Researchers Measure Casimir Torque for the First Time

Casimir crop

Physics researchers and members of the Institute for Research in Electronics and Applied Physics worked with the Department of Electrical and Computer Engineering to measure for the first time an effect that was predicted more than 40 years ago, called the Casimir torque. 

When placed together in a vacuum less than the diameter of a bacterium (one micron) apart, two pieces of metal attract each other. This is called the Casimir effect. The Casimir torque—a related phenomenon that is caused by the same quantum electromagnetic effects that attract the materials—pushes the materials into a spin. Because it is such a tiny effect, the Casimir torque has been difficult to study. The research team, which includes members from UMD's departments of electrical and computer engineering and physics and Institute for Research in Electronics and Applied Physics, has built an apparatus to measure the decades-old prediction of this phenomenon and published their results in the December 20th issue of the journal Nature.

"This is an interesting situation where industry is using something because it works, but the mechanism is not well-understood," said Jeremy Munday, the leader of the research. "For LCD displays, for example, we know how to create twisted liquid crystals, but we don't really know why they twist. Our study proves that the Casimir torque is a crucial component of liquid crystal alignment. It is the first to quantify the contribution of the Casimir effect, but is not the first to prove that it contributes."

The device places a liquid crystal just tens of nanometers from a solid crystal. With a polarizing microscope, the researchers then observed how the liquid crystal twists to match the solid's crystalline axis.

The team used liquid crystals because they are very sensitive to external forces and can twist the light that passes through them. Under the microscope, each imaged pixel is either light or dark depending on how twisted the liquid crystal layer is. In the experiment, a faint change in the brightness of a liquid crystal layer allowed the research team to characterize the liquid crystal twist and the torque that caused it.

The Casimir effect could make nanoscale parts move and can be used to invent new nanoscale devices, such as actuators or motors.

"Think of any machine that requires a torque or twist to be transmitted: driveshafts, motors, etc.," said Munday. "The Casimir torque can do this on a nanoscale."

Knowing the amount of Casimir torque in a system can also help researchers understand the motions of nanoscale parts powered by the Casimir effect.

The team tested a few different types of solids to measure their Casimir torques, and found that each material has its own unique signature of Casimir torque.

The measurement devices were built in UMD's Fab Lab, a shared user facility and cleanroom housing tools to make nanoscale devices.

In the past, the researchers also made the first measurements of a repulsive Casimir force and a measurement of the Casimir force between two spheres. They have also made some predictions that could be confirmed if the current measurement technique can be refined; Munday reports they are testing other materials to control and tailor the torque.

Munday is an associate professor of electrical and computer engineering in UMD's A. James Clark School of Engineering, and his lab is housed in UMD's Institute for Research in Electronics and Applied Physics, which enables interdisciplinary research between its natural science and engineering colleges.

"Experiments like this are helping us better understand and control the quantum vacuum. It's what one might call 'the physics of empty space,' which upon closer examination seems to be not so empty after all," said John Gillaspy, the physics program officer who oversaw National Science Foundation funding of the research.

"Classically, the vacuum is really empty—it is, by definition, the absence of anything," said Gillaspy. "But quantum physics predicts that even the most empty space that one can imagine is filled with 'virtual' particles and fields, quantum fluctuations in pure emptiness that lead to subtle, but very real, effects that can be measured and even exploited to do things that would otherwise be impossible. The universe contains many complicated things, yet there are still unanswered questions about some of the simplest, most fundamental phenomena—this research may help us to find some of the answers."

 Original story:

Four New Gravitational Wave Events Detected from Black Hole Mergers

GWTC 1 masses.croppedThe Virgo Collaboration and the LIGO Scientific Collaboration, which includes UMD Physics Professors Peter Shawhan and Alessandra Buonanno, announced the detection of four new gravitational wave events from black hole mergers. (Image: LIGO-Virgo/Frank Elavsky/Northwestern) 

 University of Maryland physicists contribute to identification of events that now total 10 black hole mergers and one neutron star merger


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.

Mountaintop Observatory Sees Gamma Rays from Exotic Milky Way Object

Wednesday, October 3, 2018

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

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