A Stable Quantum Gas of Fermionic Polar Molecules

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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.