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Physikgebäude (Neubau Otto-Hahn-Str. 4a) an der TU Dortmund © Nikolas Golsch​/​TU Dortmund
PUBLICATION IN NANO LETTERS

More photons for quantum communication

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Symbol image quantum communication © Harald Ritsch
The SUPER scheme uses two red-tuned laser pulses to generate single photons.
In the renowned journal Nano Letters, researchers from Germany and Austria present a new method for generating 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 TU physicist Dr Doris Reiter.

All over the world, physicists are working to develop new technologies that harness the principles of quantum mechanics. One key application is quantum communication: it 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 what is the best way to generate such single photon states?

Scientists use quantum dots - tiny semiconductor crystals that can be integrated into chip components. Laser light can be used to excite the quantum dot and thus generate a single photon. However, this is tricky: if the laser light has the same wavelength (colour) as the generated single photon, a complicated filtering technique is necessary. In the process, at least half of the generated photons are lost again.

To overcome this problem, theoretical physicists proposed a new method last year: the Swing-UP of quantum EmitteR population (SUPER) scheme. Dr Doris Reiter played a leading role in the theoretical considerations. Reiter has been leading her own working group in the field of condensed matter theory at the Department of Physics at TU Dortmund University since April 2022. Her team is working on understanding quantum phenomena on the smallest scales and thus making novel quantum technologies usable. Among other things, the researchers also use numerical simulations.

Number of single photons could double

At the end of 2021, when she was still working at the Westfälische Wilhelms-Universität Münster, Reiter had already proposed the new method for generating single photons as the head of a study in collaboration with physicists from the University of Bayreuth.

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

Now, in cooperation with experimental physicists from Innsbruck and Linz, she has been able to implement it in the laboratory. Doris Reiter explains: "The SUPER scheme uses two red-tuned laser pulses, i.e. pulses with lower energy than the quantum dot transition, to generate single photons. This eliminates the need for filtering and, theoretically, twice as many single photons can be generated.

To realise the experiment, the researchers therefore had to generate two different laser pulses. The team from the University of Innsbruck produced these two laser pulses from one pulse and used a special component, a spatial light modulator, for this. The quantum dots needed for the experiment came from the University of Linz. "Through the exchange between theory and practice, we were able to successfully implement the new method in the experiment," emphasises Thomas Bracht, who did his doctorate in Dr. Doris Reiter's working group and carried out the theoretical calculations.

The experiment showed that the SUPER scheme works very well and the results agree excellently with the theoretical predictions. With the implementation of this new method, which the scientists* report on in the journal Nano Letters, they are taking a big step forward in the effort to make quantum communication usable not only in the laboratory, but for real applications.

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