By IQT News posted 22 Dec 2021

(Phys.org) Experts from all over the world are developing fundamentally new technologies based on quantum physics. One key application is quantum communication, where information is written and sent in light. For many applications making use of quantum effects, the light has to be in a certain state—namely a single photon state. But what is the best way of generating such single photon states? In the PRX Quantum journal, researchers from Münster, Bayreuth and Berlin (Germany) have now proposed an entirely new way of preparing quantum systems in order to develop components for quantum technology.
In the experts’ view it is highly promising to use quantum systems for generating single photon states.
One difficulty that has to be overcome is to separate the generated single photons from the exciting laser pulse. In their work, the researchers propose an entirely new method of solving this problem. “The excitation exploits a swing-up process in the quantum system,” explains Münster University’s Thomas Bracht, the lead author of the study. “For this, we use one or more laser pulses which have frequencies which differ greatly from those in the system. This makes spectral filtering very easy.”
Scientists define the “swing-up process” as a particular behavior of the particles excited by the laser light in the quantum system—the electrons or, to be more precise, electron-hole pairs (excitons). Here, laser light from two lasers is used which emit light pulses almost simultaneously. As a result of the interaction of the pulses with one another, a rapid modulation occurs, and in each modulation cycle, the particle is always excited a little, but then dips towards the ground state again. In this process, however, it does not fall back to its previous level, but is excited more strongly with each swing up until it reaches the maximum state. The advantage of this method is that the laser light does not have the same frequency as the light emitted by the excited particles. This means that photons generated from the quantum dot can be clearly assigned.
“Our numerical simulations show that the properties of the photons generated after the swing-up process are comparable with the results of established methods for generating single photons, which are less practical,” adds Prof. Martin Axt, who heads the team of researchers from Bayreuth.
The study constitutes theoretical work. As a result of the collaboration between theoretical and experimental groups, however, the proposal is very close to realistic experimental laboratory conditions, and the authors are confident that an experimental implementation of the scheme will soon be possible. With their results, the researchers are taking a further step towards developing the quantum technologies of tomorrow.

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