Condensing toward zero Kelvin
Bose–Einstein condensates (BECs) that form at temperatures approaching the coldest possible in the Universe (absolute Kelvin) are manifestations of a quantum effect on a macro scale, providing a test bed of the quantum world. When they are not in this equilibrium ground state but close to it at finite temperature, the system consists of some condensed matter and some not. These systems exhibit complex dynamics dependent on quantum state fluctuations around the condensed ground state. Scientists sought a deeper understanding of this phenomenon with EU funding of the project 'Number conserving approaches to Bose-Einstein condensates' (NUM2BEC). Enhanced knowledge will point the way to development of accurate non-linear interferometers (exploiting wave interference for distance measures) with improved experimental sensitivity. While there are well-established mathematical descriptions of BECs at zero-temperature, description of finite-temperature BECs remains a difficult theoretical problem. This is particularly true at low temperature where particles are pushed out of the condensate (quantum depletion of the condensate) due to external driving. The team developed equations of motion describing the coupled dynamics of the condensate and non-condensate fractions ideally suited for such systems. Further, they showed that the qualitative features of the system dynamics remain the same at finite temperature compared to zero-temperature. These results have been published. Scientists also developed models of multi-component (n-component) condensates consisting of different states, isotopes or atomic species. Again, partitioning field operators into those for condensate and non-condensate for each component, researchers were able to deliver self-consistent dynamical equations governing the behaviour of each component. The work was particularly challenging and led to three publications in peer-reviewed scientific journals. NUM2BEC algorithms describing complex behaviours of non-equilibrium BECs at finite temperatures has provided important insight that should help researchers control such systems better. Control is the key to better experiments and interpretable results. Building on the above outcomes, research is now ongoing to develop concepts for non-linear interferometers and objectives are within reach.