Non-Hermitian topology has revolutionized our understanding of energy gaps and band topology, unveiling phases that do not exist within the Hermitian framework. Nonetheless, its fundamental implications for quantum interactions in open quantum systems remain largely unexplored. Here, we uncover an interaction mechanism by examining a quantum-optical system where quantum emitters interact through the photonic band gap of a dissipative photon bath with periodic boundaries, described by a nonreciprocal Su-Schrieffer-Heeger model. Although localized photons within the gap should inhibit interactions between emitters in certain regimes, we find that interactions emerge even over long distances, defying conventional expectations. These anomalous interactions are mediated by a “mirage bath”—a virtual bath that unfolds onto a distinct layer of the Riemann surface. This mirage bath generates emitter dynamics identical to those produced by the physical bath but possesses distinct band topology. Crucially, the interactions inherit the topology of the mirage bath, not the physical one. This bath duality is universal to any dissipative bath with point gaps, leading to a mechanism for interactions and correlations across the multilayered Riemann surface, unseen in traditional settings. We demonstrate that the topological mismatch between the dual baths arises necessarily from their equivalent observable dynamics. We further show how such dynamical equivalence enforces the unification of non-Hermitian topologies with different boundaries. Our findings open avenues in quantum optics, many-body quantum simulations, and offer fresh insights into non-Hermitian topology.
Non-Hermitian topology has revolutionized our understanding of energy gaps and band topology, unveiling phases that do not exist within the Hermitian framework. Nonetheless, its fundamental implications for quantum interactions in open quantum systems remain largely unexplored. Here, we uncover an interaction mechanism by examining a quantum-optical system where quantum emitters interact through the photonic band gap of a dissipative photon bath with periodic boundaries, described by a nonreciprocal Su-Schrieffer-Heeger model. Although localized photons within the gap should inhibit interactions between emitters in certain regimes, we find that interactions emerge even over long distances, defying conventional expectations. These anomalous interactions are mediated by a “mirage bath”—a virtual bath that unfolds onto a distinct layer of the Riemann surface. This mirage bath generates emitter dynamics identical to those produced by the physical bath but possesses distinct band topology. Crucially, the interactions inherit the topology of the mirage bath, not the physical one. This bath duality is universal to any dissipative bath with point gaps, leading to a mechanism for interactions and correlations across the multilayered Riemann surface, unseen in traditional settings. We demonstrate that the topological mismatch between the dual baths arises necessarily from their equivalent observable dynamics. We further show how such dynamical equivalence enforces the unification of non-Hermitian topologies with different boundaries. Our findings open avenues in quantum optics, many-body quantum simulations, and offer fresh insights into non-Hermitian topology.
We investigate quantum correlations and entanglement in a non-Hermitian quantum system modeled by a 2D stacked Su–Schrieffer–Heeger (SSH) lattice incorporating nonreciprocal hopping amplitudes and balanced on-site gain and loss. Focusing on the interplay between non-Hermiticity, topology, and open-boundary conditions, we analyze how these factors shape bipartite entanglement properties, characterized through the entanglement spectrum and entanglement negativity ( E N ). By examining the complex energy spectrum under open boundary conditions, we identify critical points associated with the closing of complex energy gaps, which significantly influence entanglement structure and signal topological phase transitions. Our findings reveal that boundary-localized non-Bloch modes and nonreciprocal couplings can induce highly tunable and unconventional entanglement features, absent under periodic boundary conditions. These results highlight the breakdown of conventional bulk-boundary correspondence and underscore the utility of entanglement measures in diagnosing non-Hermitian topological phases. Beyond advancing the fundamental understanding of quantum correlations in open systems, our study suggests potential pathways to engineer quantum devices and topological materials where gain, loss, and nonreciprocity are intrinsic elements.
Controlling atom–photon interactions in engineered environments is central to quantum optics and emerging quantum technologies. Non-Hermitian (NH) photonic baths, where dissipation fundamentally reshapes spectral and dynamical properties, provide versatile platforms for such control. Here, we investigate the relaxation dynamics of a single two-level quantum emitter coupled to the edge of a semi-infinite dissipative bosonic lattice with uniform loss. Despite the simplicity of this bath, we uncover rich dynamical phase transitions, i.e., qualitative changes in spontaneous-emission decay as system parameters are varied. In particular, we establish the existence of an optimal dissipative environment for accelerated spontaneous emission. The phase transitions are traced to spectral restructuring of the resolvent, in some cases governed by the coalescence of resonance states on the second Riemann sheet. We identify these coalescences as virtual exceptional points (EPs) of resonance origin, providing a conceptual bridge with EP physics while highlighting distinctive features of infinite-dimensional NH systems. More broadly, our results illustrate how the specific nature of dissipation—whether uniform losses, staggered losses, or dephasing—can profoundly impact emitter relaxation, pointing to dissipation engineering as a versatile tool for quantum technologies.
Photon-mediated interaction between quantum emitters in engineered photonic baths is an emerging area of quantum optics. At the same time, non-Hermitian (NH) physics is currently thriving, spurred by the exciting possibility to access new physics in systems ruled by non-trivial NH Hamiltonians—in particular, photonic lattices—which can challenge longstanding tenets such as the Bloch theory of bands. Here, we combine these two fields and study the exotic interaction between emitters mediated by the photonic modes of a lossy photonic lattice described by a NH Hamiltonian. We show in a paradigmatic case study that structured losses in the field can seed exotic emission properties. Photons can mediate dissipative, fully non-reciprocal interactions between emitters with range critically dependent on the loss rate. When this loss rate corresponds to a bare-lattice exceptional point, the effective couplings are exactly nearest neighbor, implementing a dissipative, fully non-reciprocal Hatano–Nelson model. Counterintuitively, this can occur irrespective of the lattice boundary conditions. Thus photons can mediate an effective emitter’s Hamiltonian which is translationally invariant despite the fact that the field is not. We interpret these effects in terms of metastable atom–photon dressed states, which can be exactly localized on only two lattice cells or extended across the entire lattice. These findings introduce a paradigm of light-mediated interactions with unprecedented features such as non-reciprocity, non-trivial dependence on field boundary conditions, and range tunability via a loss rate.
Non-Hermitian systems can exhibit unique quantum phases without any Hermitian counterparts. For example, the latest theoretical studies predict a new surprising phenomenon that bulk bands can localize and dissipate prominently at the system boundary, which is dubbed the non-Hermitian edge burst effect. Here we realize a one-dimensional non-Hermitian Su-Schrieffer-Heeger lattice with bulk translation symmetry implemented with a photonic quantum walk. Employing time-resolved single-photon detection to characterize the chiral motion and boundary localization of bulk bands, we determine experimentally that the dynamics underlying the non-Hermitian edge burst effect is due to the interplay of non-Hermitian skin effect and imaginary band gap closing. This new non-Hermitian physical effect deepens our understanding of quantum dynamics in open quantum systems.
The study of quantum criticality has historically been confined to Hermitian systems, where closed-system approximations neglect environmental interactions. However, a significant portion of quantum phenomena occurs in open systems, necessitating a departure from this conventional framework. This paper explores non-Hermitian quantum criticality in open many-body systems, delving into the unique phases and phase transitions that emerge due to dissipation, gain, and other non-conservative interactions. We establish a comprehensive framework that integrates concepts from biorthogonal quantum mechanics, Liouvillian dynamics, and tensor network methods to characterize critical phenomena, including dynamical quantum phase transitions and the emergence of non-Hermitian quantum many-body scars. Through an extensive literature review, we highlight how the interplay of non-Hermiticity with many-body correlations leads to novel critical behaviors, distinct from their Hermitian counterparts, and in some cases, surprisingly resilient or even enhanced. We discuss the role of exceptional points as hallmarks of non-Hermitian systems, their influence on energy spectra, and their implications for criticality and topological invariants. Furthermore, we examine the methodologies for defining observables and statistical ensembles in these non-Hermitian settings, emphasizing the distinctions and occasional controversies that arise. The findings provide critical insights into the fundamental principles governing quantum criticality in dissipative environments and pave new avenues for engineering and understanding open quantum technologies, from quantum sensing to robust quantum information processing.
The non-Hermitian skin effect (NHSE), the accumulation of eigen-wave functions at boundaries of open systems, underlies a variety of exotic properties that defy conventional wisdom. While the NHSE and its intriguing impact on band topology and dynamics have been observed in classical or photonic systems, their demonstration in a quantum gas system remains elusive. Here we report the experimental realization of a dissipative Aharonov-Bohm chain-non-Hermitian topological model with NHSE-in the momentum space of a two-component Bose-Einstein condensate. We identify signatures of the NHSE in the condensate dynamics, and perform Bragg spectroscopy to resolve topological edge states against a background of localized bulk states. Our Letter sets the stage for further investigation on the interplay of many-body statistics and interactions with the NHSE, and is a significant step forward in the quantum control and simulation of non-Hermitian physics.