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New phenomena in quantum matter observed – scientists at Universität Hamburg analyze dynamical phase transitions

For the first time, scientists of CUI, a cluster of excellence at Universität Hamburg, have observed a dynamical quantum phase transition in ultracold quantum gases. These gases, which are almost at absolute zero temperature, can be analyzed to determine the dynamics of atoms and molecules with the help of laser light. The scientists in the team of Prof. Klaus Sengstock and Dr. Christof Weitenberg as well as Prof. Ludwig Mathey report their basic research findings in the journal Nature Physics.

In conventional phase transitions order either occurs or disappears when varying an external parameter such as temperature (left). Dynamical phase transitions occur as a function of time and order is the system’s answer to a sudden change of the external parameter after a certain period of time (right).

Phase transitions are a fundamental phenomenon in nature, where matter changes from one state to another. The freezing of water, for example, is a well-known phase transition: water molecules arrange in ice crystals and consequently reach a state of higher order. A so-called “dynamical phase transition” between ordered and non-ordered states is a completely new phenomenon that can occur in isolated quantum systems, which do not exchange matter or energy with the environment. The external parameter, which in the example of water and ice is temperature, is given by time in dynamical phase transitions. With regard to this example CUI scientists would ask: What happens to water in a glass when the glass is shaken? While water in a glass cannot change dynamically between liquid and solid phases, this is, however, possible in the quantum world since particles in quantum systems have properties that do not occur in everyday life.

In order to observe such a dynamical phase transition in a quantum system, the team around Prof. Klaus Sengstock and Dr. Christof Weitenberg as well as Prof. Ludwig Mathey used ultracold atoms in an artificial crystal made of laser beams. The scientists brought this so-called optical lattice out of equilibrium, so to say by shaking it. Depending on the duration they let the system evolve, suddenly a new order occurred in the form of dynamical quantized vortices. After an even longer time period, these vortices suddenly disappeared. The system oscillated between the two different orders.

These experiments shed new light on non-equilibrium dynamics of quantum matter. In particular, they demonstrate that one can learn much about the fundamental properties of systems by strongly exciting them. The contribution to fundamental research in physics effectively increases our understanding of dynamical processes in quantum systems. Text: CUI

 

Citation:
N. Fläschner, D. Vogel, M. Tarnowski, B. S. Rem, D.-S. Lühmann, M. Heyl, J. C. Budich, L. Mathey, K. Sengstock & C. Weitenberg
“Observation of dynamical vortices after quenches in a system with topology”
Nature Physics (2017)
DOI: 10.1038/s41567-017-0013-8