These two materials, which make up the so-called ‘quantum dot’, each composed of a single atomic layer and the tip of a scanning tunnelling microscope.
These nanostructures are said to allow delicate control of individual electrons by fine-tuning their energy levels directly.
“For many applications in the field of quantum technologies one requires a quantum system were electrons occupy two states – similar to a classical switch – on or off, with the difference that quantum physics also allows for arbitrary superpositions of the on and off states,“ said Florian Libisch from TU Wien.
Libisch explained that a key property of such systems is the energy difference between those two quantum states.
“Efficiently manipulating the information stored in the quantum state of the electrons requires perfect control of the system parameters,” he continued. “An ideal system allows for continuous tuning the energy difference from zero to a large value.”
With systems found in nature, for example atoms, this is usually difficult to realise as the energies of atomic states are fixed. Tuning energies becomes possible in synthetic nanostructures engineered towards confining electrons and such structures are often referred to as quantum dots or ‘artificial atoms’.
Together with RWTH Aachen and the University of Manchester, TU Wien claimed it has developed a quantum dot that allows more accurate and widely tunable energy levels of confined electrons.
Like graphene, boron nitride forms a honeycomb lattice, but these are no of exact equal size. “If you carefully put a single layer of graphene on top of hexagonal boron nitride, the layers cannot perfectly match,” Libisch said. “This slight mismatch creates a superstructure over distances of several nanometers, which results in an extremely regular wave-like spatial oscillation of the graphene layer out of the perfect plane.”
TU Wien claimed that in its simulations, it demonstrated exactly these oscillations in graphene on hexagonal boron nitride form the ideal scaffold to control electron energies.
According to the researchers, the potential landscape created by the regular superstructure allows for accurately placing the quantum dot, or even moving it continuously and thus, smoothly changing its properties.
Depending on the exact position of the tip of the scanning tunnelling microscope, the energy levels of the electronic states inside the quantum dot change.
“A shift by a few nanometers allows for changing the energy difference of two neighboring energy levels from minus five to plus ten millielectronvolts with high accuracy – a tuning range about fifty times larger than previously possible,” continued Florian Libisch.
The researchers said that as a next step, the tip of the scanning tunneling microscope could be replaced by a series of nanoelectronic gates. This, TU Wien explained, would allow for exploiting the quantum dot states of graphene on hexagonal boron nitride for scalable quantum technologies such as ‘valleytronics’.
