MMQA: MicroKelvin Molecules in a Quantum Array

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All matter is governed by quantum physics. Even in bulk material, with huge numbers of particles, many important quantum phenomena persist. When the particles only interact appreciably with their nearest neighbours, it is usually possible to understand the bulk behaviour in terms of the quantum physics of the constituents. Often however, the interactions are long-range and strong, meaning that every particle interacts appreciably with every other particle. The behaviour of the bulk cannot then be understood from that of the constituents, and from a theoretical point of view the system is usually unsolvable. From such strongly interacting quantum systems emerge extraordinary and fascinating phenomena that are not at all well understood, such as high temperature superconductivity and exotic forms of magnetism.Modelling such a complex system on a computer is an impossible task. Instead, we need a physical model of strongly interacting quantum particles where the interactions can be controlled. We plan to build an instrument that cools polar molecules to microKelvin temperatures and below, arranges them in a regular array, and controls their motion, their orientation, and the way they interact. This instrument will be used as a quantum simulator - an ideal, tuneable and highly versatile tool for modelling strongly-interacting quantum systems and understanding the remarkable quantum phenomena they exhibit. This same device could also be used for quantum information processing, or as a multi-particle interferometer for making extremely sensitive measurements of electric, magnetic, gravitational and exotic forces.The use of ultracold polar molecules is crucial for realising this vision. Unlike atoms, the molecules have strong, tuneable, long-range interactions, an essential ingredient for the quantum simulator. While the techniques for cooling atoms to microKelvin temperatures are well established, methods to do the same for molecules are only now emerging. A large part of our programme focuses on developing these methods. We will follow two main routes. One is to start with trapped ultracold atoms, which are then paired up to form weakly-bound ultracold molecules. We will need to transfer them to deeply bound states without heating them up, using a sequence of carefully tailored laser pulses. In the second approach, a beam of molecules from a cryogenic source is decelerated to rest and trapped using electric, magnetic or optical forces. These molecules will be far too hot to form the quantum gas we need, but they could be brought to this regime by sympathetic cooling using ultracold atoms as a refrigerant. The final quantum array will be made by loading our ultracold molecules into a trap formed by laser beams. The configuration of the beams - orientation, polarisation and frequency - allows the quantum evolution to be studied for a wide variety of potentials. Low-frequency external electric fields will be used to control the interactions between molecules.The advances we make will also stimulate new and diverse areas of research: (i) Molecules allows one to test the fundamental symmetries of space and time through measurements of particle dipole moments and the constancy of molecular frequencies. (ii) We will study the collisions of molecules at temperatures where quantum reflection, tunnelling and Bose/Fermi statistics are all important. (iii) Polar molecules can interact with nano-mechanical structures through the long-range dipole interaction, allowing quantum states to be mapped from one to the other. The production of dense samples of ultracold molecules is the key step towards these goals.It will be a major milestone in quantum physics to demonstrate the molecular array. We bring together researchers from Physics and Chemistry at Durham and Imperial, each contributing the highest UK expertise on a key part of our joint programme, to tackle all the experimental and theoretical problems in a unified way.