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Scalable Two-Dimensional Quantum Integrated Photonics

Periodic Reporting for period 1 - S2QUIP (Scalable Two-Dimensional Quantum Integrated Photonics)

Reporting period: 2018-10-01 to 2020-03-31

Bringing photonic quantum technology, such as quantum communication, photonic quantum sensing, as well as photonic quantum simulation and computing, to market requires a scalable platform to increase the complexity and thus functionality of the envisioned devices. Silicon-based photonic integrated circuits emerged as a promising platform to achieve the required scalability by offering miniaturized architecture, low loss connectivity, and well-developed nanofabrication technology. However, the main building block for photonic quantum technology, namely the quantum light source producing the photonic quantum states, is challenging to monolithically integrate on such circuits. This stems from the indirect bandgap of the underlying semiconductor. Major efforts have made to build hybrid quantum photonic systems integrating optically active elements after nanofabrication of the circuit. Up to date these techniques suffer from resource heavy non-scalable transfer methods which hinders marketability of such circuits. In S2QUIP we take advantage of a new type of material, two-dimensional (2D) semiconductors. With the discovery of graphene in 2004, a monolayer of carbon atoms and the first 2D material, enormous research efforts have emerged to find other atomically flat materials. Two-dimensional semiconductors are a new class of materials, capable to emit quantum light. Furthermore, different 2D materials can be stacked, creating artificial heterostructures with tailored physical properties. Thus, 2D materials are a key enabling technology offering deterministic position control and straight forward integration into complex photonic circuits – clear advantages compared to other solid-state quantum emitters.

S2QUIP aims to develop a new platform to realize building blocks for future applications of quantum technologies using 2D materials. We develop multiplexed on-chip quantum light sources based on two different approaches using the unique properties of different 2D materials. We are focusing on three key factors for photonic quantum technologies: small, cheap, and robust, which are the current bottlenecks to bring the quantum world into our every day’s world. Our contribution will help to build sources for secure communication and for sensors with unprecedented resolution, assisting microscopy and imaging techniques.
We started the project by developing and optimizing our silicon-based photonic integrated circuit platform. We decided that we first focus on silicon nitride as the photonic platform and later transfer our knowledge to other photonic platforms for potential advantages. In close collaboration with our industrial partners we set up design libraries and process design kit for Complementary metal–oxide–semiconductor (CMOS) compatible foundry processes. Different circuit designs have been first fabricated and tested using our research facilities using electron beam lithography. We have designed new dielectric nanobeam cavities, waveguides, on-chip couplers and active elements based on microelectromechanical systems (MEMS) actuators, such as reconfigurable beam splitters and tunable ring resonators.

Simultaneously we developed efficient transfer methods for monolayers and complex heterostructures in clean environment, enhancing the performance of our 2D quantum emitters. This enabled our consortium to show charge tunable devices, tailoring the electronic properties of these quantum emitters and to fabricate devices based on Moiré pattern. We were able to proof for the first time that the confinement generated by the Moiré pattern indeed results in the emission of single-photons. This provided us with a new unforeseen resource to generate deterministic and position-controlled quantum emitters integrated on photonic circuits and cavities. Furthermore, we showed that developed another scalable and site-controlled method to generate deterministic single-photon sources on-chip. By ion bombardment we achieved nanometer precision of defect generation in a 2D material with clear single-photon emission. These two approaches are important milestones for our project because they offer scalable integration of tailorable emitters in our circuit and cavities, an important prerequisite to achieve multiplexed quantum light on a photonic chip.

Given our independent success on circuit fabrication and quantum emitters, we combined our efforts and achieved quantum emitter coupling to two different photonic platforms already. In the case of our optimized silicon nitride platform we not only showed deterministic strain-induced quantum emitter coupling to the waveguide but also for the first time single-photon propagation in such circuits. Furthermore, we were able to perform advanced quantum optics experiments with such chips. In particular, resonant excitation of 2D quantum emitters through a photonic circuit; another important step to generate on-chip multiplexed indistinguishable photons.

For the final objective to deliver a turn-key device emitting single photons we also developed a dedicated laser system capable of resonant excitation of our quantum emitters with high repetition rate.
Based on our great scientific results, which resulted in 12 high impact publications and participated in 29 conferences, we are confidant that we will reach our final objective to multiplex 20 quantum light sources on a chip at the end of the project.
Furthermore given our visibility and online presence on social media, we were invited to the Research and Innovation days in Brussels, having a booth in the Science is Wonderful Expo. We also showcased our results as atomic architects on numerous occasions, for example in front of the Scottish parliament.
Integration of 2D material on photonic integrated circuits has just started to get great scientific attention and S2QUIP is among the first to develop such a hybrid platform. We are the first European consortium to integrate deterministically strain-induced quantum emitters on silicon nitride waveguides and have achieved, as the first group worldwide, resonant excitation of emitters using on-chip coupling through waveguides. The techniques we developed to position quantum emitters with nanometer precision put us in the best position to generate indistinguishable photons on-chip by either using our unique ion bombardment or Moiré patterns and couple the site-controlled emitters into on-chip cavities. We expect to measure the indistinguishability of emitter photons from cavity coupled quantum emitters and entanglement from cavity enhanced four wave mixing experiments. Given these milestones we expect to multiplex sources on chip until the end of the project, fulfilling our main objective.
Artistic view of an 2D material integrated on a photonic circuit
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