Direct visualization of quantum dots reveals the shape of the quantum wave function of trapped electrons

Visualization of quantum dots in two-layer graphene using scanning tunneling microscopy and spectroscopy reveals a triple symmetry. In this three-dimensional image, the peaks represent places with high amplitudes in the form of trapped electron waves. Credit: Zhehao Ge, Frederic Joucken and Jairo Velasco Jr.

Researchers used a scanning microscope tunneling to visualize quantum dots in two layers Grafen, an important step towards quantum information technologies.

Blocking and controlling electrons at 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-tier graphene, revealing the shape of the quantum wave function of trapped electrons.

Results, published on November 23, 2020, in Letter Nano, provide the essential basic knowledge needed to develop quantum information technology based on two-layer graphene quantum dots.

“There has been a lot of work to develop this system for quantum information science, but we have failed to understand what electrons look like at these quantum dots,” said correspondent Jairo Velasco Jr., assistant professor of physics at UC Santa Cruz.

While conventional digital technologies encode information in bits represented as 0 or 1, a quantum bit, or qubit, can represent both states at the same time due to quantum superposition. In theory, dome-based technologies will enable a massive increase in computing speed and capacity for certain types of computing.

A variety of systems, based on materials ranging from diamond to gallium arsenide, are being explored as platforms for creating and manipulating cubes. Bilayer graphene (two-layer graphene, which is a two-dimensional arrangement of carbon atoms in a honeycomb mesh) is an attractive material because it is easy to produce and work with, and quantum dots in two-layer graphene have desirable.

“These quantum dots are an emerging and promising platform for quantum information technology because of their suppressed rotational decoherence, controllable quantum degrees of freedom, and adaptability to external control voltages,” Velasco said.

Understanding the nature of the dot quantum wave function in a two-layer graph is important because this fundamental property defines some important features for the processing of quantum information, such as electron energy spectrum, electron interactions, and electron coupling in the environment. them.

The Velasco team used a method he had previously developed to create quantum dots in single-layer graphene using a tunnel scanning microscope (STM). With graphene resting on a hexagonal boron nitrite insulating crystal, a large voltage applied to the STM tip creates charges on the boron nitride that serve to electrostatically limit the electrons in the two-layer graphene.

“The electric field creates a coral, like an invisible electrical fence, that blocks electrons at the quantum point,” Velasco explained.

The researchers then used the scanning tunneling microscope to image electron states inside and outside the pit. In contrast to the theoretical predictions, the resulting images showed a broken rotational symmetry, with three vertices instead of the expected concentric rings.

“We see symmetrical circular rings in single-layer graphene, but in two-layer graph, the quantum states of the points have a triple symmetry,” Velasco said. “Peaks represent areas with high amplitudes in the wave function. Electrons have a dual nature of wave particles and we are visualizing the properties of electron waves at the quantum point. “

This work provides the essential information, such as the energy spectrum of electrons, needed to develop quantum devices based on this system. “It is advancing the basic understanding of the system and its potential for quantum information technologies,” Velasco said. “It’s part of the mystery that is missing and taken along with the work of others. I think we are moving towards making this system useful.”

Reference: “Visualization and manipulation of Bilayer Graphene quantum dots with broken rotational symmetry and non-trivial topology” by Zhehao Ge, Frederic Joucken, Eberth Quezada, Diego R. da Costa, John Davenport, Brian Giraldo, Takashi Taniguchi, Nob Kuhiko Watan Kobayashi, Tony Low and Jairo Velasco Jr., November 23, 2020, Letter Nano.
DOI: 10.1021 / acs.nanolett.0c03453

In addition to Velasco, the authors of the paper include co-authors Zhehao Ge, Frederic Joucken and Eberth Quezada-Lopez at UC Santa Cruz, along with co-authors at the Federal University of Ceara, Brazil, the National Institute of Materials Science in Japan, the University of Minnesota and the School of Baskin Engineering of UCSC. This work was funded by the National Science Foundation and the Army Research Office.

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