Cs Experiment

Universal Scaling Symmetry

Ultracold atoms unveil a universal symmetry of systems crossing continuous phase transitions

For systems near continuous phase transitions, the details don’t matter. We can apply a universal theory to understand continuous phase transitions whether they occur in biological cell membranes, magnets, liquid crystals, or even in the entire early universe! If the theory is correct, then quantum phase transitions of our ultracold atomic gases should follow the same simple rules. But while the universal theory of static systems near continuous phase transitions has been generally successful, the degree to which we can universally explain the dynamics of crossing such transitions presents an exciting new frontier. Ultracold atoms give us the power to explore this frontier by controlling many aspects of a phase transition, including how rapidly it is crossed, and carefully recording the resulting dynamics. In this way we can look for universal features of phase transition dynamics by observing whether we can really ignore the details!

In our latest work we searched for universal features of the dynamics across a quantum phase transition of ultracold atoms in a shaking optical lattice. As we increase the lattice shaking amplitude, our atoms undergo a quantum phase transition, after which they must choose between two new ground states. This causes our gas to split into domains with atoms in one state or the other, illustrated in the left panel below as regions of red and cyan atoms. We found that the details are indeed irrelevant: the growth of domains over time and their pattern across space are independent of the rate of crossing the transition, once we account for a simple power-law scaling symmetry of space and time. This is best exemplified by the right panel below, which shows that the correlations between the states of the atoms at different locations are always the same in scaled space-time. Thus, our experiments support a universal scaling symmetry of phase transition dynamics which provides a simple, powerful prediction for the behavior of a huge variety of systems when they cross continuous phase transitions. 

arXiv:1605.01023 (2016)