Physicists Use Ultracold Quantum Gases to Model Topological Insulators

Insulators with conducting edges are promising materials that can be used for intriguing technological applications. Yet, to date, researchers have failed to gain in-depth insights into their properties.

Artificial edge in an optical lattice (blue), filled with an ultracold quantum gas that consists of “spin-up” particles (red) and “spin-down” particles (green). Along the edge—and only there—“spin-up” particles can only flow to the left, and “spin-down” particles can only flow to the right. (Image credit: Bernhard Irsigler)

At present, Goethe University physicists have modeled the so-called topological insulators using ultracold quantum gases. They have shown ways to experimentally detect the edge states in the latest issue of Physical Review Letters.

Consider an insulator made in the form of a disk, including a conducting edge along which current always flows in the same direction.

This makes it impossible for a quantum particle to be impeded, because the state of flowing in the other direction simply doesn’t exist.

Bernhard Irsigler, Study First Author, Goethe University

Put differently, there is no resistance to current flow in the edge state. This could be applied, for instance, to enhance the energy efficiency and stability of mobile devices. Studies are also being carried out to find ways to exploit this to develop highly efficient lasers.

In the recent past, ultracold quantum gases have also been used to produce topological insulators to gain better insights into these insulators. These gases are produced upon cooling a normal gas down to temperatures from one-millionth to one-billionth of a degree above absolute zero. This renders ultracold quantum gases extremely cold places in the universe. Upon producing the ultracold quantum gas also in an optical lattice formed by laser light, the gas atoms get self-arranged as uniformly as in a solid’s crystal lattice. Yet, in contrast to a solid, it is possible to change various parameters, enabling the analysis of artificial quantum states.

We like to call it a quantum simulator because this kind of system reveals many things that take place in solids. Using ultracold quantum gases in optical lattices, we can understand the basic physics of topological insulators.

Jun-Hui Zheng, Study Co-Author, Goethe University

However, a major difference between a quantum gas and a solid is that there are no defined edges in the case of cloud-shaped gases. Then in what ways does a topological insulator placed in an ultracold gas decide the location of its edge states? The scientists in Professor Walter Hofstetter’s research team from the Institute for Theoretical Physics at Goethe University have answered this question as part of their research by modeling an artificial barrier between a normal insulator and a topological insulator. This denotes the topological insulator’s edge along which the conducting edge state is formed.

We demonstrate that the edge state is characterized through quantum correlations that could be measured in an experiment using a quantum gas microscope. Harvard University, MIT and the Max-Planck-Institute for Quantum Optics in Munich all carry out these kinds of measurements.

Walter Hofstetter, Institute for Theoretical Physics, Goethe University

A quantum gas microscope is a device that can be used to detect individual atoms in experiments. “For our work, it is critical that we explicitly take into account the interaction between the particles of the quantum gas. That makes the investigation more realistic, but also much more complicated. The complex calculations could not be carried out without a supercomputer. The close collaboration with leading European scientists within the context of the DFG Research Unit ‘Artificial Gauge Fields and Interacting Topological Phases in Ultracold Atoms’ is also of particular importance for us,” added Hofstetter.

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