Whether describing the orbits of planets or the inner workings of the atom, a key paradigm in physics is the conservation of energy: While different forms of energy may be converted into one another, the total amount of energy is typically assumed to be constant over time. Along these lines, physicists tend to go through great lengths to ensure that the system they are trying to describe does not interact with its environment. Yet, as it turns out, the dynamics of a system can also be stable if the gain and loss of energy are distributed in a systematic fashion such that they cancel each other under all conceivable conditions, which can be ensured by so-called parity-time (PT) symmetry: When simultaneously exchanging gain and loss, and mirroring the geometric arrangement of its components, the system appears unchanged. Far from being a purely academic notion, PT symmetry has paved the way for a deeper understanding of open systems.
The fascinating physical phenomena associated with PT symmetry are the specialty of Prof. Alexander Szameit at the University of Rostock. His research group harnesses laser-inscribed photonic waveguides as “circuits for light” to explore the dynamics of discrete systems. In their custom photonic chips, laser light can mimic the behavior of natural and synthetic materials alike, making them an ideal testbed for a large variety of physical theories. In this vein, the scientists around Prof. Szameit managed to combine PT symmetry with the concept of topology. Szameit explains: “Topological insulators have attracted a lot of attention in the last years because of their fascinating ability to convey a lossless stream of electrons or light along their boundary. The unique capability to suppress the impact of defects and scattering makes them especially interesting for all kinds of technological applications.”
Yet, until now, such robust boundary states were thought to be fundamentally incompatible with open systems. In their joint effort, the researchers from Rostock, Würzburg and Indianapolis could show that apparent paradox can be resolved by distributing gain and loss dynamically in time. The first author, PhD student Alexander Fritzsche elaborates: “The light propagating along the boundary of our open system is like a hiker traversing mountainous terrain. Despite all ups and downs, they will inevitably end up back at the initial elevation. Similarly, the light propagating within the protected edge channel of our PT-symmetric topological insulator will never be exclusively amplified or damped, and can therefore retain its amplitude on average while enjoying the full robustness afforded by topology.”
These findings constitute an important contribution to the fundamental understanding of topological insulators and open systems, and may open the gates to a new generation of advanced circuits for electricity, light or even sound waves.
This research was funded by the German Research Foundation (DFG) and supported by the Alfried Krupp von Bohlen und Halbach Foundation.
Original Publication: Fritzsche et al., “Parity-Time-symmetric Photonic Topological Insulator”, Nature Materials (2024). Link
Contact:
Prof. Dr. Alexander Szameit
Solid-State Optics Group
Institute of Physics
University of Rostock
Phone.: +49 381 498-6790
E-Mail: alexander.szameituni-rostockde