Electron and hole wavefunctions: TEM image of a self-assembled quantum dot with a pictorial schematic of the experiment: a quantum dot loaded with a single hole is subject to an in-plane magnetic field (along the x-direction) and optical excitation
A research team from Germany, France and Switzerland has realized qubits in a new form. One day, they might become the information units of quantum computers. To date, researchers have realized qubits in the form of individual electrons. However, this led to interferences and rendered the information carriers difficult to programme and read. The group has solved this problem by utilising electron holes as qubits, rather than electrons.
In order to realize qubits in the form of electrons, an electron is locked in a tiny semiconductor volume, ie quantum dot. The spin turns the electron into a permanent magnet. Researchers manipulate spin via an external magnetic field and initiate precession. The direction of spin is used to code information.
Coherent population trapping on a single hole spin.
The problem: the nuclear spins of the surrounding atoms also generate magnetic fields, which distort the external magnetic field in a random, unpredictable manner. This, in turn, interferes with programming and reading qubits. Consequently, the team searched for another method. The solution: rather than locking individual electrons in the quantum dot, the team removed specific electrons. Thus, positively charged vacancies were generated in the electron structure, ie electron holes.
Electron holes have a spin, too. Researchers can manipulate it via the magnetic field in order to code information. As the holes are positively charged, they are decoupled from the nuclei of surrounding atoms, which are likewise positive. This is why they are virtually immune against the interfering forces of nuclear spin. “This is important if we one day want to manufacture reproducible components that are based on quantum bits,” explains Andreas Wieck. However, this method is only applicable at low temperatures, as the holes are more likely to be disturbed by warmth than the electrons.
PhD student Sascha René Valentin used this equipment to generate quantum dots with electron holes. Credit: RUB, Marquard
At Ruhr-Universität, researchers are able to generate quantum dots of outstanding quality. The experiment could be conducted thanks to a structural design developed by Arne Ludwig in Basel and subsequently realized at the RUB Department headed by Andreas Wieck. It enabled the researcher to apply not just individual electrons to quantum dots, but also electron holes. http://aktuell.ruhr-uni-bochum.de/pm2016/pm00106.html.en
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