Quantum noise suppressed – a step towards quantum communications
Quantum dots can be implemented in semiconductors by, for example, confining an electron and a p-hole – i.e. a positively charged defect in the electron quantity – in a very limited area. The electron and the hole together form an excited state. When they recombine, the excited state disappears and a photon is emitted. This photon could serve as an information carrier for quantum communication over long distances, presumes Dr. Arne Ludwig of the Bochum Chair of Solid State Physics.
The institute produces quantum dots from the semiconductor material indium arsenide. The researchers grow this material on a carrier of gallium arsenide. This initially produces a uniform layer of indium arsenide that is only one and a half atom thick – the so-called wetting layer. The researchers then create elevations on this layer: small islands 30 nanometers in diameter and only a few nanometers high. They form the quantum dots.
However the wetting layer, which has to be applied in the first step, is problematic. There are also excited electron-hole states in this layer that can decay and release photons. In the wetting layer these states decay even more easily than in the quantum dots. However, the photons emitted cannot be used for quantum communication; they only generate noise. Since the wetting layer covers the entire surface of the semiconductor chip, whereas the quantum dots only cover one thousandth of this surface, the disturbing light is about a thousand times stronger than the light from the quantum dots. In addition, the wetting layer emits photons with much higher intensity than the quantum dots. The result is interference that distorts the signal and makes its evaluation more difficult.
The research team has now eliminated this interference by adding an additional layer of aluminum arsenide, which the scientists allowed to grow above the quantum dots and the wetting layer. This eliminates the energy states in the wetting layer, making it less likely for electrons and holes to recombine and emit photons.
Dr. Sven Scholz at the Chair of Applied Solid State Physics at the RUB, who was awarded the dissertation prize of the Wilhelm and Else Heraeus Foundation in June 2019 for this work, produced the sample for the present paper. The measurements of the magnitude of the interferences with and without aluminium arsenide layer were carried out by the team of the University of Basel around Matthias Löbl, Dr. Immo Söllner and Prof. Dr. Richard Warburton. The group at Jülich Research Center produced high-resolution microscopic images of the samples.
For more information, contact arne.ludwig@rub.de
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