The behavior of electrons in tiny quantum "prisons" is very different from their performance in free space. They can only occupy discrete energy levels, just like the electrons in an atom - for this reason, the electronic "prison" is often called the "artificial atom." Artificial atoms may also have properties that traditional atoms do not have, giving them potential for many applications such as quantum computing. Some additional properties of this class now appear in the artificial atoms of graphene. The study was published in the journal Nano Letters, a collaboration of scientists from the Vienna University of Technology in Vienna, Austria, RWTH Aachen (Germany) and the University of Manchester (UK).

Create artificial atoms

"Artificial atoms open up new and exciting possibilities because we can directly adjust their performance," said Professor Joachim Burgdorfer at the Technical University of Vienna. In semiconducting materials such as gallium arsenide, it has proven possible to trap electrons in tiny confinement regions. These structures are often called "quantum dots." Just as the electrons move around the nucleus only in some orbits, the electrons in these quantum dots are forced into discrete quantum states.

And by using graphene, a single-layer carbon atom that has garnered a lot of attention in the past few years, more interesting possibilities have also been uncovered. "In most materials, electrons can occupy two different quantum states for a given energy, but the high symmetry of the graphene lattice allows the electrons to occupy four different quantum states, which deals with quantum information and Storage opens up new avenues, "explained Florian Libisch at the Technical University of Vienna. However, making well-controlled artificial atoms in graphene has proven to be very challenging.

Cutting the edge is not enough

There are different ways to make artificial atoms: the easiest way is to put the electrons into tiny pieces and cut a thin layer of material. Although this method also works on graphene, the symmetry of the material is ruined by the edges of the fragments because the edges can not be perfectly smooth. As a result, the specific four-quantum state of graphene is reduced to the double quantum state of the traditional material.

Therefore, a different approach must be found: it is not necessary to capture electrons using small pieces of graphene. Using a clever combination of electric and magnetic fields is a smarter option. By scanning the tip of a tunneling microscope, an electric field can be locally applied. In this way, a tiny area is formed on the surface of graphene, and the low-energy electrons can be bound in this area. At the same time, by applying a magnetic field, these electrons are forced into tiny circular orbits. "If we only use the electric field, the quantum effect causes the electrons to quickly leave the trap," explains Libisch.

These artificial atoms were measured at the RWTH Aachen University by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Its modeling and theoretical model was developed by the Technical University of Vienna at Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean samples of graphene come from the team of Andre Geim and Kostya Novoselov at the University of Manchester, England, who won the 2010 Nobel Prize for the first graphene film.

These new artificial atoms now open up new possibilities for many quantum technology experiments: "Four-state electronic states with the same energy make it possible to store information between different quantum states," Joachim Burgdöfer. Electrons can hold any superposition for a long time, which is an ideal feature required for quantum computers. In addition, the new approach has the big advantage of scalability: it should be possible to place many of these artificial atoms on a small chip to use them in quantum information applications.

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