New light control technology

Quantum computers are one of the key technologies of the future 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 light control technology that could be the basis of future optical quantum computers. The results have just been published in the journal Photonics of nature.

New optical elements for controlling light will enable more advanced applications in modern information technology, especially in quantum computers. However, the main problem remains the non-reciprocal propagation of light through nanostructured surfaces when these surfaces are manipulated on a tiny scale. Professor Thomas Zentgraf, head of the ultrafast nanophotonics working group at the University of Paderborn, explains: “With mutual propagation, light can follow the same path back and forth through the structure; however, non-reciprocal propagation is comparable to a one-way street. where it can only propagate in one direction. Nonreciprocity is a special characteristic of optics that causes light to create different material characteristics when its direction is reversed. An example is a glass window, which is transparent on one side and allows light to pass through, and on the other acts as a mirror and reflects light. This is called duality. “In the field of photonics, this duality can be very useful for developing innovative optical elements for controlling light,” Zentgraf says.

In an ongoing collaboration between his working group at the University of Paderborn and researchers at the Australian National University and the Singapore University of Technology and Design, non-reciprocal light propagation has been combined with laser frequency conversion, i.e. in frequency and therefore in color of light. “We used frequency conversion in specially designed structures with dimensions on the order of several hundred nanometers to convert infrared light invisible to the human eye into visible light,” explains Dr. Sergei Kruk, a researcher at the Marie Curie Institute. Group Zentgraf. Experiments show that this conversion process only occurs in one illumination direction for a nanostructured surface, while it is completely suppressed in the opposite illumination direction. This duality of frequency conversion characteristics was used to encode images on a transparent surface. “We positioned different nanostructures so that they gave different images depending on whether the surface of the sample was illuminated from the front or from behind,” Zentgraf explains, adding: “The images only became visible when we used an infrared laser. light for illumination. »

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

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