Research News - UMD Physics

Glass Fibers and Light Offer New Control Over Atomic Fluorescence

Details: Published: Friday, February 01 2019 14:47

Electrons inside an atom whip around the nucleus like satellites around the Earth, occupying orbits determined by quantum physics. Light can boost an electron to a different, more energetic orbit, but that high doesn’t last forever. At some point the excited electron will relax back to its original orbit, causing the atom to spontaneously emit light that scientists call fluorescence.

Scientists can play tricks with an atom’s surroundings to tweak the relaxation time for high-flying electrons, which then dictates the rate of fluorescence. In a new study, researchers at the Joint Quantum Institute observed that a tiny thread of glass, called an optical nanofiber, had a significant impact on how fast a rubidium atom releases light. The research, which appeared as an Editor’s Suggestion in Physical Review A, showed that the fluorescence depended on the shape of light used to excite the atoms when they were near the nanofiber.

“Atoms are kind of like antennas, absorbing light and emitting it back out into space, and anything sitting nearby can potentially affect this radiative process,” says Pablo Solano, the lead author on the study and a University of Maryland graduate student at the time this research was performed.

To probe how the environment affects these atomic antennas, Solano and his collaborators surround a nanofiber with a cloud of rubidium atoms. Nanofibers are custom-made conduits that allow much of the light to travel on the outside of the fiber, enhancing its interactions with atoms. The atoms closest to the nanofiber—within 200 nanometers—felt its presence the most. Some of the fluorescence from atoms in this region hit the fiber and bounced back to the atoms in an exchange that ultimately modified how long a rubidium atom’s electron stayed excited.

The researchers found that the electron lifetime and subsequent atomic emissions depended on the wave characteristics of the light. Light waves oscillate as they travel, sometimes slithering like a sidewinder snake and other times corkscrewing like a strand of DNA. The researchers saw that for certain light shapes the electron lingered in the excited state, and for others, it made a more abrupt exit.

“We were able to use the oscillation properties of light as a kind of knob to control how atomic fluorescence near the nanofiber turned on,” Solano says.

The team originally set out to measure the effects the nanofiber had on atoms, and compare the results to theoretical predictions for this system. They found disagreements between their measurements and existing models that incorporate many of the complex details of rubidium’s internal structure. This new research paints a simpler picture of the atom-fiber interactions, and the team says more research is needed to understand the discrepancies.

"We believe this work is an important step in the on-going quest for a better understanding of the interaction between light and atoms near a nanoscale light-guiding structure, such as the optical nanofiber we used here," says JQI Fellow and NIST scientist William Phillips, who is also one of the lead investigators on the study.

Written by Emily Edwards

Solano is currently a postdoctoral researcher at the MIT-Harvard University Center for Ultra Cold Atoms. In addition, the following researchers were authors on this study.

Cold Atoms Offer a Glimpse of Flat Physics

Details: Published: Monday, January 07 2019 09:45

These days, movies and video games render increasingly realistic 3-D images on 2-D screens, giving viewers the illusion of gazing into another world. For many physicists, though, keeping things flat is far more interesting.

One reason is that flat landscapes can unlock new movement patterns in the quantum world of atoms and electrons. For instance, shedding the third dimension enables an entirely new class of particles to emerge—particles that that don’t fit neatly into the two classes, bosons and fermions, provided by nature. These new particles, known as anyons, change in novel ways when they swap places, a feat that could one day power a special breed of quantum computer.

But anyons and the conditions that produce them have been exceedingly hard to spot in experiments. In a pair of papers published this week in Physical Review Letters, JQI Fellow Alexey Gorshkov and several collaborators proposed new ways of studying this unusual flat physics, suggesting that small numbers of constrained atoms could act as stand-ins for the finicky electrons first predicted to exhibit low-dimensional quirks.

"These two papers add to the growing literature demonstrating the promise of cold atoms for studying exotic physics in general and anyons in particular," Gorshkov says. "Coupled with recent advances in cold atom experiments—including by the grou gorshkov anyon 1a Simulated images from two papers showing anyons spreading preferentially to the left in a 1-D grid (left) and a novel phase of matter that may arise from atoms constrained to move in 2-D (right). (Images courtesy of the authors)p of Ian Spielman at JQI—this work hints at exciting experimental demonstrations that might be just around the corner."

In the first paper, which was selected as an Editors’ Suggestion, Gorshkov and colleagues proposed looking for a new experimental signature of anyons—one that might be visible in a small collection of atoms hopping around in a 1-D grid. Previous work suggested that such systems might simulate the swapping behavior of anyons, but researchers only knew of ways to spot the effects at extremely cold temperatures. Instead, Fangli Liu, a graduate student at JQI, along with Gorshkov and other collaborators, found a way to detect the presence of anyons without needing such frigid climes.

Ordinarily, atoms spread out symmetrically over time in a 1-D grid, but anyons will generally favor the left over the right or vice versa. The researchers argued that straightforward changes to the laser used to create the grid would make the atoms hop less like themselves and more like anyons. By measuring the way that the number of atoms at different locations changes over time, it would then be possible to spot the asymmetry expected from anyons. Furthermore, adjusting the laser would make it easy to switch the favored direction in the experiment.

"The motivation was to use something that didn’t require extremely cold temperatures to probe the anyons," says Liu, the lead author of the paper. "The hope is that maybe some similar ideas can be used in more general settings, like looking for related asymmetries in two dimensions."

In the second paper, Gorshkov and a separate group of collaborators found theoretical evidence for a new state of matter closely related to a Laughlin liquid, the prototypical example of a substance with topological order. In a Laughlin liquid, particles—originally electrons—find elaborate ways of avoiding one another, leading to the emergence of anyons that carry only a fraction of the electric charge held by an electron.

"Anyons are pretty much still theoretical constructs," says Tobias Grass, a postdoctoral researcher at JQI and the lead author of the second paper, "and experiments have yet to conclusively demonstrate them."

Although fractional charges have been observed in experiments with electrons, many of their other predicted properties have remained unmeasurable. This makes it hard to search for other interesting behavior or to study Laughlin liquids more closely. Grass, Gorshkov and their colleagues suggested a way to manipulate the interactions between a handful of atoms and discovered a new state of matter that mixes characteristics of the Laughlin liquid and a less exotic crystal phase.

The atoms in this new state avoid one another in a similar way as electrons in a Laughlin liquid, and they also fall into a regular pattern like in a crystal—albeit in a strange way, with only half of an atom occupying each crystal site. It’s a unique mix of crystal symmetry and more complex topological order—a combination that has received little prior study.

"The idea that you have a bosonic or fermionic system, and then from interactions there emerges completely different physics—that’s only possible in lower dimensions," Grass says. "Having an experimental demonstration of any of these phases is just interesting from a fundamental perspective."

Story by Chris Cesare

Reference Publication

"Asymmetric Particle Transport and Light-Cone Dynamics Induced by Anyonic Statistics," Fangli Liu, James R. Garrison, Dong-Ling Deng, Zhe-Xuan Gong, Alexey V. Gorshkov, Phys. Rev. Lett., 121, 250404 (2018)

"Fractional Quantum Hall Phases of Bosons with Tunable Interactions: From the Laughlin Liquid to a Fractional Wigner Crystal," Tobias Graß, Przemyslaw Bienias, Michael J. Gullans, Rex Lundgren, Joseph Maciejko, Alexey V. Gorshkov, Phys. Rev. Lett., 121, 253403 (2018)

Research Contact: Alexey Gorshkov, This email address is being protected from spambots. You need JavaScript enabled to view it.; Tobias Grass, This email address is being protected from spambots. You need JavaScript enabled to view it.; Fangli Liu, This email address is being protected from spambots. You need JavaScript enabled to view it.

Media Contact: Chris Cesare, This email address is being protected from spambots. You need JavaScript enabled to view it.

Original story: https://jqi.umd.edu/news/cold-atoms-offer-glimpse-flat-physics

Researchers Measure Casimir Torque for the First Time

Details: Published: Thursday, December 20 2018 14:03

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: https://energy.umd.edu/news/story/researchers-make-liquid-crystals-do-the-twist

Four New Gravitational Wave Events Detected from Black Hole Mergers

Details: Published: Monday, December 03 2018 12:20

GWTC 1 masses.cropped The 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

Scientists announced four new observations of gravitational waves—ripples in the fabric of spacetime—from the final moments of black hole mergers.

The twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors—located in Livingston, Louisiana, and Hanford, Washington—and the Virgo detector located near Pisa, Italy detected the gravitational wave events. The Virgo Collaboration and the LIGO Scientific Collaboration (LSC) announced the discoveries on December 1, 2018, at the Joint Space-Science Institute’s Gravitational Wave Physics and Astronomy Workshop hosted by the University of Maryland in College Park, Maryland. Two scientific papers describing these new findings have been initially published on the arXiv repository of electronic preprints and include a catalog of all gravitational wave detections and candidate events observed to date.

“These results mark an evolution in the way we are thinking about binary black hole mergers detected by LIGO and Virgo,” said Peter Shawhan, a UMD professor of physics and an LSC principal investigator who serves as the LSC’s data analysis committee chair. “While we carefully determine the properties of the individual events, such as the masses and spins of the black holes, we are also looking at the big picture: the distribution of these properties and what that can tell us about how massive stars live and die.”

Gravitational waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained. During a two-year span, physicists on the LIGO and Virgo teams detected gravitational waves from 10 black hole mergers and one merger of neutron stars, which are the dense, spherical remains of stellar explosions. The four new observations—named GW170729, GW170809, GW170818 and GW170823 for the dates they were detected—include some record breakers.

One of the new events, GW170729, is the most massive and distant gravitational wave source ever observed. This black hole merger, which happened roughly 5 billion years ago, transformed an equivalent energy of almost five solar masses into gravitational energy.

Another new event, GW170818, was triangulated well in the sky by the LIGO and Virgo detectors, making it the second-best localized gravitational wave source after the neutron star merger. The position of the binary black holes, located 2.5 billion light-years from Earth, was identified in the sky with a precision of 39 square degrees.

A major contributor to this accomplishment was Alessandra Buonanno—a UMD College Park Professor of Physics and LSC principal investigator who also has an appointment as director at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. Buonanno has led the effort to develop highly accurate models of gravitational waves that black holes generate in the final process of orbiting and colliding with each other. The scientists use these waveform models to localize the source in the sky and identify it as a pair of orbiting black holes.

“State-of-the-art waveform models, advanced data processing and better calibration of the instruments have allowed us to infer astrophysical parameters of previously announced events more accurately and discover four new gravitational wave transients from black hole mergers,” Buonanno said. “I look forward to the next observing run in spring 2019, where we expect to detect more than two black hole mergers per month of collected data.”

The scientific papers describing the new findings include a catalog of all gravitational wave detections and candidate events observed from September 12, 2015 to August 25, 2017.

Scientists observed GW170817—the merger of two neutron stars—in both gravitational waves and light. Shawhan and his students at UMD worked with other LIGO and Virgo team members to establish a program to quickly share information about each gravitational wave event candidate, including sky location, with astronomers. This enabled astronomers to look for the event with their telescopes and other instruments, marking an exciting new chapter in multi-messenger astronomy, a field in which cosmic objects are observed simultaneously in different forms of radiation.

“The one neutron star merger in the catalog, GW170817, may look a bit lonely, but we have learned so many things about it by looking at the gravitational wave data together with the incredibly rich range of follow-up observations,” said Shawhan, who is also a fellow of the Joint Space-Science Institute. “Still, we have a lot of unanswered questions about the population of binary neutron stars that future data should fill in for us.”

In one of the two new papers, the scientists carefully investigate the characteristics of the merging black hole population. Most notably, the researchers found that almost all black holes formed from stars are lighter than 45 times the mass of the sun.

“The LIGO and Virgo collaborations have worked hard to release the event properties and also the data in which these signals were found so that other scientists can analyze them with their own tools and compare them with theories of stellar evolution and gravitational wave emission,” Shawhan added.

The first detection of gravitational waves, observed on September 14, 2015, was a milestone in physics and astronomy. It confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity and marked the beginning of the new field of gravitational wave astronomy.

###

This press release was adapted from text provided by the LIGO and Virgo Collaborations.

The research paper, “GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs,” by the LIGO Scientific Collaboration and the Virgo Collaboration, was presented December 1, 2018 at the Joint Space-Science Institute–Gravitational Wave Physics and Astronomy Workshop and is available on the arXiv.

The research paper, “Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo,” by the LIGO Scientific Collaboration and the Virgo Collaboration, was presented December 1, 2018 at the Joint Space-Science Institute–Gravitational Wave Physics and Astronomy Workshop and is available on the arXiv.

Media Relations Contact: Abby Robinson, 301-405-5845, This email address is being protected from spambots. You need JavaScript enabled to view it.

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About LIGO and Virgo
LIGO is funded by the National Science Foundation (NSF) and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. More than 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo collaboration consists of more than 300 physicists and engineers belonging to 28 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; 11 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in the Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with IFAE and the Universities of Valencia and Barcelona; two in Belgium with the Universities of Liege and Louvain; Jena University in Germany; and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy, funded by CNRS, INFN, and Nikhef. A list of the Virgo Collaboration can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at www.virgo-gw.eu.

About the College of Computer, Mathematical, and Natural Sciences
The College of Computer, Mathematical, and Natural Sciences at the University of Maryland educates more than 9,000 future scientific leaders in its undergraduate and graduate programs each year. The college's 10 departments and more than a dozen interdisciplinary research centers foster scientific discovery with annual sponsored research funding exceeding $175 million.

Fast-flowing electrons may mimic astrophysical dynamos

Details: Published: Monday, October 29 2018 13:06

dynamo galitski1 blue gallery Certain 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

Glass Fibers and Light Offer New Control Over Atomic Fluorescence

Cold Atoms Offer a Glimpse of Flat Physics

Researchers Measure Casimir Torque for the First Time

Four New Gravitational Wave Events Detected from Black Hole Mergers

Fast-flowing electrons may mimic astrophysical dynamos

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