An international cooperation of physicists from the University of Rostock, the Cluster of Excellence ct.qmat, the Julius-Maximilians University of Würzburg and the Indiana University Indianapolis (IUPUI) have shown for the first time that light can propagate without any loss in systems that interact with their environment. Previously, it was assumed that such open systems inevitably would exhibit exponential amplification or damping of light and thus lead to the instability of the system. The new results could become the basis for future development of new, robust circuits for electricity, light and sound waves.

While different forms of energy may be converted into one another, the total amount of energy is typically assumed to be constant over time. Therefore, physicists usually tend to make sure 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 out under all conceivable conditions, which can be ensured by parity-time (PT) symmetry: The components in the system are arranged in such a way that an exchange of gain and loss of light through the simul­taneous mirroring and time reversal makes the system appear unchanged. Far from being a purely academic notion, PT symmetry has paved the way for a deeper under­standing of open systems.

Physical phenomena associated with PT symmetry are the specialty of Alexander Szameit at the University of Rostock. In their custom photonic chips, laser light can mimic the behavior of natural and synthetic materials, that are arranged in periodic lattice structures, alike, making them an ideal testbed for a large variety of physical theories. In this way, Szameit and his team have managed to combine PT symmetry with the concept of topology. Topology studies properties that do not change despite the under­lying system being conti­nuously deformed. Such properties then make a system parti­cularly robust against external influences.

For their experiments, Szameit's research group uses laser-inscribed photonic waveguides. In these circuits for light, topo­logical insulators are realized. Szameit explains: “These insulators have attracted a lot of attention in recent 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 parti­cularly interesting for all kinds of techno­logical appli­cations.” However, 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 were able to show that apparent paradox can be resolved by dynamically distributing gain and loss over time. PhD student Alexander Fritzsche elaborates: “The light propagating along the boundary of our open system is like a hiker traversing moun­tainous terrain. Despite all the ups and downs, they will inevitably end up back at the initial elevation of the starting point. 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 average amplitude while enjoying the full robustness afforded of the topology.” These findings are 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. (Source: U. Rostock)

Reference: A. Fritzsche et al.: Parity–time-symmetric photonic topological insulator. Nat. Mater., online 9 January 2024; DOI: 10.1038/s41563-023-01773-0

Link: Solid-State Optics Group, Institute of Physics, University of Rostock, Rostock, Germany