Visualization
of quantum dots in bilayer graphene using scanning tunneling microscopy and
spectroscopy reveals a three-fold symmetry. In this three-dimensional image,
the peaks represent sites of high amplitude in the waveform of the trapped
electrons.
Image: Zhehao Ge, Frederic Joucken, and Jairo Velasco Jr.).
Trapping
and controlling electrons in bilayer graphene quantum dots yields a promising
platform for quantum information technologies. Researchers at UC Santa Cruz
have now achieved 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 in Nano Letters ("Visualization and Manipulation of
Bilayer Graphene Quantum Dots with Broken Rotational Symmetry and Nontrivial
Topology"), provide important fundamental knowledge needed to develop
quantum information technologies based on bilayer graphene quantum dots.
"There
has been a lot of work to develop this system for quantum information science,
but we've been missing an understanding of 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 as
either 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
enable a massive increase in computing speed and 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 emergent and promising platform for quantum information
technology because of their suppressed spin decoherence, controllable quantum
degrees of freedom, and tunability 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 relevant features 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 developed previously to create quantum dots in
monolayer graphene using a scanning tunneling microscope (STM). With the
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 corral, like an invisible electric fence, that traps
the electrons in the quantum dot," Velasco explained.
The
researchers then used the scanning tunneling microscope to image the electronic
states inside and outside of the corral. 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 symmetric rings in monolayer graphene, but in bilayer graphene
the quantum dot states have a three-fold symmetry," Velasco said.
"The peaks represent sites of high amplitude 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 the electrons,
needed to develop quantum devices based on this system. "It is advancing
the fundamental understanding of the system and its potential for quantum
information technologies," Velasco said. "It's a missing piece of the
puzzle, and taken together with the work of others, I think we're moving toward
making this a useful system."