Using scanning tunneling microscopy, researchers have seen the morphology of quantum dots on bilayer graphene for the first time, an important step forward in the development of quantum information technology.
Electrons confined within bilayer graphene quantum dots are promising platforms for quantum information technology. In the study, published Nov. 23 in Nano Letters, researchers at the University of California, Santa Cruz, visualized for the first time the morphology of quantum dots within a bilayer graphene material, demonstrating the wave function characteristics of confined electrons.
Jairo Velasco, assistant professor of physics, one of the researchers, said, “Although a lot of work has been done previously to develop this system (the bilayer graphene quantum dot system) for the development of quantum information technology, we have never known the morphology of electrons within these quantum dots.”
While traditional digital information is encoded in the basic unit of 0 and 1, the digit bits, quantum computers use quantum digits on top of these two encodings and a quantum superposition state that is both 0 and 1. This will dramatically increase the speed and ability of quantum computers to process information.
From diamond to gallium arsenide, scientists are experimenting with many different materials to build quantum bits. Double-layered graphene is also an attractive one, with each layer being a two-dimensional structure of carbon atoms arranged in a honeycomb crystal form. This material is easy to fabricate and manipulate, and the quantum dots in it have all the desired properties.
Velasco said, “Such quantum dots have suppressed spin decoherence, controllable quantum degrees of freedom, and these properties that can be tuned by an applied voltage, making them promising as a recent emerging platform for quantum information technology.”
And understanding the wavefunction of quantum dots within graphene materials is one of the key foundations for the development of quantum information technology. This is because this fundamental property determines many relevant features of the quantum information processing process, such as the electron energy spectrum, the interaction between electrons, and the binding of electrons to their surroundings.
The researchers saw from the imaging of the scanning tunneling microscope that “the electric field creates a wall, as if it were an invisible electronic fence, confining the electrons within the quantum dot.” Velasco said.
Quantum dots exhibit a triple symmetry structure, meaning that every 120 degrees in a plane has the same shape. Inside the “electron wall,” three spike patterns appear. This was surprising to the researchers because it was a completely different shape from the concentric rings they saw within the single-layer graphene.
“We see symmetrical concentric rings within single-layer graphene, but the quantum dots within bilayer graphene show a triple symmetry pattern.” Velasco said, “These spikes represent the high points of amplitude of the wave function.”
The study provides important information needed to develop quantum devices based on such systems. Velasco said, “We have taken our understanding of this system one step further, adding another small piece to the puzzle, and combined with the work of other colleagues, I believe we are well on our way to building a useful system.”
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