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

Researchers used a scanning tunneling microscope to visualize bilayer quantum dots graphene, an important step towards quantum information technologies.

The trapping and control of electrons in quantum dots of bilayer graphene offers a promising platform for quantum information technology. Researchers at UC Santa Cruz have now performed the first direct visualization of quantum dots in bilayer graphene, revealing the shape of the quantum wave function of trapped electrons.

The results, published on November 23, 2020, in Nano Letters, provide important fundamental knowledge necessary to develop quantum information technologies based on quantum dots of bilayer graphene.

“There has been a lot of work to develop this system for quantum information science, but we haven’t figured out what the electrons look like in these quantum dots,” said corresponding author Jairo Velasco Jr., assistant professor of physics at UC Santa Cruz.

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

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

“These quantum dots are an emerging and promising platform for quantum information technology due to 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 graphene is important because this basic property determines several characteristics relevant to quantum information processing, such as electron energy spectrum, 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 graphene using a scanning tunneling microscope (STM). Since the graphene sits on an insulating hexagonal boron nitride crystal, a high voltage applied with the STM tip creates charges in the boron nitride which serve to electrostatically confine the electrons in the bilayer graphene.

“The electric field creates a corral, like an invisible electric fence, that traps electrons in the quantum dot,” Velasco explained.

The researchers then used the scanning tunneling microscope to image the electronic states inside and outside the corral. Contrary to theoretical predictions, the resulting images showed broken rotational symmetry, with three peaks instead of the expected concentric rings.

“We see circularly symmetrical rings in single-layer graphene, but in bilayer graphene the quantum dot states have triple symmetry,” Velasco said. “The peaks represent high amplitude sites 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 crucial information, such as the energy spectrum of electrons, needed to develop quantum devices based on this system. “It advances the fundamental understanding of the system and its potential for quantum information technology,” Velasco said. “This is a missing piece of the puzzle, and given the work of others, I think we are heading towards a useful system.”

Reference: “Visualization and Manipulation of Bilayer Graphene 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 article include co-first authors Zhehao Ge, Frederic Joucken and Eberth Quezada-Lopez of UC Santa Cruz, as well as co-authors of the Federal University of Ceara, Brazil, the Institute National Materials Sciences of Japan, University of Minnesota and Baskin School of Engineering at UCSC. This work was funded by the National Science Foundation and the Army Research Office.