A new technology for manipulating light

Quantum computers are one of the key future technologies of the 21st century. Researchers from the University of Paderborn, under the direction of Professor Thomas Zentgraf and in cooperation with colleagues from the Australian National University and the Singapore University of Technology and Design, have developed a new technology for manipulating the light that can be used as the basis for future optical quantum computers. The results have just been published in the journal Nature Photonics.

New optical elements for manipulating light will allow more advanced applications in modern information technologies, especially in quantum computers. However, a major challenge that remains is the non-reciprocal propagation of light through nanostructured surfaces, where these surfaces have been manipulated at a tiny scale. Professor Thomas Zentgraf, leader of the ultrafast nanophotonics working group at the University of Paderborn, explains: “In reciprocal propagation, light can take the same path forwards and backwards through a structure; however, non-reciprocal propagation is comparable to a one-way street. where it can only extend in one direction. Non-reciprocity is a particular characteristic of optics that causes light to produce different material characteristics when its direction is reversed. An example would be a glass window that is transparent on one side and lets light through, but acts like a mirror on the other side and reflects light. This is called duality. “In the field of photonics, such duality can be very useful for developing innovative optical elements for manipulating light,” says Zentgraf.

In a current collaboration between his working group at the University of Paderborn and researchers from the Australian National University and the Singapore University of Technology and Design, non-reciprocal propagation of light has been combined with a conversion frequency of the laser light, i.e. a change in frequency and therefore also the color of the light. “We used frequency conversion in specially designed structures, with dimensions on the order 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 Zentgraf group. Experiments show that this conversion process only takes place 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 characteristics has been used to encode images into an otherwise transparent surface. “We arranged the different nanostructures in such a way that they produced a different image depending on whether the sample surface was illuminated from the front or from the back,” explains Zentgraf, adding: “The images only became visible only when we used infrared laser light for illumination. »

In their first experiments, the intensity of frequency-converted light in the visible range was still very low. So the next step is to further improve the efficiency so that less infrared light is needed for frequency conversion. In future optically integrated circuits, direction control for frequency conversion could be used to directly switch light with different light, or to produce specific photonic conditions for quantum optics calculations directly on a small chip. “Perhaps we will see an application in future optical quantum computers where the directed production of individual photons using frequency conversion will play an important role,” says Zentgraf.

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Materials provided by University of Paderborn. Note: Content may be edited for style and length.

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