The charged tip of a scanning tunneling microscope and an additional magnetic field lead to localized stable electron states in graphene. Credit: Nils Freitag, RWTH Aachen
Scientists have created tiny quantum dots in graphene. In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms.” Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications eg quantum computing. Such additional properties have now been shown for artificial atoms in graphene.
“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties,” says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown in “quantum dots.” Just like in an atom, where electrons only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.
“In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for 4 different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene is extremely challenging.
There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special 4-fold multiplicity of states in graphene is reduced to the conventional 2-fold one.
It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.
The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) — these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.
The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information,” says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications. http://www.tuwien.ac.at/en/news/news_detail/article/124248/
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