Towards low-dimensional Bose-Fermi mixtures on a microchip

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.

Since the proposed project focuses on fundamental research, direct applications will emerge on a longer timescale (> 10 years).
The research carried out with this proposal will help to model and better understand condensed matter physics especially transport and conductivity in extremely thin wires or planes, which in the future could lead to applications such as the design of new and more efficient materials or the possibility of modelling traffic jams and classical 1D transport. Quantum physics in 2 dimensional planes might also help to shed light on the unsolved problem of high temperature superconductivity. A better understanding of these questions might lead to the development of tailored materials with reduced resistivity, which may have applications in nano-sized motors or sensors and generally increase our understanding towards reducing losses and energy consumption in transport mechanisms.
Additionally the proposed research scheme will shed light onto the interactions between cold quantum gases and surfaces. This information is of fundamental importance e.g. for the creation of a chip based quantum interface in the context of information storage, quantum communication and quantum computing.

Technical developments necessary for the setup of the experiment might have an impact on small and intermediate sized companies in related areas. For example we are currently developing the design for our glass cell in collaboration with Torr Scientific. Other examples of knowledge exchange between industry and experiment might include the construction of an optical transport scheme, the improvement of our homebuilt lasers, used feedback stabilisation and electronic controllers. While these technical projects have overall only small significance for the scientific community, they might have the potential to stimulate new developments for companies on the long run.
These links to companies are further enhanced through a European Initial Training Network (ITN) on 'Quantum Technologies and Applications' (QTea), which will start in October 2012 and acts as a platform between five academic and five industrial partners.

In this context, the exploration of new atom trapping technologies on micro-chips and the influence of decoherence mechanisms will also contribute towards the development of chip-based sensors such as rotation and gravity sensors or atomic clocks. With the view of these applications being commercialised in the intermediate future (10-20 years) this research potentially contributes to the economic growth of the UK.

The proposed research will change our understanding of nature and in this way will have impact on the UK society. In order to make our findings accessible to a wide audience, we will disseminate our results through the University's homepage, through press releases on our research highlights, through articles for the general public, through undergraduate and A-level student projects (Nuffield foundation), at the University's open days and through online outreach projects (www.sixtysymbols.com).

In summary this project will have impact on the wealth, the well-being and the security of the UK society by creating fundamental understanding which will lead to applications on a long-term scale, by direct applications which can influence the UK economy on a shorter time scale and by increasing the understanding and awareness of the public.