Thursday, November 26, 2009

Würzburg Quantum Dot Towers Signal Ultra Safe Data Transmission, Quantum Cryptography Possible


Quantum Dot Towers, a hundred times thinner than a human hair, with quantum cryptographic properties. The nanostructures are produced by researchers at the Department of Applied Physics, University of Würzburg.
Image Credit: Monika Emmerling / Adriana Wolf

German physicists at the University of Würzburg have created a method of broadcasting data over nanotowers with "quantum cryptography" making messages and data transmitted over the Internet impossible to intercept.

Contained inside the Würzburg towers are special structures capable of emitting light: these are known as quantum dots, and their electronic and optical properties can be customized during production. Quantum dots, in the same way as single atoms, possess precisely defined energy states. This enables them to send out photons (light particles) with an exact amount of energy.

What is special about the Würzburg quantum dot towers is that "with them it is possible to 'fire off' single photons in a targeted fashion. It is structural elements like these that are needed for the tap-proof transmission of data in the field of quantum cryptography," explains Würzburg physicist Stephan Reitzenstein.

However, to date, the production of single photons in these structures has only been achieved with temperatures well below minus 100 degrees Celsius. So, there are still hurdles to overcome before the concept can be routinely applied.

Thanks to the tiny towers developed in Würzburg, there are now new insights into quantum dots. Physicists on Professor Peter Michler's team (Institute of Semiconductor Optics and Functional Interfaces of the University of Stuttgart) have published these jointly with their Würzburg colleagues in the journal Nature Photonics.

Those involved in the publication from Würzburg's Department of Applied Physics were Stephan Reitzenstein, Andreas Löffler, Sven Höfling, and Professor Alfred Forchel. The Stuttgart team included Serkan Ates, Sven M. Ulrich, Ata Ulhaq, and Professor Peter Michler.

New tool for analyzing quantum dots: The Stuttgart physicists studied the Würzburg nano-towers as part of a venture sponsored by the German Research Foundation (DFG). "The towers serve as a new tool for analyzing the properties of quantum dots in a way never seen before," explains Reitzenstein.

The Stuttgart team discovered an unexpected effect, known as non-resonant coupling. This suggests strong light-matter interactions in such solid-state systems. According to Peter Michler, "this will have major repercussions on the design and functionality of future quantum emitters that are based on quantum dots."

Structure of the Würzburg towers: The nanotowers consist of a sophisticated sequence of layers made from the semiconductors aluminum arsenide and gallium arsenide. "Their special structure makes them into high-quality optical resonators, which confine single photons on a light wavelength scale in all three spatial dimensions," says Stephan Reitzenstein.

Embedded in the center of the towers are some 100 quantum dots made from the semiconducting material indium gallium arsenide. Reitzenstein: "Using special spectroscopic procedures, however, a single quantum dot can purposefully be brought into resonance with the optical mode of a tower in order to conduct fundamental physics experiments on the interaction between light and matter."

Non-resonant dot-cavity coupling and its potential for resonant single-quantum-dot spectroscopy, by S. Ates, S. M. Ulrich, A. Ulhaq, S. Reitzenstein, A. Loffler, S. Hofling, A. Forchel, and P. Michler, Nature Photonics, was published online on Nov. 22, 2009, doi:10.1038/nphoton.2009.215

Contact:
Dr. Stephan Reitzenstein, University of Würzburg,
phone +49 931 31-85116,
stephan.reitzenstein@physik.uni-wuerzburg.de

Dr. Sven M. Ulrich, University of Stuttgart,
phone +49 711 685-65226,
s.ulrich@ihfg.uni-stuttgart.de




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