A quantum network is based on the transmission of individual photons, which serve as “mobile” quantum bits. The probability that such a light particle arrives at the receiver generally decreases as the optical fiber distance increases. In order to exchange data over distances of 10 to 100 km or more, the photons must therefore have a certain wavelength. But even then, intermediate stations are necessary for a continental network in which the signal is processed. However, these quantum repeaters differ fundamentally from signal amplifiers used in classical communication technology.
Quantum repeaters have to bridge the sections using quantum effects. They are based on the interference of individual light particles emitted by spatially separated, independent emitters. “In our case, we use semiconductor nanostructures as emitters of the light particles. They emit photons at a very high frequency,” explains Jonas Weber, who was responsible for photon generation and interference in the project. “This is important for fast data transmission,” says the PhD student at the Institute of Semiconductor Optics and Functional Interfaces (IHFG) at the University of Stuttgart.
However, the common nanostructures usually emit light particles whose wavelength is not adapted to transmission with glass fibers. In order to make use of the many advantages of quantum dots, two independent quantum frequency converters were set up by the quantum optics working group led by Prof. Christoph Becher at Saarland University. These converters contain special crystals. If the individual light particles in them are superimposed with strong laser light, their wavelength can be manipulated. Then the light particles can be transmitted over the targeted 10-100 km fiber optic line. Without this preparation, signal amplifiers would have to be set up at 1-km intervals – which would drive up the costs of such a quantum network enormously.
The physicists were able to show that this necessary manipulation does not damage the elementary quantum effect. Thus, the individual light particles were sent through a 2-km fiber optic line and then successfully interfered with. This is not self-evident in view of the extremely fragile nature of quantum states. “This very complex experiment shows that semiconductor quantum dots in combination with quantum frequency conversion represent a veritable platform for quantum repeaters,” comments Professor Peter Michler, head of the IHFG.
The research results have now been published in the journal Nature Nanotechnology. See https://doi.org/10.1038/s41565-018-0279-8