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Visualization of quantum dots in bilayer graphene using microscopy and scanning tunneling spectroscopy reveals triple symmetry. In this three-dimensional image, the peaks represent high-amplitude sites in the waveform of the trapped electrons. Credit: Zhehao Ge, Frederic Joucken and Jairo Velasco Jr.
The researchers used a scanning tunnel microscope to visualize the quantum dots in double layers graphene, an important step towards quantum information technologies.
The entrapment and control of electrons in quantum dots of bilayer graphene produces a promising platform for quantum information technologies. UC Santa Cruz researchers have now obtained the first direct visualization of quantum dots in bilayer graphene, revealing the shape of the quantum wave function of the trapped electrons.
The results, published on 23 November 2020, in Nano Letters, provide important fundamental knowledge needed to develop quantum information technologies based on double-layer graphene quantum dots.
“There has been a lot of work to develop this system for quantum information science, but we lacked an understanding of what 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 as 0 or 1, a quantum bit, or qubit, can represent both states simultaneously due to quantum superposition. In theory, qubit-based technologies will allow for a huge increase in speed and processing capacity for certain types of computations.
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 manufacture and process, 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 tuning with external control voltages,” Velasco said.
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 the energy spectrum of electrons, interactions between electrons and the coupling of electrons to their environment.
Velasco’s team used a method it had previously developed to create quantum dots in single-layer graphene using a scanning tunneling microscope (STM). With graphene resting on an insulating hexagonal boron nitride crystal, a large voltage applied with the STM tip creates charges in the boron nitride that serve to electrostatically confine electrons in the bilayer graphene.
“The electric field creates a fence, like an invisible electric fence, that traps electrons in the quantum dot,” Velasco explained.
The researchers then used the scanning tunnel microscope to visualize the electronic states inside and outside the enclosure. In contrast to theoretical predictions, the resulting images showed a broken rotational symmetry, with three peaks instead of the expected concentric rings.
“We see circularly symmetrical rings in single-layer graphene, but in double-layer graphene the quantum dot states have triple symmetry,” Velasco said. “The peaks represent high-amplitude sites in the wave function. Electrons have a dual wave-particle nature and we are visualizing 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. “The fundamental understanding of the system and its potential for quantum information technologies is improving,” Velasco said. “It is a missing piece of the puzzle and, taken together with the work of others, I think we are moving to make it a useful system.”
Reference: “Visualization and manipulation of bilayer graphene quantum dots with interrupted rotational symmetry and non-trivial topology” by Zhehao Ge, Frederic Joucken, Eberth Quezada, Diego R. da Costa, John Davenport, Brian Giraldo, Takashi Taniguchi, Kenji Watanabe, Nobuhiko Fr. 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 early authors Zhehao Ge, Frederic Joucken and Eberth Quezada-Lopez of UC Santa Cruz, along with co-authors of the Federal University of Ceara, Brazil, the National Institute for Materials Science in Japan, University of Minnesota and UCSC Baskin School of Engineering. This work was funded by the National Science Foundation and the Army Research Office.
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