Photonic Crystals for Coupling Quantum Circuits

All over the world, researchers are engaged in intensive work on the individual components of quantum tech­nologies – these include circuits that process infor­mation using single photons instead of electricity, as well as light sources producing such individual quanta of light. Coupling these two components to produce integrated quantum optical circuits on chips presents a particular challenge. Researchers at the University of Münster have now developed an interface that couples light sources for single photons with nano­photonic networks.

Photonic crystals in various designs concentrate light in a cavity. (Source: J. Olthaus, P. Schrinner et al. / Adv. Quantum Technol.)

This interface consists of photonic crystals, i.e. nano­structured dielectric materials that can enhance a certain wavelength range when light passes through. Such photonic crystals are used in many areas of research, but they had not previously been optimized for this type of interface. The researchers took particular care to achieve this feat in a way that allows for replicating the photonic crystals straight­forwardly by using established nano­fabrication processes. “Our work shows that it is not only in highly specia­lized labora­tories and unique experiments that complex quantum technologies can be produced,” says physicist Carsten Schuck, an assistant professor at Münster University who headed the study together with Doris Reiter, likewise an assistant professor, who works in the field of solid state theory. The results could help to make quantum tech­nologies scalable.

As single photons obey the laws of quantum physics, researchers talk of quantum emitters with respect to the light sources involved. For their study, the researchers considered quantum emitters which are embedded in nano­diamonds and emit photons when they are stimulated by means of electro­magnetic fields. In order to produce the interfaces desired, the researchers’ aim was to develop optical structures tailored to the wavelength of the quantum emitters. Cavities or holes in photonic crystals are well suited for trapping light in minute volumes and getting it to interact with matter such as, in this case, nano­diamonds. Jan Olthaus, a PhD student in physics in Doris Reiter’s junior research group, developed theoretical concepts and special computer-assisted simu­lation techniques in order to compute the designs for these photonic crystals.

The theo­retically developed designs were produced by physicists in the junior research group headed by Carsten Schuck at the Center for Nano­Technology and the Center for Soft Nanoscience at Münster University. PhD student Philipp Schrinner manu­factured the crystals from a thin film of silicon nitride. For this purpose, he used modern electron beam litho­graphy and special etching methods on the equipment at the Münster Nano­fabrication Facility and succeeded in producing high-quality crystals directly on the base material of silicon dioxide. In structuring the crystals, the researchers varied not only the size and the arrange­ment of the cavities, but also the width of the waveguide on which the cavities were placed. The results measured showed that photonic crystals which demons­trated a special variation in hole sizes were best suited for the interfaces.

“Our collaboration – between theoretical and experi­mental physicists – is an ideal one for physics research,” says Doris Reiter. “This type of colla­boration isn’t always easy, as our respective methods of working are often very different indeed – which is why we are all the more delighted that it turned out so well in the case of our two junior research groups.” “What’s special about our work,” adds Carsten Schuck, “is that our designs don’t require any additional processing steps, because they are compatible with established thin-film technology for inte­grated photonic circuits.” This cannot be taken for granted in the development of complex quantum tech­nologies, because although researchers often succeed in producing an important, high-quality component as a one-off, they are not able to produce multiple copies of the same component again.

The next steps for the researchers involve trying to position the quantum emitters, embedded in the nano­diamonds, at certain spots on the photonic crystals – with the aim of putting the results of the study into practice. To this end, the team headed by Carsten Schuck is already developing a special nano­fabrication technique which is able, for example, to place a diamond just 100-nanometers in size with an accuracy of less than 50 nano­meters. The team of theo­retical physicists led by Doris Reiter wants to extend the studies to other materials systems and more complex geometries of photonic crystals and, for example, use elliptical holes instead of round ones. (Source: WWU)

Reference: J. Olthaus et al.: Optimal Photonic Crystal Cavities for Coupling Nanoemitters to Photonic Integrated Circuits, Adv. Quantum Tech., online 1 October 2019; DOI: 10.1002/qute.201900084

Links: Research group Carsten Schuck, Westfälischen Wilhelms-Universität Münster WWU, Münster, Germany Research group Doris Reiter, Westfälischen Wilhelms-Universität Münster WWU, Münster, Germany

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