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In a quantum simulator, an international team of researchers was able to investigate the properties of different quantum phase transitions. Due to the complexity of the processes, these dynamics were not experimentally accessible until now. The researchers report their findings in the journal Nature.

Complex processes in the quantum world of atoms and molecules are difficult to understand, and observing them in nature is even more difficult. That is why scientists in the laboratory use well-controlled quantum systems - so-called quantum simulators - to investigate quantum processes. In recent years, programmable quantum simulators have been developed. Compared to a fully programmable quantum computer, they have a much more limited instruction set, but can be scaled to a much larger number of particles. In the USA, researchers have now used such a programmable quantum simulator to study so-called quantum phase transitions. In the experiment, 51 atoms were trapped in optical tweezers and manipulated their internal degrees of freedom with lasers. By exploiting the properties of Rydberg states, the researchers generated interactions between the atoms and thus realized a quantum-multiparticle system. "Interestingly, this system can be brought into different quantum phases, all separated by different quantum phase transitions. The dynamic transition from one phase to another is a complex process", explains Peter Zoller from the Institute of Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences.

Hypotheses experimentally confirmed

Theorist Peter Zoller and his colleagues hypothesized years ago that the transition between quantum phases can be described by "universal" laws that depend only on the type of quantum phase transition, but not on microscopic details of the system. This prediction has now been impressively confirmed in the experiment at Harvard University. "Since different types of quantum phase transitions are accessible in this experiment, it has been possible to investigate this not only in relatively well understood types of quantum phase transitions, but also in more exotic ones," says Hannes Pichler. Pichler, who was born in South Tyrol, received his doctorate with Peter Zoller and has been an ITAMP Postdoctoral Fellow at Harvard University, USA, since 2016. In close cooperation with Peter Zoller and his Innsbruck colleagues, Pichler collaborated on the conception of the experiment and, together with his colleagues, theoretically modelled the quantum system and analyzed the properties of the various phase transitions.

The research work now published in Nature was the result of a collaboration between researchers from Harvard University, MIT, Caltech and the University of Innsbruck as well as IQOQI Innsbruck and was led by Mikhail Lukin, Markus Greiner and Vladan Vuletic.

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