Physicists Observe the Emergence of Collective Behavior

Phase transitions describe dramatic changes in the properties of a macroscopic system – such as the transition from a liquid to a gas. Starting with ultra-cold individual atoms, physicists at the University of Heidelberg were able to observe the occurrence of such a transition with an increasing number of particles. The research work was carried out in the field of quantum physics under the direction of Prof. Dr. Selim Jochim from the Institute of Physics.

In order to formulate effective theories in physics, microscopic details have been set aside in favor of quantities observed macroscopically. A cup of water can be described by properties such as pressure, temperature and density of the liquid, while the position and speed of individual water molecules are irrelevant. A phase transition describes the change of a macroscopic system from one state of matter, such as liquid, to another state of matter, such as gas. The characteristics of macroscopic systems – the so-called multi-body systems – can be described as emergency because they result from the interaction of individual components which themselves do not have these properties.

“I have long been interested in how this dramatic macroscopic change in a phase transition comes out of the microscopic description,” says Selim Jochim. To answer this question, the researchers designed an experiment in which they assembled a system of individual ultra-cold atoms. Using this quantum simulator, they investigated how collective behavior arises in a microscopic system. For this purpose, they blocked up to twelve atoms in a strongly concentrated laser beam. In this artificial system it is possible to continuously tune the force of interaction between atoms from non-interaction to what is the largest rate of energy in the system. “On the one hand, the number of particles in the system is small enough to describe the system microscopically. On the other hand, the collective effects are already visible, ”explains Luca Bayha, a postdoc in Prof.’s team. Jochim.

In their experiment, Heidelberg physicists configured the quantum simulator in such a way that atoms attract each other, and if the attraction is strong enough, form pairs. These pairs of atoms are the essential ingredient for a phase transition into a superfluid – a state in which particles flow without friction. Actual experiments focus when pair formation appears as a function of the force of interaction and the number of particles. “The surprising result of our experiment is that only six atoms show all the signatures of a phase transition expected for a multi-particle system,” adds Marvin Holten, a doctoral student in Prof. Jochim.

In their work, researchers have benefited from the Framework of Excellence framework “STRUCTURES – A unifying approach to emerging phenomena in the physical world, mathematics and complex data” and the Collaborative Research Center “Isolated Quotation Systems and Universality in Extreme Conditions” (ISOQUANT) ”of the University of Heidelberg. Crucial to the success of the experiments was a collaboration with researchers from the universities in Lund (Sweden) and Aarhus (Denmark). The research results were published in Nature.

Reference: “Observation of the appearance of a quantum phase transition shell” by Luca Bayha, Marvin Holten, Ralf Klemt, Keerthan Subramanian, Johannes Bjerlin, Stephanie M. Reimann, Georg M. Bruun, Philipp M. Preiss and Selim Jochim, 25 November 2020 , Nature.
DOI: 10.1038 / s41586-020-2936-years