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Cosmology in the lab

Especially interesting for
  • developers of optical ion clocks
  • fundamental research

At the QUEST Institute at PTB, a novel ion trap has been developed in which ion chains are to be used as an optical frequency standard with improved stability. Due to the very good control over the dynamics of the particles trapped therein, PTB scientists have succeeded, for the first time, in inducing spontaneous symmetry breaking in crystalline structures and in demonstrating the occurrence of crystal defects with lasercooled ions. This allows the dynamics of phase transitions and symmetry breaking in nature to be studied in the way in which they are encountered in the most various solid-state systems and in which they have led to the creation of matter in the young universe, shortly after the Big Bang.

Image of Coulomb crystals consisting of ytterbium ions, taken with a camera. The ionized atoms fluoresce in the laser light; the distance between the ions is approx. 10 μm to 20 μm. The rows (a) and (b) exhibit symmetrical arrangements (linear, zigzag). In the case of (c) and (d), the symmetry is broken due to topological defects.

Within the scope of an international cooperation project with colleagues from the Los Alamos National Lab (USA), from the University of Ulm (Germany) and from the Hebrew University in Jerusalem (Israel), PTB researchers have now, for the first time, succeeded in demonstrating topological defects in an atomic- optical experiment in the laboratory. Thereby, an accurately laser-structured ion trap allows long chains of positively charged ions to be trapped and provides optimum optical access to observe single ions. The ions are loaded into this trap under ultra-high vacuum and cooled down to temperatures of a few mK. Hereby, the trapped, charged particles repel each other inside the trap due to the Coulomb interaction and, at such ultra-low temperatures, take on a crystalline structure (Figs. a−b).

If the trap properties are modified faster than information can spread due to the sound-propagation velocity, then develindividual areas inside the crystal are not able to “communicate” with each other, so that topological defects may occur (Figs. c-d). Thereby, the spontaneous re-orientation of the Coulomb crystal follows the same rules as those describing the early universe after the Big Bang. The probability that defects may occur has been measured as a function of the quench rate and compared with predictions of the Kibble-Zurek mechanism in different regimes.

This theory was based on Kibble's thoughts about topological defects in the early universe: fractions of seconds after the Big Bang, a symmetry breaking took place, and the young universe had to “decide” which new state to adopt. Everywhere where individual areas of the universe could not communicate with each other, topological defects such as, e.g., cosmological strings and domain walls may have been created. But the Kibble-Zurek mechanism also allows statistic statements on the occurrence of defects in phase transitions in general. Due to its universal character, this theory is applicable to many fields of physics such as, e.g., the study of the transition from metals to superconductors, or the transition from ferromagnetic to paramagnetic systems. The new system now demonstrated will soon allow further experiments on phase transitions in classical systems and in the quantum world.

Scientific publication

K. Pyka, J. Keller, H.L. Partner, R. Nigmatullin, T. Burgermeister, D.-M. Meier, K. Kuhlmann, A. Retzker, M.B. Plenio, W.H. Zurek, A. del Campo, T.E. Mehlstäubler: Topological defect formation and spontaneous symmetry breaking in ion Coulomb crystals. Nat. Commun. 4, 2291 (2013)