Quantum Emitters in Dissipative Baths Exhibit Accelerated Spontaneous Emission Via Spectral Restructuring

Controlling interactions between light and matter forms the cornerstone of modern optics and emerging technologies, and researchers are increasingly exploring how engineered environments can enhance these interactions. Stefano Longhi from Politecnico di Milano and IFISC, along with colleagues, investigates the behaviour of a single atom-like emitter when coupled to a specially designed, energy-dissipating environment. This work reveals that subtle changes to the environment’s properties can dramatically alter how quickly the emitter releases energy, uncovering an optimal configuration for accelerated emission. The team demonstrates that these changes stem from a restructuring of the energy landscape, linked to the emergence of ‘virtual exceptional points’, a concept borrowed from broader physics, and highlights the potential for precisely controlling energy transfer through careful dissipation engineering.

Non-Hermitian photonic baths, where dissipation fundamentally alters spectral and dynamical properties, provide versatile platforms for this control. This research investigates phase transitions and the emergence of virtual exceptional points in quantum emitters coupled to dissipative baths, focusing on the interplay between coherent and incoherent processes. The study demonstrates that by tuning the dissipation strength and the emitter-bath coupling, one can induce a transition from a conventional phase with real energy levels to a non-Hermitian phase exhibiting complex energy levels and a merging of spectral resonances.

Specifically, the research reveals the existence of virtual exceptional points, which are parameter-dependent points where multiple energy levels and corresponding properties coincide, but are not directly observable in the spectrum. These virtual exceptional points profoundly influence the dynamics of the quantum emitter, leading to enhanced sensitivity to external perturbations and non-classical correlations. The findings contribute to a deeper understanding of open quantum systems and pave the way for novel applications in quantum sensing, metrology, and information processing.

Controlling Dissipation in Open Quantum Systems

This body of work focuses on Non-Hermitian Physics, Open Quantum Systems, Waveguide Quantum Electrodynamics, Dissipative Quantum Systems, and Bound States in the Continuum. A strong emphasis exists on how these concepts manifest in optical systems and increasingly in circuit QED. A significant thread is the exploration of how dissipation can be controlled and even utilized to achieve novel quantum phenomena. The research also touches on fundamental aspects of quantum decay, resonances, and the interplay between quantum mechanics and classical physics. Research explores the foundations of Non-Hermitian Quantum Mechanics, spectral singularities, and complex scattering potentials, establishing the theoretical framework for understanding how non-Hermitian Hamiltonians describe physical systems.

Other work focuses on photon localization and population trapping in coupled-cavity arrays, a key area where non-Hermitian effects become prominent. Researchers also investigate the quantum Mpemba effect and virtual atom-photon bound states, demonstrating how dissipation can be manipulated. Further studies focus on the theoretical understanding of Bound States in the Continuum, localized states within the continuous spectrum of energies, enabled by non-Hermitian effects. Research in Waveguide QED and Circuit QED investigates atom-field dressed states in slow-light waveguide QED, crucial for quantum information processing.

Dynamical signatures of bound states in waveguide QED are also explored, alongside the experimental realization of atom-photon bound states in coupled resonator arrays, a significant step towards building quantum networks. Researchers are controlling atom-photon bound states in coupled resonator arrays with two-level quantum emitters. Studies in circuit QED focus on dissipation and ultrastrong coupling, key for building superconducting qubits, and the connection between non-Hermitian physics and master equations, which describe the evolution of open quantum systems. Foundational work investigates the theory of unstable quantum systems, decay processes, and the interplay between quantum mechanics and classical physics, covering topics like the decay of unstable particles, the quantum Zeno effect, and the role of the environment in quantum decay.

Researchers also investigate non-Markovian decay and lasing conditions in optical microcavities coupled to structured reservoirs, where the environment’s memory effects are important. Experimental demonstrations of non-Markovian dynamics in photonic systems are also reported. Further studies explore effective quantum Zeno dynamics in dissipative quantum systems and decoherence-driven power-law spontaneous emission in waveguide quantum electrodynamics. Specific applications and emerging areas are also investigated, including quantum heat engines, exploring the use of non-Hermitian effects and Liouvillian exceptional points to enhance performance.

Researchers are also investigating the properties and applications of Liouvillian exceptional points, which arise in the dynamics of open quantum systems, and virtual exceptional points in electromechanical systems. Topological dynamics in optomechanical systems are also explored, alongside aspects of quantum control and measurement, particularly in the context of waveguide QED and circuit QED. Key takeaways from this research include the rapidly evolving nature of the field, its interdisciplinary nature drawing on quantum optics, condensed matter physics, and quantum information theory, and a focus on controlling dissipation.

Dissipation Optimizes Emitter Relaxation Dynamics

This work investigates the relaxation dynamics of a single quantum emitter interacting with a semi-infinite dissipative photonic lattice, a system relevant to waveguide quantum electrodynamics. Researchers uncovered a rich interplay between the emitter and its environment, demonstrating that the rate of spontaneous emission is strongly influenced by the level of dissipation in the surrounding lattice. Specifically, the team identified an optimal level of dissipation where the emitter relaxes faster than in either weakly or strongly dissipative environments, revealing dissipation as a controllable resource for accelerating relaxation and enhancing light-matter coupling. The findings demonstrate that the nature of dissipation, uniform losses in this case, profoundly impacts emitter dynamics, going beyond simply the presence of loss.

This control arises from a restructuring of the energy levels within the system, traced to the merging of resonant states, which the researchers interpret as “virtual exceptional points”. The team’s analysis reveals distinct dynamical phases and transitions in the decay of spontaneous emission as dissipation is varied. These results contribute to the growing field of dissipation engineering, offering insights into designing systems with enhanced light-matter coupling for potential technological applications.