Investigation of universal non-equilibrium dynamics using coupled 2-D quantum systems

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Systems that are not in equilibrium are ubiquitous but can be complex to describe. Systems at equilibrium are described with great success by statistical mechanics but there are no general theoretical framework for how closed many-body quantum systems evolve to reach such thermalised states. Examples range from the cooling of a cup of coffee to the emergence of structures in the early universe. Non-equilibrium (NEQ) processes are also important for quantum systems including quantum computers such as those based on superconducting qubits. Our experimental techniques allow many-body quantum systems to be prepared in precisely defined NEQ situations and then track their evolution towards equilibrium in unprecedented level of detail.

The system that we will use to gain a better understanding of NEQ physics is a two-dimensional (2D) gas of atoms at temperatures of tens of nanokelvin. The properties of 2D systems are of central importance in physics and part of the Nobel prize for Physics (2016) was awarded to Kosterlitz and Thouless for their work on a phase transition in 2D systems that is named after them, the Berezinskii-Kosterlitz-Thouless (BKT) transition. This transition occurs as the 2D quantum gas is cooled and, at a certain temperature, it changes into a superfluid that flows without friction amongst other fascinating properties.

The ultracold atoms are trapped in extremely well-controlled conditions thus enabling us to make definitive quantitative comparisons with theoretical expectations. Quantum systems confined to 2D are especially interesting for studying NEQ processes because the fluctuations, that are an inherent part of quantum mechanics, play a large role in preventing true long-range order. This approach will provide insights into similar phase transitions in other 2D systems such as thin-film superconductors and liquid crystals, and the quantum gas acts as a quantum simulator of 2D quantum physics in general.
A key factor that enables the proposed investigation is the double-well potential for ultracold rubidium atoms that we have created by an innovative use of combined radiofrequency (RF) and static magnetic fields. With this technique we have realised a bilayer of 2D quantum gases where the inter-layer distance is controlled with a precision of tens of nanometres, which is impossible with alternative (optical) methods that are widely used. This allows the quantum coupling between two layers to be set to precise values, and we use the programmability of modern RF electronics to implement dynamical control of the double-well potential with nanosecond resolution. A further advantage of having two layers, is that we can use matter-wave interference of the ultracold atoms to probe the microscopic phase fluctuations of the system that are intrinsic in 2D quantum gases.

This allows us to probe the local vortex density and first-order correlation functions which are the key to understanding BKT physics. Further technical improvement will allow the detection of higher-order correlations, as well as the full probability distribution function of the fluctuating observables, which represent the essence of quantum observables. Using this cold-atom apparatus as a 'quantum simulator' of many-body phases in 2D systems will provide fresh insights. These experimental techniques have been developed and refined to the level at which the quantum tunnelling between the two wells is controllable and this state-of-the-art apparatus enables the experimental investigation of long-standing research questions.


10 25 50