Researchers have found that a small stretch is enough to unleash the exotic electrical properties of a newly discovered topological insulator, unshackling a behavior previously locked away at cryogenic temperatures.

The compound, called samarium hexaboride, has been studied for decades. But recently it has enjoyed a surge of renewed interest as scientists first predicted and then discovered that it was a new type of topological insulator—a material that banishes electrical currents from its interior and forces them to travel along its periphery. That behavior only emerges at around 4 degrees above absolute zero, though, thwarting potential applications.

Now, experimentalists at the University of California, Irvine (UCI), working with JQI Fellow Victor Galitski and former JQI postdoctoral researcher Maxim Dzero (now at Kent State University), have found a way to activate samarium hexaboride's cryogenic behavior at much higher temperatures. By stretching small crystals of the metal by less than a percent, the team was able to spot the signature surface currents of a topological insulator at 240 K (minus 33 C)—nearly room temperature and, in any case, a far cry from 4 K. The currents even persisted once the strain was removed.

Their technique, which was recently reported in Nature Materials, uses piezoelectric elements that bend when they are fed with an electric current. By suspending a sample of samarium hexaboride between two titanium supports and pulling on one side, researchers could measure the crystal's electrical properties for different temperatures and amounts of stretch.

Last year, Galitski partnered with the same experimental group at UCI and discovered a potential application for samarium hexaboride's unusual surface currents. They found that holding a small crystal at a fixed voltage could produce oscillating currents on its surface. Such tick-tock signals are at the heart of modern digital electronics, but they typically require clocks that are much larger than the micron-sized crystals.

The new result might make such applications more likely, and it could even be achieved without any piezo elements. It may be possible to grow samarium hexaboride as a thin film on top of another material that would naturally cause it to stretch, the researchers say.

REFERENCE PUBLICATION
"Surface-dominated conduction up to 240K in the Kondo insulator SmB6 under strain," A. Stern, M. Dzero, V.M. Galitski, Z. Fisk, J. Xia, Nature Materials, advance online publication, – (2017)

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Victor Galitski

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An artist's rendering of many linked trapped-ion modules. Researchers at JQI put one of their modules to the test against an IBM superconducting device. (Credit: E. Edwards/JQI)

The race to build larger and larger quantum computers is heating up, with several technologies competing for a role in future devices. Each potential platform has strengths and weaknesses, but little has been done to directly compare the performance of early prototypes. Now, researchers at the JQI have performed a first-of-its-kind benchmark test of two small quantum computers built from different technologies.

The team, working with JQI Fellow Christopher Monroe and led by postdoctoral researcher Norbert Linke, sized up their own small-scale quantum computer against a device built by IBM. Both machines use five qubits—the fundamental units of information in a quantum computer—and both machines have similar error rates. But while the JQI device relies on chains of trapped atomic ions, IBM Q uses coupled regions of superconducting material.

To make their comparison, the JQI team ran several quantum programs on the devices, each of which solved a simple problem using a series of logic gates to manipulate one or two qubits at a time. Researchers accessed the IBM device using an online interface, which allows anyone to try their hand at programming IBM Q.

Both computers have strengths and weaknesses. For example, the superconducting platform has quicker gates and may be easier to mass produce, but its man-made qubits are all slightly different and have shorter lifetimes. Monroe says that the slower gates of ions might not be a major hurdle, though. "Because there is time," Monroe says. "Trapped ion qubit lifetimes are way longer than any other type of qubit. Moreover, the ion qubits are identical, and they can be better replicated without error."

When put to the test, researchers found that the trapped-ion module was more accurate for programs that involved many pairs of qubits. Linke and Monroe attribute this to the simple fact that every qubit in their device is connected to every other—meaning that a logic gate can connect any pair of qubits. IBM Q has fewer than half the connections of its JQI counterpart, and in order to run some programs it had to shuffle information between qubits—a step that introduced errors into the calculation. When this shuffling wasn't necessary, the two computers had similar performance. "As we build larger systems, connectivity between qubits will become even more important," Monroe says.

The new study, which was recently published in Proceedings of the National Academy of Sciences, provides an important benchmark for researchers studying quantum computing. And such head-to-head comparisons will become increasingly important in the future. "If you want to buy a quantum computer, you'll need to know which one is best for your application," Linke says. "You'll need to test them in some way, and this is the first of this kind of comparison."

By Erin Marshall

REFERENCE PUBLICATION
"Experimental comparison of two quantum computing architectures," N.M. Linke, D. Maslov, M. Roetteler, S. Debnath, C. Figgatt, K.A. Landsman, K. Wright, C. Monroe, Proceedings of the National Academy of Sciences, 114, 3305-3310 (2017)
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Norbert Linke|

Christopher Monroe|

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In our own galaxy and beyond, violent collisions fling a never-ending stream of stuff at the earth, and astrophysicists are eager to learn more about the processes that produce this cosmic barrage.

Researchers from around the world have teamed up to build the High-Altitude Water Cherenkov (HAWC) gammy-ray observatory, an array of hundreds of huge water tanks on a mountain in Mexico. HAWC helps astrophysicists spot active cosmic neighborhoods by capturing the shower of particles created when high-energy packets of light smash into the earth's atmosphere.

Jordan Goodman, HAWC's lead investigator, and Dan Fiorino, a postdoctoral researcher at UMD, tell Chris Cesare about the details of the HAWC experiment and how it promises to fill some gaps in our understanding of the universe. To learn more about HAWC, please visit www.hawc-observatory.org. The collaboration is preparing to publish the first results of its search, and you can read about the details in an upcoming source catalog or a paper about high-energy gamma rays from the Crab Nebula.

This episode of Relatively Certain was produced by Chris Cesare, Sean Kelley and Emily Edwards and edited by Chris Cesare and Kate Delossantos, featuring music by Dave Depper, Podington Bear, Kevin MacLeod and Chris Zabriskie. Relatively Certain is a production of the Joint Quantum Institute and the University of Maryland, and you can find it on iTunes, Google Play or Soundcloud. 

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Consider, for a moment, the humble puddle of water. If you dive down to nearly the scale of molecules, it will be hard to tell one spot in the puddle from any other. You can shift your gaze to the left or right, or tilt your head, and the microscopic bustle will be identical—a situation that physicists call highly symmetric.  

That all changes abruptly when the puddle freezes. In contrast to liquid water, ice is a crystal, and it gains a spontaneous rigid structure as the temperature drops. Freezing fastens neighboring water molecules together in a regular pattern, and a simple tilt of the head now creates a kaleidoscopic change.

In 2012, Nobel-prize winning physicist Frank Wilczek, a professor at the Massachusetts Institute of Technology, proposed something that sounds pretty strange. It might be possible, Wilczek argued, to create crystals that are arranged in time instead of space. The suggestion prompted years of false starts and negative results that ruled out some of the most obvious places to look for these newly named time crystals.

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Cosmic rays are not rays at all, but highly energetic particles that zoom through space at nearly the speed of light.  The particles range in size, from subatomic protons to the atomic nuclei of elements such as carbon and boron. Scientists suspect that the particles are bits of subatomic shrapnel produced by supernovae, but could also be signatures of other cataclysmic phenomena.

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