QSUM: Quantum Science with Ultracold Molecules

For over a century, scientists have been fascinated, and at times mystified, by quantum mechanics, the theory that governs atoms, molecules and, indeed, all matter at a microscopic level. Central to this theory are two concepts: (1) Wave-particle duality - the idea that particles, such as electrons in an atom, can behave like waves and that light waves can behave like particles, and (2) entanglement - the concept that once two (or more) particles have interacted, they cannot be treated as independent entities no matter how far apart they are. These inherently quantum phenomena are at the heart of a wide range of physical effects, but their role is often extremely difficult to elucidate. For example, in solid materials, where every atom interacts with many other atoms, it is very challenging to predict and understand how the quantum behaviour will manifest itself, and yet it leads to effects, such as high-temperature superconductivity and special forms of magnetism. Our Programme will advance the understanding of these complex quantum systems by studying the behaviour of molecules cooled to very low temperatures where we can isolate their quantum behaviour. In this respect, the use of molecules is crucial. Their rich internal structure means they couple strongly to electric and microwave fields, and interact with each other over a much greater distance compared with atoms. In advancing our understanding of the quantum science of molecules, we will also learn how to harness their properties to build new devices, including sensors of exceptional sensitivity, computers capable of solving previously unsolvable problems, and simulators that can design new materials, magnets and superconductors. To study the quantum science of molecules in a controlled and systematic way, we need to develop the ability to manipulate the quantum properties of individual molecules. The first step towards this goal is to remove the thermal motion that normally hides their quantum behaviour. We have already developed methods to achieve this both using molecules in the solid state and in the gas-phase. In the solid state, we have demonstrated that certain organic dye molecules, when embedded in a suitable solid cooled to cryogenic temperatures, behave as near-ideal two-level quantum systems. Such molecules have the perfect properties to act as interfaces between quantum light and quantum matter - an essential building block of many future quantum devices. We will learn how to exploit these properties to generate single photons on demand, control individual photons, and store quantum information. In the gas phase, we have extended the methods of laser cooling and developed new techniques to cool molecules to within a millionth of a degree above absolute zero. In this quantum regime, it is possible to exert complete control over the internal state and motion of the molecules. With this control we can learn how to couple molecules to microwave and optical waveguides, to trap molecules on chips, to assemble ordered arrays of molecules that replicate the crystalline structure of real materials, and to explore how the interactions between molecules govern the behaviour of the many-particle system. These ambitious goals calls for radical advances, which we will deliver through a set of interconnected experiments intimately linked to state-of-the-art theory. With isolated molecules we will develop the control of single molecules and their coupling to single photons; with small arrays of interacting molecules we will control interactions and entanglement in simple geometries; and with two- and three-dimensional lattices we will understand the complex behaviour of strongly interacting many-particle systems. Through these projects, our Programme will lay the foundations for a broad range of future scientific advances and technological applications based on the quantum control of molecules.