What makes a university physics lab tick? Sean Kelley grabs a mic and heads to a lab that's trying to build an early quantum computer out of atomic ions. Marko Cetina and Kai Hudek, two research scientists at the University of Maryland who run the lab, explain what it takes to keep things from burning down and muse about the future of quantum computers.

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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)

RESEARCH CONTACT
Victor Galitski

MEDIA CONTACT
Chris Cesare

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The Lorraine DeSalvo Chair's Endowed Award for Outstanding Service recognizes employees who provide benefit beyond their regular duties, promote positive professional and personal exchanges among colleagues, and work effectively within the Department of Physics and/or with outside contacts. Nominees must have been employed within the Department of Physics for a minimum of one year.

The Sibylle Sampson Award was established by Sibylle Sampson, a long-time Physics employee and valued assistant to former Chair John S. Toll. It spotlights an employee whose one-time accomplishment otherwise might go unrecognized. The employee’s undertaking will have benefited the entire Department or a specific unit, and shown creativity, initiative or self-motivation.  Nominees must have been employed within the Department of Physics for a minimum of one year. Two nominators are needed for the Sibylle Sampson Award.

The Staff Excellence Award recognizes employees who excel in job performance, provide a friendly and supportive atmosphere, and display a personal commitment to the Department and to positive workplace morale. Up to three awards may be given.  Nominees must have been employed within the Department of Physics for a minimum of one year.

Christopher Monroe, Distinguished University Professor & Bice Seci-Zorn Professor was elected to the U.S. National Academy of Sciences 2016. See recorded video http://www.nasonline.org/NAS154presentation.

Membership of the National Academy of Sciences is one the greatest and highest honors that a scientist can receive.  "The NAS membership totals approximately 2,290 members and 490 foreign associates, of whom approximately 200 have received Nobel prizes."  - NAS

Read more about Monroe's accomplishments.

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)
RESEARCH CONTACT
Norbert Linke|

Christopher Monroe|

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