Light-controlled ‘quantum Etch-a-Sketch’ could lead to advanced computers and quantum microchips

October 14, 2015

Artist’s rendition of optically defined quantum circuits in a topological insulator (credit: Peter Allen)

Penn State University and University of Chicago researchers say an accidental discovery of a “quantum Etch-a-Sketch” may lead to a new way to use beams of light to draw and erase quantum circuits, and that could lead to the next generation of advanced computers and quantum microchips.

The new technique is based on “topological insulators” (a material that behaves as an insulator in its interior but whose surface contains conducting states, meaning that electrons can only move along the surface of the material). The electrons in topological insulators have unique quantum properties that many scientists believe will be useful for developing spin-based electronics (such as disk drives) and quantum computers.

However, making even the simplest experimental circuits with topological insulators has proved difficult because traditional semiconductor engineering techniques tend to destroy their fragile quantum properties. Even a brief exposure to air can reduce their quality.

The researchers have now discovered a rewriteable “optical fabrication” process that allows them to “tune” the energy of electrons in these materials using light instead of chemicals — without ever having to touch the material itself. They used this effect to draw and erase one of the central components of a transistor — the p-n junction — in a topological insulator for the first time.

An accidental discovery

Optical fabrication (draw/erase) of a topological insulator (credit: Andrew L. Yeats et al./Science Advances)

Curiously, the scientists made the discovery when they noticed that a particular type of fluorescent light in the lab caused the surface of strontium titanate (the substrate material on which they had grown their samples) to become electrically polarized by ultraviolet light. The room lights happened to emit it at just the right wavelength. It turned out that the electric field from the polarized strontium titanate was leaking into the topological insulator layer, changing its electronic properties.

They found by intentionally focusing beams of light on their samples, they could draw electronic structures that persisted long after the light was removed. “It’s like having a sort of quantum Etch-a-Sketch in our lab,” said said David D. Awschalom, Liew Family Professor and deputy director in the Institute of Molecular Engineering at the University of Chicago. They also found that bright red light counteracted the effect of the ultraviolet light, allowing the researchers to both write (with UV) and erase (with red light).

“Instead of spending weeks in the clean room and potentially contaminating our materials, now we can sketch and measure devices for our experiments in real time,” said Awschalom. “When we’re done, we just erase it and make something else. We can do this in less than a second.”

To test whether the new technique might interfere with the unique properties of topological insulators, the team measured their samples in high magnetic fields. They found promising signatures of an effect called “weak anti-localization,” which arises from quantum interference between the different simultaneous paths that electrons can take through a material when they behave as waves.

To better understand the physics behind the effect, the researchers conducted a number of control measurements, which showed that the optical effect is not unique to topological insulators; it can also act on other materials grown on strontium titanate.

“In a way, the most exciting aspect of this work is that it should be applicable to a wide range of nanoscale materials such as complex oxides, graphene, and transition metal dichalcogenides,” said Awschalom. “It’s not just that it’s faster and easier. This effect could allow electrical tuning of materials in a wide range of optical, magnetic, and spectroscopic experiments where electrical contacts are extremely difficult or simply impossible.”

The research was published October 9, 2015 in an open-access paper in a new AAAS journal, Science Advances.

Abstract of Persistent Optical Gating of a Topological Insulator

The spin-polarized surface states of topological insulators (TIs) are attractive for applications in spintronics and quantum computing. A central challenge with these materials is to reliably tune the chemical potential of their electrons with respect to the Dirac point and the bulk bands. We demonstrate persistent, bidirectional optical control of the chemical potential of (Bi,Sb)2Te3 thin films grown on SrTiO3. By optically modulating a space-charge layer in the SrTiO3 substrates, we induce a persistent field effect in the TI films comparable to electrostatic gating techniques but without additional materials or processing. This enables us to optically pattern arbitrarily shaped p– and n-type regions in a TI, which we subsequently image with scanning photocurrent microscopy. The ability to optically write and erase mesoscopic electronic structures in a TI may aid in the investigation of the unique properties of the topological insulating phase. The gating effect also generalizes to other thin-film materials, suggesting that these phenomena could provide optical control of chemical potential in a wide range of ultrathin electronic systems.