A Stable Quantum Gas of Fermionic Polar Molecules

The theory of quantum mechanics provides an excellent description of isolated atoms and has allowed us to develop our understanding of the physics of such systems to unprecedented levels. The same quantum physics ultimately governs all matter, even in bulk materials, and leads to many important and interesting phenomena, such as high temperature superconductivity and exotic forms of magnetism. However, in solid materials individual atoms are no longer isolated from one another and commonly experience strong long-range interactions with many other particles in the material. In this case, the nature of quantum mechanics means that an exact solution of the many-body system is usually impossible. Instead we must develop a simple model of the system which captures the essential physics and then try to solve this model for a finite number of particles. However, even this approach becomes intractable on a classical computer for more than 10 to 100 particles. An alternative strategy, originally proposed by Richard Feynman, is to use another quantum system to 'simulate' the model or Hamiltonian describing the system of interest. Developing such 'quantum simulators' has become a major theme of research, as they have the potential to change the way we understand new materials and could ultimately impact on future devices and technologies of benefit to all of society.

The development of laser cooling has allowed us to cool atomic gases to temperatures less than a millionth of a degree above absolute zero where the quantum mechanical nature of particles dominates over their thermal motion. In this regime new states of matter emerge in the form of Bose-Einstein condensates and Fermi-degenerate gases. Such gases are highly controllable and offer a promising platform to implement quantum simulation protocols. In particular, ultracold heteronuclear molecules possess the controllable long-range interactions needed to engineer an important class of problems relevant to condensed-matter physics. Moreover, the crystalline structures of real materials can easily be replicated using standing waves of laser light to confine the molecules in optical lattices.

The laser cooling and trapping techniques developed for atoms do not generally work, however, for molecules due to their complex internal rotational and vibrational structure. Nevertheless, they can still be exploited by carefully assembling ultracold molecules from ultracold atoms. This approach has proved remarkably successful, with a number of different molecules having been created. The technique uses two distinct steps to associate the molecules. First, weakly bound molecules are formed using a collision resonance, known as a Feshbach resonance, which couples the free atoms into a near threshold molecular state. Secondly, the molecules are transferred to the absolute ground state using a two-photon optical transfer process, known as stimulated Raman adiabatic passage (STIRAP). Remarkably, the overall conversion process can be highly efficient with negligible heating so that the temperature and density of the resulting molecular quantum gas mirror the initial parameters of the atomic mixture.

The goal of this proposal is to realise a gas of ultracold fermionic KCs molecules by associating pre-cooled atoms of K and Cs. This molecule has the advantage over other bi-alkali molecules of being stable against reactive collisions and offers both fermionic and bosonic isotopes. By confining the molecules in an array of two-dimensional pancake traps we will deliver a test platform for quantum simulation applications. To achieve this ambitious objective we propose to combine state-of-the-art experiments in synergy with world leading theoretical support into a transformative program of research that stands to 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, both in the short and long term:

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 at least 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 highly sought after in the current job market (e.g. project and time management, communication and presentation skills). The skills they acquire could be applied in the education, defence, R&D, technology and finance sectors, for example.

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 or lasers. The nature of 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; helping ensure that the public is fully 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 internationalisation efforts (principally via the planned mini-conference on ultracold molecules where we will invite participants from the leading international groups).

4. Knowledge generation.
In the short term our proposal will yield a better fundamental understanding of interacting quantum gases, molecular physics and non-equilibrium dynamics. At the same time we will develop new experimental and theoretical techniques for manipulating and controlling quantum systems. Ultimately the realisation of a quantum simulator using fermionic particles has the potential to contribute to our understanding of a range of problems in condensed-matter physics (e.g. novel regimes of superfluidity, quantum magnetism, strongly correlated many-body systems). Our research will 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 future applications of quantum technology.