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Panoramic view on the North Campus © Jürgen Huhn​/​TU Dortmund
PUBLICATION IN NANO LETTERS

More Photons for Quantum Communication

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On a black background there are many colored lines running in different directions. In the center of the image, a thicker red ray and a thicker blue ray meet in a circular formation of light rays. © Harald Ritsch
The SUPER scheme uses two red-tuned laser pulses to generate single photons.
In the prestigious journal Nano Letters, researchers from Germany and Austria have presented a new method for producing photons that can double the information rate in future quantum communication networks. The work is based on a theoretical prediction by the team led by Dr. Doris Reiter, a physicist at TU Dortmund University.

Physicists throughout the world are working to develop new technologies that utilize the principles of quantum mechanics. A key application is quantum communication, which is based on sending light in its smallest unit, the photon. For many applications, however, the light must be in a certain state, namely a single-photon state. But which is the best way to produce such single-photon states?

Scientists use quantum dots for this – tiny semiconductor crystals that can be integrated into chip components. The quantum dot is excited by means of a laser light source and produces a single photon in the process. However, this is tricky: If the laser light has the same wavelength (color) as the single photon, complex filtering is required, where at least half the photons are lost again.

Last year, theoretical physicists proposed a new method to overcome this problem: The Swing-UP of quantum EmitteR population (SUPER) scheme. Dr. Doris Reiter played an instrumental role in the theoretical considerations. Reiter has headed her own research group in the area of condensed matter theory at the Department of Physics of TU Dortmund University since April 2022. Her team is endeavoring to understand quantum phenomena on the smallest scales in order to harness new quantum technologies.

Photo of Dr. Doris Reiter © Oliver Schaper​/​TU Dortmund
Dr. Doris Reiter

Number of single photons could double

Reiter had already proposed the new method for producing single photons at the end of 2021, when she was still working at the University of Münster and heading a study in collaboration with physicists from the University of Bayreuth.

In collaboration with experimental physicists from Innsbruck and Linz, she has now been able to put the method into practice in the laboratory. Doris Reiter explains: “The SUPER scheme uses two red-tuned laser pulses, that is, ones with lower energy than the quantum dot transition, to produce single photons.” This renders filtering redundant and, in theory, makes it possible to produce twice as many single photons.

To conduct the experiment, the researchers therefore had to generate two different laser pulses. Using a special component, a spatial light modulator, the team from University Innsbruck produced these two laser pulses from one pulse. The quantum dots required for the experiment came from the JKU (University of Linz). “Through the exchange between theory and practice, we were able to apply the new method successfully in the experiment,” says Thomas Bracht, who is doing his doctoral studies in Reiter’s research group and conducted the theoretical calculations.

The experiment has shown that the SUPER scheme works very well, and the results match the theoretical predictions perfectly. By implementing this new method, which the scientists have reported in the scientific journal Nano Letters, they are taking a big step forward in endeavors to make quantum communication usable not only in the laboratory but also for real applications.

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