Photons - the elementary particles of light - are devoid of any type of mutual interaction in free space. While this fundamental property of light is crucial to the success of modern day optical communication, the ability to induce synthetic interactions between photons, like the ones between electrons in electronic circuits, may open up new possibilities for quantum science and technology. "Quantum control of light" (Qcool) is a Carlsberg foundation "Semper Ardens" research project that aims to realize this challenging endeavor. The project brings together the joint efforts of theoretical and experimental groups at Aarhus University and the University of Southern Denmark to develop, realize and utilize a unique approach to photon-photon interactions will enable entirely new levels of control over individual quanta of light. 

Our approach to engineering photon interactions is based on light propagation through a novel type of optical medium composed strongly interacting Rydberg atoms in an ultracold environment. Understanding how quantum light propagates under such extreme conditions presents an outstanding problem that remains to be solved in its full complexity and is one of the ambitious goals of QCooL. We will develop new theory to identify the fundamental mechanisms behind induced photon interactions and devise novel schemes for their application. Combining theory and experiment, we will thereby develop and demonstrate the key concepts for next generation all-optical quantum devices, such as single-photon routers, quantum transistors and optical amplifiers of quantum information. At the same time, the envisioned nonlinear medium studied in the QCooL project can promote forms of light that acquire matter-like properties, and thereby enable the exploration of collective optical phenomena into uncharted territory, where the spontaneous emergence of strong quantum correlations and even a crystallization of photons may occur.