Fractional quantised Thouless pumping of solitons demonstrated: On the trail of hidden properties of solids
Deciphering and exploiting mechanisms that influence the state of matter is often the catalyst for scientific progress. Junior Professor Christina Jörg, who conducts research at the Rhineland-Palatinate University of Technology Kaiserslautern-Landau (RPTU), focuses on the interaction processes that underlie such phenomena. In a research paper, she and her colleagues from Pennsylvania State University and the Indian Institute of Science have for the first time experimentally demonstrated fractional Thouless pumping of solitons (dimensionally stable light wave packets). The study was recently published in the prestigious journal Nature Physics.
One-dimensional Thouless pumps serve as a model for the two-dimensional quantum Hall effect. This occurs when electrons can only move in two spatial directions at low temperatures and in a strong magnetic field. Under these conditions, the voltage perpendicular to a flowing current does not increase continuously with the magnetic field, but grows gradually. Klaus von Klitzing was awarded the Nobel Prize in Physics in 1985 for the discovery of this extraordinary state. The associated Hall resistance, the ratio of Hall voltage to Hall current, is quantized; it is determined by an integer topological constant (Chern number). Thouless pumps (named after David J. Thouless, another Nobel Prize winner in physics) make this effect tangible: Charge is pumped around a constant (integer) number of lattice sites in a time-periodically modulated lattice during one period. In analogy to the quantum Hall effect, this constant is described by the Chern number. If we consider time as an "artificial" dimension, we obtain the two-dimensional quantum Hall system.
Further research has shown that at elevated magnetic fields not only phases defined by an integer constant occur. Constants corresponding to fractions of integers can also describe such phases. The cause of this so-called fractional quantum Hall effect is the interaction between electrons.
This is the starting point of first author Marius Jürgensen's study, carried out in collaboration with Sebabrata Mukherjee and Christina Jörg during their postdoctoral research in the group of Prof. Mikael C. Rechtsman at Pennsylvania State University. The team created a model system driven by photons, or light particles, to better understand interactions in quantum Hall systems. The system is based on coupled optical fibres, similar to glass fibres. "Light particles in light waves behave in a fundamentally different way to electrons in a solid," says junior professor Dr Christina Jörg, who leads the "Topology in 3D photonic quantum simulators" working group at RPTU. "They fly through each other and don't collide." So the researchers used a trick to get the photons to interact with each other: They exploited the intensity-dependent change in the refractive index of the material, glass, they were using for the experiments. "The refractive index increases the more light we introduce into the system via a pulsed laser," says the physicist. "This creates an interaction. Because the focusing due to the increased refractive index and the coupling to the neighbouring waveguides balance each other out, a dimensionally stable wave packet is formed, a so-called soliton. In the Thouless pumps, the solitons, just like the light waves in the non-interacting case, have each moved on by an entire lattice constant, which is given by the Chern number".
"We had already shown in 2021 that solitons are pumped using this method," says Marius Jürgensen, first author of the study and a PhD student at Pennsylvania State University. "In the joint research, we also observed that solitons can also move fractionally. At higher light powers, the solitons only travel half the distance. In the end, we were able to show pumping by different fractional numbers depending on the strength of the light field".
In summary, the team has succeeded in experimentally demonstrating fractional Thouless pumping of solitons for the first time. The researchers have also created a model system that provides fascinating insights into the interplay between interacting particles and the physics of the quantum Hall system. "In follow-up studies, we will investigate the exact relationship between the fractional numbers and the required interaction strength. We want to understand whether there is a direct link to the fractional quantum Hall effect.
The study can be viewed at: https://www.nature.com/articles/s41567-022-01871-x
Press contact:
Jun.-Prof. Dr. Christina Jörg
RPTU, AG Optical Technologies and Photonics
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Phone: 0631 205-4400