Magneto-optical trapping and sympathetic cooling of molecules

In a magneto-optical trap (MOT), a combination of precisely-tuned laser light and a magnetic field is used to cool atoms to temperatures below 1 milli-Kelvin and trap them for minutes at a time. For over 25 years the MOT has been at the heart of all applications that use ultracold atoms. These include state-of-the-art instruments such as atomic clocks, magnetometers, gravimeters and accelerometers, measurements of constants, and a wide range of studies into the properties and behaviour of matter in the quantum regime. The potential applications of ultracold molecules go even further. They can be used as sensitive field sensors, and for making extremely precise measurements that test our most fundamental models of physics. Because molecules interact more strongly than atoms they can be used to study how quantum matter behaves when every particle is interacting with every other. This is important for understanding and designing new materials and chemical processes. Ultracold molecules can also be used to study fundamental processes in chemistry at the quantum level, and to make components of a quantum processor. To realize these applications, we first need to learn how to make a MOT for molecules. This is more difficult than for atoms because the laser light tends to set molecules rotating and vibrating, heating them up instead of cooling them down. Our previous work has shown how to overcome these difficulties, and we are now ready to make the MOT, which is the main subject of this proposal.

We will focus on calcium fluoride (CaF) molecules. These will be slowed to rest and then captured in the MOT where multiple laser frequencies will be used to cool and trap them. Our simulations show that the CaF will cool to about 1 milli-Kelvin. This is an excellent starting point for many applications, but still not cold enough for others. To reach even colder temperatures, the CaF will be mixed with Rb atoms which are easy to cool to micro-Kelvin temperatures. We will investigate the collisions between these two species in a magnetic field and in a microwave field. Under optimum conditions, the CaF will thermalize with the Rb, allowing us to reduce their temperature to about 1 micro-Kelvin.

1. Immediate Impact.

(i) Transfer of new knowledge and techniques within the academic community.
There are just a few groups worldwide developing these new methods to cool molecules to ultracold temperatures, but there are many others that could benefit from these techniques and apply them to problems in quantum chemistry, quantum information processing, metrology, and tests of fundamental physics.

(ii) Supply of highly-trained personnel.
Industry and business need a supply of professionals with strong technical and analytical skills, who are independent thinkers and creative problem solvers. This technically demanding project and our approach to managing staff encourage all these skills. We will train the two PDRAs funded directly by the project. The research fits perfectly with the aims of the Centre for Doctoral Training in Controlled Quantum Dynamics and we expect to train several PhD students through this route.

(iii) Public engagement with all
School pupils, teachers, undergraduate students, and the general public, will benefit from our outreach programme which aims to present the science we do to a wide audience. They will benefit from discussions with scientists working on an exciting project, and from the opportunities to visit our laboratories. This will contribute to the wider UK and global effort to ensure that the public is engaged with science and recognises its importance in the economy and society. Engagement with science drives curiosity, stimulates creativity, expands horizons, and encourages an appreciation of nature.

2. Long-term impact

In the long term, we anticipate our work bringing benefits to industry and society through the development of quantum simulation as an important new technology, and by advancing the precision of measurements and sensors.

(i) Quantum simulation
An important future technology is the design of materials and complex molecules at the atomic level. This technology is held back because large-scale quantum systems, where each particle interacts with all the others, cannot be modelled by a computer. The properties may only emerge once millions of particles are involved, but the best computers can only cope with a few tens of particles as the size of the problem scales exponentially. A quantum simulator is a physical quantum system that can be engineered to simulate another quantum system whose behaviour we wish to understand. A lattice of ultracold polar molecules, all interacting through dipole-dipole interactions, would make a versatile, highly-controllable, quantum simulator. Our proposal will enable us to trap polar molecules in a lattice at useful densities, and this will be a major step towards a quantum simulator. In the long term, we anticipate that quantum simulation will transform our understanding of quantum phase transitions, quantum magnetism and high-temperature superconductors and aid in the design of new materials, molecules, chemical processes and superconductors.

(ii) Measurement and sensing.
Polar molecules are exceptionally sensitive to static and microwave electric fields. We aim for an ensemble of polar molecules with near-perfect control over the internal and external motions. This can make an extremely sensitive field sensor which could be a useful tool for probing surfaces or detecting extremely weak signals. Measurement precision can be enhanced by using entangled particles instead of an ensemble of independent particles. This has so far proved difficult to achieve. The gas of ultracold polar molecules will be an excellent system for studying entanglement on both a small scale (a few molecules) and a large scale (the entire gas), and for understanding how entanglement can be used to improve measurement precision. In the long term, this understanding could be used to improve the performance of clocks and sensors of all kinds. This could bring benefits to surface science, health monitoring, metrology, mineral detection and navigation.