Quantum technologies are of utmost importance for the European community to create practical applications in many different fields, from computing and communications to health, finance, transport, and national security and defence. This critical technology will generate a multi-billion Euro market in new technological solutions for business and citizens. For exploitation of quantum effects in sensing, computing and communication, coherence is the most important parameter, and highly coherent laser sources are required both for the development of more advanced devices and for the widespread applications of quantum sensors outside specialist laboratories. This project aims at opening the route to the next generation of ultrastable lasers, realising optical fractional frequency instabilities of 10-17 and below, corresponding to millihertz linewidth and tens of seconds coherence time.