The first detections of gravitational waves since 2015 and the dawn of the multi-messenger astronomy era have demonstrated the enormous scientific potential of gravitational-wave observatories and provided a solid basis for the future of gravitational-wave (astro-)physics. The Einstein Telescope is envisioned as the pioneer of a new generation of gravitational wave detectors designed to realize this potential. An EU-funded Conceptual Design Study has identified the Belgian-Dutch-German border area as a promising location to site the Einstein Telescope. With this application we aim to conduct forefront research in gravitational-wave science and engineering in support of the realisation of the Einstein Telescope. The focus of this proposal is to leverage existing research strengths combined with industrial partnerships to develop a limited number of essential technologies for the Einstein Telescope, including new promising mirror coating materials, custom-made sensors for the low-temperature systems, the beampipe vacuum system, tracking and characterising systems for ground vibrations, and high-performance computing.