UCSB Researchers Break New Ground in Quantum Mechanics

In a recent study, UCSB physicists were the first to successfully demonstrate how defects in silicon carbide can be controlled quantum mechanically at room temperature. The physicists’ achievement is particularly important since the ability to control electrons at a fundamental level is the key to developing quantum computers and ultra sensitive nanoscale detectors.

Lead researcher professor David Awschalom, director of both the California NanoSystems Institute and UCSB’s Center for Spintronics & Quantum Computation, worked on the study with graduate students William Koehl, Bob Buckley, Joseph Heremans and Greg Calusine. The results were published this month in Nature.

Until now, command over the spins of electrons could only be realized near absolute zero, when the electrons are in their ground state. Some control, however, was available in diamonds, where electrons are trapped in holes between carbon atoms called nitrogen-vacancy centers. The Awschalom group found that silicon carbide, a widely used semiconductor, contains defects allowing electrons in these nitrogen-vacancy centers to be manipulated using laser light and microwave pulses. Upon exposure to either laser light or microwave pulses, electrons are elevated to higher energy levels and emit a photon that provides scientists with information regarding the excited electrons’ spin.

According to Awschalom, the team implemented previous knowledge and existing quantum mechanics theories in order to successfully control electrons in silicon carbide.

“One of the reasons we went to silicon carbide was the result of a pretty significant theoretical effort we undertook a year or two ago to understand what kind of materials could host quantum mechanical effects,” Awschalom said. “We worked with professor Chris van de Walle … who embraced this idea of screening. We used density functional theory … a theory that enables you to calculate the structure of materials. [Density functional theory] was what Walter Kohn at UCSB worked on to win the Nobel Prize. We’re delighted with how well it worked.”

The defects found in silicon carbide may be especially important for their use in quantum mechanic technologies.

“The defects, which normally trap electrons and are problematic for classical electronics, are exactly what you want for quantum technology, where you want to hold the electron and not allow it to flow,” Koehl said. “I’m excited to see what other people can do, and what kinds of novel devices people can make that exploit quantum mechanics to do something useful.”

Despite the experiment’s achievement, involved researchers did not expect to succeed at the outset. According to Koehl, the project was a gamble.

“The only defect people knew of [like this] was the diamond one,” Koehl said. “We kind of had a hunch that there could be defects in other systems that had similar properties, that maybe it wasn’t such a rare thing.”

The future of the group will be in controlling single electrons rather than an ensemble and engineering devices with individually addressable divacancy defects to enable qubit reliance. If both future goals are met, the use of defects in silicon carbide may further the development of quantum mechanic technology, including its role in computers.