Understanding Collisions of Ultracold Polar Molecules

At temperatures about a millionth of a degree above absolute zero, atoms and molecules enter a new regime where all their motions are governed by the laws of quantum mechanics. Over the last 20 years, there has been a renaissance in atomic physics based on the new phenomena that emerge in this regime. Experiments with atomic Bose-Einstein condensates (recognised by the 2001 Nobel Prize in Physics) have explored a wide range of topics including nonlinear atom optics, quantum vortices, and phase-transitions in optical lattices. At the same time, degenerate Fermi gases have been used, for example, to improve our understanding of Fermionic superfluidity and the physics of polarons. At the heart of all these advances is a deep and detailed understanding of ultracold atomic collisions that has developed over the last two decades through the close interplay of theory and experiment.

Attention in this field is now turning to the new possibilities offered by ultracold molecules. Unlike atoms, molecules can possess an electric dipole moment, with one end positively charged and the other negatively charged. These dipoles mean that molecules can interact with one another more strongly than atoms, and crucially at longer range. Molecules also have more complicated internal structure than atoms: they have multiple spinning nuclei, and they can rotate and be oriented by external fields. Because of this, ultracold molecules offer many new possibilities for the study of novel physical phenomena and the development of new quantum technologies. Examples include the study of exotic forms of quantum magnetism and the potential to design new material properties using so-called "quantum simulators" based on arrays of molecules confined in optical lattices.

We have recently become only the third group in the world to succeed in forming a sample of ultracold polar molecules. In our case, this was done by cooling gases of rubidium and cesium atoms to ultracold temperatures, and then pairing up the atoms to form molecules. The molecules we formed were initially nonpolar and only very weakly bound, but we succeeded in transferring them to deeply bound, polar states using a two-photon optical transfer process, known as stimulated Raman adiabatic passage (STIRAP). The entire process occurs without heating, so that the temperature of the resulting molecular gas mirrors that of the atomic mixture.

Before our molecules can be used in new quantum devices, we need to understand a lot more about their interactions and collisions. A key question, which does not arise for atoms, is whether pairs of molecules "stick together" for a long time when they collide. If they do, then a third molecule may come along and destroy the first two, shortening the lifetime of the molecular sample. The objective of this project is to investigate collisions of ultracold molecules, both experimentally and theoretically, in order to understand the processes involved. We will investigate both 2-body and 3-body collisions, and distinguish between them by loading our molecules into optical lattices formed by standing waves of laser light. These can confine the molecules in stacks of flat pancakes, bundles of tubes, or even individual boxes.

Even if molecular collisions are "sticky", we expect to be able to find ways to control them and thereby preserve the molecular sample for sufficient time to perform interesting experiments. We will investigate orienting the confined molecules with applied electric fields and dressing them with microwave photons, allowing us to create forces that will hold the molecules apart and prevent collisions.

This ambitious project, combining state-of-the-art experiments with world-leading theory, will cement the UK's position at the forefront of an exciting international field.

The research outlined in this proposal will have an impact on society and the economy in a number of ways, in both short and long terms:

1. Supply of highly trained personnel.
Modern high-tech industry requires personnel with strong technical backgrounds and highly developed problem-solving skills. Through this proposal two PDRAs will acquire expertise in a range of state-of-the-art experimental and theoretical techniques in quantum science. Additionally, we have an exceptionally strong track record of training postgraduates and undergraduates within the Joint Quantum Centre and we will endeavour to involve students in the project wherever possible. All personnel connected with the project will gain professional and transferable skills that are highly sought after in the current job market (e.g. project and time management, communication and presentation skills). The skills they acquire are needed in many sectors, including education, defence, R&D, technology and finance.

2. The development of high-tech equipment.
This proposal will drive the development of high-tech equipment with potential benefits to UK companies in areas such as photonics and lasers. Our research often requires us to develop new techniques or devices which can lead to commercial exploitation. For example, in the past we have developed a simple resonant electro-optic modulator which is now commercially available from Photonic Technologies, a small UK based company. Similarly, working closely with manufacturers, our research can drive the improvement of existing products and the development of new product lines.

3. Presentation of our research to the wider community.
The general public will benefit from our efforts to communicate our research in simple terms. This helps to ensure that the public is engaged with science and recognises its enormous importance in the economy and society. In addition to public lectures, laboratory tours and outreach activities, we will add videos and non-technical synopses of our research publications to our Web pages. Further societal impacts will be realised through our extensive international links.

4. Knowledge generation.
In the short term our proposal will yield a better fundamental understanding of the collisions of ultracold molecules, and we will learn how to maintain samples of molecules so that they are not destroyed by inelastic collisions. This will underpin nearly all future uses of ultracold molecules in quantum science and technology. Ultimately the realisation of a quantum simulator using polar molecules has the potential to contribute to our understanding of a range of problems in condensed matter physics (e.g. quantum magnetism and strongly correlated many-body systems). As such, our research will ultimately be relevant to two of the current Physics Grand Challenges: "Emergence and Physics Far From Equilibrium" and "Quantum Physics for New Quantum Technologies". Our efforts will contribute to the competitiveness of science research in the UK, which in turn will help attract highly skilled personnel, funding and even companies into the UK economy. It will probably take more than a decade for the knowledge generated in our research to spread beyond the academic community and on this time-scale it is hard to assess the full impact. Nevertheless we can expect potential impacts in the areas of quantum metrology, precision measurement and quantum technology. Indeed, this research will contribute to maintaining the strong base of world-leading quantum science in the UK which is vital to the development of future applications of quantum technology.