Quantum computers are one of the most important future technologies of the 21st century. Researchers at the University of Paderborn, led by Professor Thomas Zentgraf and in collaboration with colleagues from the Australian National University and the Singapore University of Technology and Design, have developed a new technology for manipulating light that can serve as the basis for future optical quantum computers. The results have now been published in the journal Nature Photonics.
New optical elements to manipulate light will enable more advanced applications in modern information technology, especially in quantum computers. However, a major challenge that remains is non-reciprocal light propagation through nanostructured surfaces, where these surfaces have been manipulated on a tiny scale.
Professor Thomas Zentgraf, head of the working group for ultrafast nanophotonics at the University of Paderborn, explains: With reciprocal propagation, light can take the same path forwards and backwards through a structure; However, non-reciprocal propagation is like a one-way street where it can only propagate in one direction. Non-reciprocity is a peculiarity in optics that causes light to produce different material properties when it reverses direction. An example would be a window made of glass that is transparent on one side and lets light through, but acts like a mirror on the other side and reflects the light.
This is called duality. In the field of photonics, such a duality can be very helpful in developing innovative optical elements for manipulating light, says Zentgraf.
In a current collaboration of his working group at the University of Paderborn with researchers from the Australian National University and the Singapore University of Technology and Design, the non-reciprocal light propagation was combined with a frequency conversion of laser light, i.e. a change in frequency and thus also the color of the light.
We used frequency conversion in the specially designed structures with dimensions in the range of a few hundred nanometers to convert infrared light – which is invisible to the human eye – into visible light, explains Dr. Sergey Kruk, Marie Curie Fellow in the Zentgrafs group.
The experiments show that this conversion process takes place only in one direction of illumination for the nanostructured surface, while it is completely suppressed in the opposite direction of illumination.
This duality in frequency conversion properties has been used to encode images into an otherwise transparent surface. We arranged the different nanostructures in such a way that they produce a different image depending on whether the sample surface is illuminated from the front or from behind, says Zentgraf, adding: The images only became visible when we used infrared laser light for illumination.
In their first experiments, the intensity of the frequency-converted light in the visible range was still very low. The next step is therefore to further improve the efficiency so that less infrared light is required for frequency conversion.
In future optical integrated circuits, directional control for frequency conversion could be used to switch light directly with other light, or to create specific photon conditions for quantum optical calculations directly on a small chip. Perhaps we can see an application in future optical quantum computers, in which the targeted generation of individual photons using frequency conversion plays an important role, says Zentgraf.
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