Boron nitride with a twist could lead to a new way of making qubits

– Rachel Berkowitz

Achieving scalability in quantum processors, sensors and networks requires novel devices that can be easily manipulated between two quantum states. A team led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has now developed a method that uses a ‘twisted’ crystalline layered material in the solid state, creating tiny light-emitting dots called color centers. These color centers can be switched on and off simply by applying an external voltage.

“This is a first step toward a color-center device that engineers could build or convert into true quantum systems,” said Shaul Aloni, a research associate at Berkeley Lab’s Molecular Foundry who co-led the study. The work is described in detail in the journal natural materials.

For example, the research could lead to a new way to make quantum bits, or qubits, that encode information in quantum computers.

Color centers are microscopic defects in a crystal, such as B. a diamond, which normally emit bright and stable light of a certain color when illuminated with a laser or other energy source such. B. an electron beam are taken. Their integration with waveguides, devices that guide light, can connect operations via a quantum processor. A few years ago, researchers discovered that color centers in a synthesized material called hexagonal boron nitride (hBN), commonly used as a lubricant or additive in paints and cosmetics, emit even brighter colors than color centers in diamond. However, engineers had difficulty using the material in applications because of the difficulty in creating the defects at a specific location and the lack of a reliable way to turn the color centers on and off.

The Berkeley Lab team is now solving these problems. Cong Su, a postdoc in the research group of Alex Zettl, senior faculty scientist at Berkeley Lab and professor of physics at UC Berkeley, studied how color centers behave in various advanced forms of hBN. The researchers found that two stacked and twisted layers of the material resulted in the activation and enhancement of ultraviolet (UV) emission from a color center, which can be turned off when a voltage is applied across the structure. “It’s like a sandwich with two pieces of bread, but turned one against the other,” says Zettl. The rotation between the two layers makes the color centers at the interface extremely bright. The applied voltage then simply and reversibly regulates the intensity from light to completely dark without the halves being “unrotated”.

Aloni’s development of a modified electron microscope that not only examined the structure of the material but also collected the emitted light for analysis proved key to this study. The setup uses an electron beam to excite the color centers; The researchers also found they could use the electron beam to activate color centers and create patterns, such as B. a smiley to draw on hBN. “People usually zap the material with lasers or ions, but we tapped it with an electron beam instead,” Zettl said.

The study achieves three steps to realize a scalable quantum device. First, the UV color centers in hBN can be reliably activated to exceptional maximum brightness by twisting the crystal interface. Second, these color centers can then be gradually and reversibly dimmed by a simple applied voltage. Finally, the electron beam treatment enables further precise spatial positioning of these color centers.

Theoretical calculations led by Steven Louie, senior faculty scientist at Berkeley Lab and distinguished physics professor at UC Berkeley, provided candidates for the atomic configuration of the UV color center to explain why their brightness depends on the twist angle. In the light emission process, an excited electron wanders around and recombines with a hole in the color center. But a typical hBN structure has many traps that could trap the electrons and prevent light emission. “Twisting the crystal layers removes a lot of these traps or ‘quantum parking spots’ near the interface,” Louie said.

Next, the team intends to prepare samples that will allow atomic characterization to localize the specific atomic structures behind this mechanism and add additional layers of control. “The work points us in the direction of novel mechanisms by which we can control the emission even better, and this is very important for many applications in quantum information science,” said Aloni.

The Molecular Foundry is a user facility of the Office of Science at Berkeley Lab.

This work was supported by the DOE Office of Science.

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Lawrence Berkeley National Laboratory was founded in 1931 on the belief that the greatest scientific challenges are best tackled by teams, and its scientists have been awarded 14 Nobel Prizes. Today, researchers at the Berkeley Lab develop sustainable energy and environmental solutions, create useful new materials, push the frontiers of computing, and explore the mysteries of life, matter, and the universe. Scientists from around the world rely on the laboratory’s facilities for their own science of discovery. Berkeley Lab is a national multiprogram laboratory administered by the University of California for the US Department of Energy’s Office of Science.

The DOE Office of Science is the largest single funder of basic science research in the United States and works to address some of the most pressing challenges of our time. Visit energy.gov/science for more information.