In a groundbreaking advance in the realm of quantum physics, an interdisciplinary team led by Professors Guo-Yong Xiang and Wei Yi at the University of Science and Technology of China (USTC) has experimentally demonstrated chiral switching between collective steady states within a dissipative Rydberg gas. This pioneering work, recently published in Science Bulletin, unveils the role of a complex “Liouvillian exceptional structure,” a concept emerging from non-Hermitian quantum dynamics, which controls the system’s evolutionary pathways based purely on the direction it traverses in the parameter space. This insight pushes the frontier of non-Hermitian many-body physics far beyond single-particle systems, providing a novel mechanism for directional state control in open quantum systems.
The foundational framework of this research lies in non-Hermitian physics, which describes open quantum systems exchanging energy and information with their surroundings. Unlike closed quantum systems described by Hermitian operators guaranteeing real eigenvalues and thus stable energies, non-Hermitian systems can host exceptional points (EPs). These EPs are unique singularities in parameter space where not only the eigenvalues but the associated eigenvectors simultaneously coalesce, giving rise to rich and often counterintuitive physical phenomena, including chiral state evolution, where the final system state depends intricately on the trajectory taken around such EPs.
Traditionally, chiral dynamics emerging from exceptional points have been analyzed and observed primarily in single-particle or few-particle quantum systems, which are amenable to direct experimental control and theoretical modeling. However, the realization of such non-Hermitian topological effects in genuine many-body systems, especially those involving strong interactions and dissipations, has remained elusive due to the complex interplay of interactions and decoherence. The work by the USTC team closes this crucial gap by engineering a strongly correlated Rydberg atomic ensemble serving as a prototypical many-body open quantum system with tunable parameters.
In their experimental setup, the researchers utilized a room-temperature vapor cell containing Rubidium atoms. The atoms were excited to high-lying Rydberg states by precisely controlled laser fields. These Rydberg states are characterized by exaggerated dipole moments, which engender substantial long-range interactions between atoms, mediating a strongly correlated many-body environment. The continuous laser drive and the inevitable dissipation through spontaneous emission and dephasing render the system inherently non-Hermitian, thus effectively embedding themselves in a dissipative quantum many-body framework.
Key to the observed phenomena is the manifestation of optical bistability within the system—a regime in which the system can stably reside in one of two distinct steady states distinguished by their Rydberg populations and associated optical transmission properties. Crucially, this bistability does not arise from simple metastability or local nonlinearities but instead corresponds to a richer underlying non-Hermitian structure in parameter space. The team demonstrated that the boundary of this bistable region corresponds to a complex exceptional structure comprising exceptional lines, which converge at a higher-order exceptional point, reflecting a sophisticated topology of the system’s dynamical generator, known as the Liouvillian superoperator.
Exploring the parameter landscape constructed from laser detuning and laser power, the researchers employed slow cyclic variations encircling this exceptional structure. Remarkably, they recorded chiral state-switching behavior: traversing the loop in one direction transformed the system from a high optical transmission state to a low transmission state, while reversing the cycle direction restored the original state. This directional dependence is a striking manifestation of the global topological properties of the exceptional structure rather than a trivial hysteresis typically encountered in nonlinear optical systems.
The unique topology of the Liouvillian exceptional structure endows the system with a form of path-dependent memory and control, reminiscent of a molecular-scale “revolving door” that permits state transitions only upon encircling the exceptional structure in a particular direction. This chiral switching differs fundamentally from conventional bistable hysteresis, as the final state depends on a global geometric property of the trajectory in the multi-dimensional control parameter space, reflecting the underlying non-Hermitian topology rather than local local-bistable energy landscape features.
Prof. Wei Yi emphasized that this phenomenon is intrinsically many-body in nature, arising from the collective interactions among the Rydberg atoms and their interplay with coherent driving and dissipation. The many-body effects enrich the topology and spectral properties of the Liouvillian operator governing the system’s time evolution, enabling control paradigms that surpass single-particle scenarios. This non-trivial many-body topological control could pave new pathways for designing quantum devices that exploit dissipative engineering and non-Hermitian dynamics.
The experimental flexibility was further showcased by demonstrating external tunability of the chiral dynamics. By varying the atomic density through temperature control, the researchers modulated the effective interaction strength and dissipation rates, thereby tuning the exceptional structure’s landscape. Additionally, dressing the Rydberg states with microwave fields introduced further control knobs, allowing for modulation of energy levels and interaction pathways. This tunability offers practical avenues for harnessing and exploiting chiral state-switching in future quantum technologies.
The significance of this discovery lies not only in advancing fundamental understanding of non-Hermitian topology in many-body open quantum systems but also in establishing a new paradigm for controlling quantum states dynamically via their non-Hermitian spectral singularities. These insights inform future explorations into dissipative phase transitions, quantum information processing, and the design of novel optical and quantum devices with inherent directionality and robust control via parameter-space topology.
This research bridges experimental quantum optics, atomic physics, and mathematical physics by unveiling how complex exceptional-point structures govern emergent many-body quantum dynamics. By manipulating the topology of the Liouvillian spectrum, the team realized directional switchable states in a Rydberg gas, bringing theoretical predictions of non-Hermitian topological phenomena firmly into the realm of experimental reality. This opens exciting prospects for further exploring how topology, dissipation, and many-body interactions interplay to produce rich quantum phenomena inaccessible in closed or weakly interacting systems.
The experimental observation of chiral switching via encircling exceptional points in a dissipative Rydberg gas thus sets a precedent for future exploration of non-Hermitian many-body systems. It spotlights the importance of topological structures within Liouvillian frameworks and outlines strategic control methods using accessible parameters such as laser characteristics, atomic density, and microwave dressing. This novel approach deepens our grasp of open quantum system dynamics, particularly in strongly interacting regimes, and promises transformative applications in quantum optics and photonic device engineering.
As researchers delve further into the underlying mathematics and physics of these exceptional structures, potential applications include robust quantum memories, direction-dependent optical switches, and sensors with unprecedented sensitivity reliant on non-Hermitian topological features. The emergent control methodology through many-body parameter modulation provides a versatile platform for engineering quantum states with bespoke dynamical behaviors, a crucial step towards practical quantum technologies leveraging the subtle balance between coherence and dissipation.
This pioneering work embodies an elegant synergy between experiment and theory, highlighting how fundamental concepts such as exceptional points and non-Hermitian topology can manifest spectacularly in complex quantum many-body systems. The integration of Rydberg physics with non-Hermitian dynamics opens new horizons for understanding and harnessing quantum phenomena shaped by their environment, interactions, and global parameter-space geometry.
Subject of Research: Experimental observation and control of chiral state-switching in a dissipative Rydberg gas through Liouvillian exceptional structures in non-Hermitian many-body quantum systems.
Article Title: Chiral Switching Between Collective Steady States in a Dissipative Rydberg Gas via Liouvillian Exceptional Topology
News Publication Date: Not explicitly provided
Web References:
DOI: 10.1016/j.scib.2025.08.051
References: Published in Science Bulletin, University of Science and Technology of China (USTC) research team.
Image Credits: USTC
Keywords
Non-Hermitian physics, exceptional point, chiral state-switching, dissipative quantum systems, Rydberg atoms, Liouvillian exceptional structure, many-body quantum dynamics, optical bistability, quantum topology, open quantum systems, laser-driven atomic vapor, topological control, quantum optics.
Tags: chiral state switchingcollective steady statesdirectional state controldissipative quantum systemsexceptional points in physicsinterdisciplinary quantum researchLiouvillian exceptional structuremany-body quantum systemsnon-Hermitian quantum physicsopen quantum systemsquantum state evolutionRydberg gas dynamics
