000 years or so. "The dynamics of the Earth’s dynamo are not well understood, and neither are the dynamics of these flips," says Galitski, who is also a physics professor at the University of Maryland. "If we had experiments that could reproduce some aspects of that dynamo, that would be very important."

Such experiments wouldn’t be possible but for the fact that electrons, which carry current through a material, can sometimes be thought of as a fluid. They flow from high potential to low potential, just like water down a hill, and they can flow at different speeds. The trick to spotting the dynamo effect in an electron fluid is getting them to flow fast enough without melting the material.

"People haven’t really thought about doing these experiments in solids with electron fluids," Galitski says. "In this work we don’t imagine having a huge system, but we do think it’s possible to induce very fast flows."

Those fast flows would be interesting in their own right, Galitski says, but they are especially important for realizing the dynamo effect in the lab. Despite the many lingering unknowns about dynamos, it seems that turbulence plays a crucial role in their creation. This is likely because turbulence, which leads to chaotic fluid motion, can jostle the magnetic field loose from the rest of the fluid, causing it to twist and bend on top of itself and increase its strength.

But turbulence only arises for very fast flows—like the air rushing over the wing of an airplane—or for flows over very large scales—like the liquid metal in the Earth’s core or the plasma shell of the sun. To create a dynamo using a small piece of solid matter, the electrons would need to move at speeds never before seen, even in materials known for having highly mobile electrons.

Galitski and his collaborators think that a material called a Weyl semimetal may be able to host an electron fluid flowing at more than a kilometer per second—potentially fast enough to generate the turbulence necessary to bootstrap a dynamo. These materials have received broad attention in recent years due to their unusual characteristics, including anomalous currents that arise in the presence of magnetic fields and that may reduce the speed required for turbulence to emerge.

"It might seem that turbulence isn’t particularly extraordinary," says Sergey Syzranov, a co-author and former JQI postdoctoral researcher who is now an assistant professor of physics at the University of California, Santa Cruz. "But in solids it has never been demonstrated to our knowledge. A major achievement of our work is that turbulence is realistic in some solid-state materials."

The authors say that it’s not yet clear how best to kickstart a dynamo on a small sliver of Weyl semimetal. It may be as simple as physically rotating the material. Or it could require pulsing an electric or magnetic field. Either way, Galitski says, the experimental signature would show a totally nonmagnetic system spontaneously form a magnetic field. "Controlled experiments like these with turbulence in electrons are totally unheard of," Galitski says. "I can’t really say what will come out of it, but it could be really interesting."

Mehdi Kargarian, a former JQI postdoctoral researcher who is now an assistant professor of physics at the Sharif University of Technology, was also a co-author of the new paper.

Story by Chris Cesare

Read more information on this and the Joint Quantum Institute.

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Modified Superconductor Synapse Reveals Exotic Electron Behavior

JQI researchers modified a Josephson junction to include a sliver of topological crystalline insulator (TCI). Using this circuit, they detected signs of exotic quantum states lurking on the TCI's surface. (Credit: E. Edwards/JQI)

Electrons tend to avoid one another as they go about their business carrying current. But certain devices, cooled to near zero temperature, can coax these loner particles out of their shells. In extreme cases, electrons will interact in unusual ways, causing strange quantum entities to emerge.

At the Joint Quantum Institute (JQI), a group, led by Jimmy Williams, is working to develop new circuitry that could host such exotic states. “In our lab, we want to combine materials in just the right way so that suddenly, the electrons don’t really act like electrons at all,” says Williams, a JQI Fellow and an assistant professor in the University of Maryland Department of Physics. “Instead the surface electrons move together to reveal interesting quantum states that collectively can behave like new particles.”

These states have a feature that may make them useful in future quantum computers: They appear to be inherently protected from the destructive but unavoidable imperfections found in fabricated circuits. As described recently in Physical Review Letters, Williams and his team have reconfigured one workhorse superconductor circuit—a Josephson junction—to include a material suspected of hosting quantum states with boosted immunity.

Josephson junctions are electrical synapses comprised of two superconductors separated by a thin strip of a second material. The electron movement across the strip, which is usually made from an insulator, is sensitive to the underlying material characteristics as well as the surroundings. Scientists can use this sensitivity to detect faint signals, such as tiny magnetic fields. In this new study, the researchers replaced the insulator with a sliver of topological crystalline insulator (TCI) and detected signs of exotic quantum states lurking on the circuit’s surface.

Physics graduate student Rodney Snyder, lead author on the new study, says this area of research is full of unanswered questions, down to the actual process for integrating these materials into circuits. In the case of this new device, the research team found that beyond the normal level of sophisticated material science, they needed a bit of luck.

“I'd make like 16 to 25 circuits at a time. Then, we checked a bunch of those and they would all fail, meaning they wouldn’t even act like a basic Josephson junction,” says Snyder. “We eventually found that the way to make them work was to heat the sample during the fabrication process. And we only discovered this critical heating step because one batch was accidentally heated on a fluke, basically when the system was broken.”

Once they overcame the technical challenges, the team went hunting for the strange quantum states. They examined the current through the TCI region and saw dramatic differences when compared to an ordinary insulator. In conventional junctions, the electrons are like cars haphazardly trying to cross a single lane bridge. The TCI appeared to organize the transit by opening up directional traffic lanes between the two locations. 

The experiments also indicated that the lanes were helical, meaning that the electron’s quantum spin, which can be oriented either up or down, sets its travel direction. So in the TCI strip, up and down spins move in opposite directions. This is analogous to a bridge that restricts traffic according to vehicle colors—blue cars drive east and red cars head west. These kinds of lanes, when present, are indicative of exotic electron behaviors.

Just as the careful design of a bridge ensures safe passage, the TCI structure played a crucial role in electron transit. Here, the material’s symmetry, a property that is determined by the underlying atom arrangement, guaranteed that the two-way traffic lanes stayed open. “The symmetry acts like a bodyguard for the surface states, meaning that the crystal can have imperfections and still the quantum states survive, as long as the overall symmetry doesn’t change,” says Williams.

Physicists at JQI and elsewhere have previously proposed that built-in bodyguards could shield delicate quantum information. According to Williams, implementing such protections would be a significant step forward for quantum circuits, which are susceptible to failure due to environmental interference.

In recent years, physicists have uncovered many promising materials with protected travel lanes, and researchers have begun to implement some of the theoretical proposals. TCIs are an appealing option because, unlike more conventional topological insulators where the travel lanes are often given by nature, these materials allow for some lane customization. Currently, Williams is working with materials scientists at the Army Research Laboratory to tailor the travel lanes during the manufacturing process. This may enable researchers to position and manipulate the quantum states, a step that would be necessary for building a quantum computer based on topological materials.

In addition to quantum computing, Williams is driven by the exploration of basic physics questions. “We really don't know yet what kind of quantum matter you get from collections of these more exotic states,” Williams says. “And I think, quantum computation aside, there is a lot of interesting physics happening when you are dealing with these oddball states.”

Written by E. Edwards and S. Elbeshbishi

RESEARCH CONTACT
Jimmy Williams  This email address is being protected from spambots. You need JavaScript enabled to view it.

Pristine Quantum Light Source Created at the Edge of Silicon Chip

hafezi mittal goldschmidt nature 09 2018 psResearchers configure silicon rings on a chip to emit high-quality photons for use in quantum information processing. Credit: E. Edwards/JQI.

Protected Pathways for Light Offer a way to Streamline Single Photon Production.

The smallest amount of light you can have is one photon, so dim that it’s pretty much invisible to humans. While imperceptible, these tiny blips of energy are useful for carrying quantum information around. Ideally, every quantum courier would be the same, but there isn’t a straightforward way to produce a stream of identical photons. This is particularly challenging when individual photons come from fabricated chips.

Now, researchers at the Joint Quantum Institute (JQI) have demonstrated a new approach that enables different devices to repeatedly emit nearly identical single photons. The team, led by JQI Fellow Mohammad Hafezi, made a silicon chip that guides light around the device’s edge, where it is inherently protected against disruptions. Previously, Hafezi and colleagues showed that this design can reduce the likelihood of optical signal degradation. In a paper published online on Sept. 10 in Nature , the team explains that the same physics which protects the light along the chip’s edge also ensures reliable photon production.

Single photons, which are an example of quantum light, are more than just really dim light. This distinction has a lot to do with where the light comes from. “Pretty much all of the light we encounter in our everyday lives is packed with photons,” says Elizabeth Goldschmidt, a researcher at the US Army Research Laboratory and co-author on the study.  “But unlike a light bulb, there are some sources that actually emit light, one photon at time, and this can only be described by quantum physics,” adds Goldschmidt.

Many researchers are working on building reliable quantum light emitters so that they can isolate and control the quantum properties of single photons. Goldschmidt explains that such light sources will likely be important for future quantum information devices as well as further understanding the mysteries of quantum physics. “Modern communications relies heavily on non-quantum light,” says Goldschmidt. “Similarly, many of us believe that single photons are going to be required for any kind of quantum communication application out there.”

Scientists can generate quantum light using a natural color-changing process that occurs when a beam of light passes through certain materials. In this experiment the team used silicon, a common industrial choice for guiding light, to convert infrared laser light into pairs of different-colored single photons.

They injected light into a chip containing an array of miniscule silicon loops. Under the microscope, the loops look like linked-up glassy racetracks. The light circulates around each loop thousands of times before moving on to a neighboring loop. Stretched out, the light’s path would be several centimeters long, but the loops make it possible to fit the journey in a space that is about 500 times smaller. The relatively long journey is necessary to get many pairs single photons out of the silicon chip.  

Such loop arrays are routinely used as single photon sources, but small differences between chips will cause the photon colors to vary from one device to the next. Even within a single device, random defects in the material may reduce the average photon quality. This is a problem for quantum information applications where researchers need the photons to be as close to identical as possible.

The team circumvented this issue by arranging the loops in a way that always allows the light to travel undisturbed around the edge of the chip, even if fabrication defects are present. This design not only shields the light from disruptions—it  also restricts how single photons form within those edge channels. The loop layout essentially forces each photon pair to be nearly identical to the next, regardless of microscopic differences among the rings. The central part of the chip does not contain protected routes, and so any photons created in those areas are affected by material defects.

The researchers compared their chips to ones without any protected routes. They collected pairs of photons from the different chips, counting the number emitted and noting their color. They observed that their quantum light source reliably produced high quality, single-color photons time and again, whereas the conventional chip’s output was more unpredictable.

“We initially thought that we would need to be more careful with the design, and that the photons would be more sensitive to our chip’s fabrication process,” says Sunil Mittal, a JQI postdoctoral researcher and lead author on the new study. “But, astonishingly, photons generated in these shielded edge channels are always nearly identical, regardless of how bad the chips are.”

Mittal adds that this device has one additional advantage over other single photon sources. “Our chip works at room temperature. I don’t have to cool it down to cryogenic temperatures like other quantum light sources, making it a comparatively very simple setup.”

The team says that this finding could open up a new avenue of research, which unites quantum light with photonic devices having built-in protective features. “Physicists have only recently realized that shielded pathways fundamentally alter the way that photons interact with matter,” says Mittal. “This could have implications for a variety of fields where light-matter interactions play a role, including quantum information science and optoelectronic technology.”

Written by D. Genkina and E. Edwards

Author Affiliations 

Sunil Mittal, Joint Quantum Institute, Institute for Research in Electronics and Applied Physics (IREAP)

Elizabeth Goldschmidt, Joint Quantum Institute and US Army Research Laboratory

Mohammad Hafezi, Joint Quantum Institute, UMD Electrical and Computer Engineering, IREAP and Department of Physics

Reference Publication

"A topological source of quantum light," S. MittalElizabeth A. GoldschmidtMohammad HafeziNature, , (2018)

Research Contact

S. Mittal:  This email address is being protected from spambots. You need JavaScript enabled to view it.