Quasinormal modes solve challenges in Quantum Nanophotonics
Trinity researchers unify magnetodielectric cavity dynamics, a quantum nanophotonics breakthrough.
Quantum Nanophotonics A theoretical breakthrough from Trinity College Dublin's School of Physics and CRANN Institute answers a significant quantum nanophotonics challenge. In APL Quantum, Lars Meschede, Daniel D. A. Clarke, and Ortwin Hess presented a unified theoretical framework for the quantization of electromagnetic resonances, or quasinormal modes (QNMs), in spatially inhomogeneous, dissipative, and dispersive magnetodielectric resonators.
Challenge of Lossy Nanostructures Nanoscale cavity quantum electrodynamics (cQED) designs have advanced faster than theoretical models. Traditional semiconductor quantum optics uses high-quality-factor dielectric microcavities to imitate “normal modes” with genuine frequencies and limitless lifetimes. Modern nanostructures, especially plasmonic and magnonic cavities, are fundamentally different.
Plasmonic resonators are valued for confining light to sub-wavelength scales to raise local electromagnetic fields exceeding the diffraction limit. Even single molecules can achieve room-temperature cQED. Spin-photon interfaces are created by collective microwave excitations of spins in ferrimagnetic structures in cavity magnonics, a new field.
They are “lossy” because to radiative leakage and material absorption (Ohmic dissipation). Non-Hermitian systems often fail standard quantization methods or use ad hoc protocols that are not field-theoretically justifiable.
Conquering Modal Divergence Lossy cavities' natural resonances, quasinormal modes (QNMs), have intricate eigenfrequencies, making math difficult. Light leakage from these “open” cavities causes the QNM fields to diverge dramatically at long distances. This discrepancy makes orthogonality, normalization, and completeness difficult to express.
Exterior complex coordinate transformations—equivalent to PMLs—fixed the issue for Trinity. These specialised layers enclosed the computer domain, transforming unlimited space into a confined domain. This method converts unphysical radiative losses into non-radiative material dissipation to strictly regularize modes inside and outside the resonator chamber.
A New Quantum Formalism The researchers used macroscopic quantum electrodynamics to quantify these regularized modes. Their method finds canonical field variables in complex dielectric and magnetic media.
The breakthrough relies on the concept of creation and annihilation operators, which generate modal Fock states. These states describe the excitations of “field-dressed matter”—a blend of material polaritonic excitations and the electromagnetic field.
Researchers found an effective Lindblad master equation. This equation controls the quantum dynamics of modes and their interactions with quantum emitters (QEs) such molecular spins and semiconductor quantum dots. By accounting for coherent and dissipative coupling, the master equation captures complex interference effects like Fano-like features that occur when multiple modes interact with a single emitter.
Results, Numerical Validation The researchers demonstrated their framework's predictive power using two rigorous numerical examples:
Scientists tested a nitrogen-vacancy center QE diamond resonator. Even with low cavity quality factors, the quantum quasinormal modes theory predicted the Purcell factor (the amplification of spontaneous emission) with good agreement with exact semi-classical models. The theory accurately represented the system's temporal Rabi oscillations in the strong coupling region, matching mQED computations. 3D Spherical Cavity: Applying the theory to a silicon sphere in a vacuum eliminated PML layer unphysical gain contributions. Modelling resonance shapes from Lorentzian peaks to Fano-like decay rate suppressions was successful. Making Quantum 2.0 Possible The consequences of this discovery go beyond “Quantum 2.0”. This rigorous and mathematically possible nanophotonic technology evaluation helps build near-field multipartite entanglement creation, ultrafast all-optical switching, and on-demand single-photon sources.
The group also hopes its quantum quasinormal modes theory will let them use dissipation as a tool rather than a nuisance. The results include advancements in dissipative photonic time crystal engineering, non-Hermitian topological magnonics, and quantum nanoplasmonic coherent perfect absorption.