The visualization of quantum dots in two-layer graphs using scanning tunneling microscopy and spectroscopy reveals a triple symmetry. In this three-dimensional image, the peaks represent locations of high amplitude in the waveform of the trapped electrons. Photo credit: Zhehao Ge, Frederic Joucken, and Jairo Velasco Jr.
The researchers used a scanning tunneling microscope to make quantum dots visible in a double layer Graph, an important step towards quantum information technologies.
Trapping and controlling electrons in two-layer graphene quantum dots provides a promising platform for quantum information technologies. Researchers at UC Santa Cruz have now achieved the first direct visualization of quantum dots in two-layer graphs, revealing the shape of the quantum wave function of the trapped electrons.
The results published on November 23, 2020 in Nano lettersprovide important basic knowledge that is required for the development of quantum information technologies based on two-layer graph quantum dots.
“Much work has been done to develop this system for quantum information science, but we have no understanding of what the electrons in these quantum dots look like,” said author Jairo Velasco Jr., assistant professor of physics at UC Santa Cruz.
While conventional digital technologies encode information in bits that are represented as either 0 or 1, a quantum bit or qubit can represent both states at the same time due to the quantum superposition. In theory, qubit-based technologies will allow massive increases in computing speed and capacity for certain types of computation.
A variety of systems based on materials from diamond to gallium arsenide are being explored as platforms for the creation and manipulation of qubits. Bilayer graphene (two layers of graphene, a two-dimensional array of carbon atoms in a honeycomb lattice) is an attractive material because it is easy to make and process, and quantum dots in bilayer graphs have desirable properties.
“These quantum dots are an emerging and promising platform for quantum information technology because of their suppressed spin decoherence, controllable quantum degrees of freedom and tunability with external control voltages,” said Velasco.
Understanding the nature of the quantum dot wave function in bilayer graphs is important as this fundamental property determines several relevant characteristics for quantum information processing, such as the electron energy spectrum, the interactions between electrons, and the coupling of electrons to their environment.
Velasco’s team used a method he had previously developed to create quantum dots in monolayer graphs using a scanning tunneling microscope (STM). When the graph rests on an insulating hexagonal boron nitride crystal, a large voltage applied to the STM tip creates charges in the boron nitride that serve to electrostatically trap electrons in the bilayer graph.
“The electric field creates a corral like an invisible electric fence that traps the electrons in the quantum dot,” explained Velasco.
The researchers then used the scanning tunneling microscope to map the electronic states inside and outside the corral. Contrary to theoretical predictions, the resulting images showed a broken rotational symmetry with three peaks instead of the expected concentric rings.
“We see circularly symmetric rings in monolayer graphs, but in bilayer graphs the quantum dot states have triple symmetry,” Velasco said. “The peaks represent locations with a high amplitude in the wave function. Electrons have a double wave particle nature and we visualize the wave properties of the electron in the quantum dot. ”
This work provides important information such as the energy spectrum of the electrons, which is required for the development of quantum devices based on this system. “It promotes a basic understanding of the system and its potential for quantum information technologies,” said Velasco. “It’s a missing piece of the puzzle, and along with the work of others, I think we are trying to make this system a useful one.”
Reference: “Visualization and manipulation of two-layer graph quantum dots with broken rotational symmetry and nontrivial topology” by Zhehao Ge, Frederic Joucken, Eberth Quezada, Diego R. da Costa, John Davenport, Brian Giraldo, Takashi Taniguchi, Kenji Watanabe, Nobuhiko P. Kobayashi, Tony Low and Jairo Velasco Jr., November 23, 2020 Nano letters.
DOI: 10.1021 / acs.nanolett.0c03453
In addition to Velasco, the authors of the paper include co-first authors Zhehao Ge, Frederic Joucken and Eberth Quezada-Lopez from UC Santa Cruz as well as co-authors from the Federal University of Ceara, Brazil, the National Institute for Materials Science in Japan, the University of Minnesota and Baskin School of Engineering from UCSC. This work was funded by the National Science Foundation and the Army Research Office.
