Periodic Reporting for period 2 - S2QUIP (Scalable Two-Dimensional Quantum Integrated Photonics) Reporting period: 2020-04-01 to 2022-03-31 Summary of the context and overall objectives of the project 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. 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. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far 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 with active elements based on microelectromechanical systems (MEMS) and cavities have been first fabricated and tested using our research facilities using electron beam lithography. Simultaneously we developed efficient transfer methods for monolayers and complex heterostructures in clean environment, enhancing the performance of our 2D quantum emitters. We automated the process and started a spin-off company. Our advanced methods allowed 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 key technologies developed in our project because they offer scalable integration of tailorable emitters in our circuit and cavities. To improve the quantum optical properties of the emitted photons we developed three new excitation techniques to coherently drive a quantum system, overcoming for example the intrinsic time-correlation in a three ladder quantum ladder system. Given our independent success on circuit fabrication and quantum emitters, we combined our efforts and achieved quantum emitter coupling to two different photonic platforms (silicon nitride and lithium niobate). 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, realizing multiplexing of three quantum emitters in a single-mode waveguide. We achieved several breakthroughs to generate on-chip photon pairs. We miniaturized the gain medium for nonlinear down-conversion using only a single quantum emitter, resulting in down-converted single photons with polarization control. In addition, we generated for the first time on-demand entangled photon pairs at 1.55µm and integrated the emitters on complex 6leg strain-tunable substrates to control the emitted entangled state.Despite the ongoing pandemic, we have achieved impressive progress on all work packages. S2QUIP clearly established itself at the forefront of 2D material research by developing several key disruptive technologies since October 2018. We were invited to the Research and Innovation days in Brussels, having a booth in the Science is Wonderful Expo and we participated in several Quantum Flagship outreach activities. We also showcased our results as atomic architects on numerous occasions, for example in front of the Scottish parliament.S2QUIP was an integral part of the starting phase of the Quantum Flagship with members in the science and engineering board, the joint working group between the Flagship and Photonics21, and the gender equality working group. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Integration of 2D material on photonic integrated circuits has just started to get great scientific attention and S2QUIP is among the pioneers 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 by either using our unique ion bombardment or moiré patterns put us in the best position to couple site-controlled emitters into on-chip cavities. Our results path the way to finally generate highly indistinguishable photons from 2D materials, which so far has been elusive. With our acquired knowledge about 2D materials, heterostructures, novel excitation techniques, and quantum emitters, the Flagship is well prepared for the next phase, taking advantage of 2D materials for the upcoming quantum technologies.We also advanced the nanofabrication of quantum photonic integrated circuits, in particular fabricated the first silicon nitride MEMS reconfigurable circuit hosting superconducting single photon detectors. These developments might be beneficial for several other quantum flagship projects, since half of all consortia work with integrated photonics, requiring miniaturized power meters, power stabilizers and detectors. Artistic view of an 2D material integrated on a photonic circuit S2QUIP logo
