Towards low-dimensional Bose-Fermi mixtures on a microchip

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Atoms at ultra-low temperatures form degenerate quantum states, depending on their spin either a Bose-Einstein-Condensate or a degenerate Fermi gas. Bose-Einstein-Condensation occurs when the wavelength of an individual atom becomes comparable to the distance between the atoms and the behaviour of the ensemble is therefore dictated by the wave. This is a premier example for the predictions of quantum mechanics. As a form of an extreme many body system, degenerate quantum gases are fascinating on their own and provide insight into the fundamental building blocks of our world. Because we can control these systems comparatively easily they are also extremely useful to study and model other systems, which are less well controlled or more difficult to access. Examples are the still open problem of high-temperature superconductivity, the physics of neutron stars, analogue models for gravity or modelling of a black hole and Hawking radiation. The notation of an analogue quantum simulator was formed by Richard Feynman, where he proposed that instead of trying to theoretically predict the behaviour of a complex system one could model model its behaviour with another quantum system. Such a simulator would exactly be implemented by a cold atoms mixture experiment. We propose here to study mixtures of two ultracold species formed from bosonic (caesium, with integer spin) and fermionic (lithium, with half-integral spin) atoms. This will allow for the first time the study of one-dimensional fermionic atoms (similar to electrons in a solid) in a degenerate superfluid background (similar to the phonons in a crystal). We will implement these systems on a micro-chip, which provides a particularly stable and reliable environment for studying ultracold quantum gases.For these and related experiments it will also be relevant to know the interactions between the two sorts of atoms, which we will map out over a large range of magnetic-fields. For atom-atom interactions a so-called Feshbach-resonance can exist, which enables tuning of the interaction strength from strongly attractive to non-interacting and to strongly repulsive, knowing the position and shape of this resonance is therefore of great importance.Because one species can act as the background medium for the other, this system is also extremely well-suited to the investigation of transport and non-equilibrium physics in low-dimensions. The importance of understanding these phenomena has been highlighted in EPSRC's Physics Grand Challenges.