A research team led by Prof. Guo-Yong Xiang and Prof. Wei Yi from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, has reported the experimental observation of chiral switching between collective steady states in a dissipative Rydberg gas. This phenomenon, underpinned by a unique "Liouvillian exceptional structure" inherent to non-Hermitian physics, allows the state of the system to be controlled by the direction in which it is tuned through the parameter space, much like a revolving door that only allows exit in one direction. The results were published in Science Bulletin .
Open quantum systems, which interact with their environment, are intrinsically described by non-Hermitian physics. A hallmark of such systems is the exceptional point (EP), a singularity where eigenvalues and eigenvectors coalesce. Encircling an EP in the parameter space can lead to chiral behavior, where the final state depends on the direction of the parameter change. Conventionally observed in single-particle systems, demonstrating this effect in a genuine many-body setting has remained a significant challenge.
This new work bridges that gap. The team used a room-temperature vapor cell of Rubidium atoms. By exciting the atoms to high-lying Rydberg states using lasers, they created a strongly interacting many-body system. The interplay between the external drive, dissipation, and strong interactions leads to optical bistability, where the system can settle into one of two steady states with distinct Rydberg populations and optical transmissions.
Crucially, the team revealed that the boundary of this bistable region is not a simple transition line but forms an exceptional structure—specifically, exceptional lines that merge at a higher-order exceptional point. This structure in the parameter space of laser detuning and power dictates the system's long-time dynamics.
By slowly varying the system parameters along a closed loop around this exceptional structure, the researchers demonstrated chiral state-switching. When the parameters were tuned clockwise, the system would switch from a high-transmission state to a low-transmission state. Tuning counter-clockwise from the same starting point would return the system to its original state. The chirality of this switching could be reversed by starting from the other steady state.
"This is distinct from the simple hysteresis," explained Prof. Wei Yi. "The outcome is determined by the global topology of the exceptional structure, not just the local parameter sweep. It's the many-body effect that endows us with this novel form of control."
Furthermore, the team showed that this chiral dynamics, born from many-body effects, can be efficiently controlled by tuning the atomic density via temperature or by using a microwave field to dress the Rydberg states, offering practical knobs for manipulation.
This work introduces a novel non-Hermitian perspective to the rich physics of dissipative Rydberg gases. It provides a new paradigm for chiral state-switching controlled by many-body parameters, with potential applications in the development of novel optical device, and for exploring fundamental non-Hermitian physics in the many-body regime.
Science Bulletin
Experimental study