Chiral Magnon-Photon Coupling

• Chiral magnon-photon coupling is a phenomenon where engineered symmetry breaking creates directional, nonreciprocal interactions between spin-wave excitations and electromagnetic modes.
• The coupling exploits tailored magnetic textures and photonic structures, such as cavities and waveguides, to enable selective and robust interactions.
• This mechanism underpins advanced quantum networks and topologically protected devices, offering measurable signatures like mode splitting and chiral edge states.

Chiral magnon-photon coupling refers to engineered, symmetry-breaking regimes in which the interaction between magnons (quantized spin-wave excitations) and photons (microwave or optical electromagnetic modes) becomes directional, nonreciprocal, or selective with respect to propagation, angular momentum, or polarization. This phenomenon exploits unique features of both the magnetic systems (including topological spin textures, chiral order, or chirality-encoded symmetry) and photonic environments (such as cavities, waveguides, or metamaterials with tailored polarization properties) to realize robust, tunable, and topology-enabled functionalities in classical and quantum information devices.

1. Fundamental Mechanisms and Symmetry Breaking

Chiral magnon-photon coupling emerges from the interplay of magnetic excitations with electromagnetic modes under deliberately engineered spatial and polarization asymmetry. The presence of chirality can originate from the underlying magnetic order—as in noncollinear or topological magnets, skyrmion crystals, or altermagnets—or from the electromagnetic environment, such as waveguides or cavities with polarization–momentum locking.

Key Interactions

• Chiral Field Overlap: When magnets are placed at special positions inside a waveguide or cavity that supports modes with locked polarization and propagation direction (e.g., TE₁₀ in rectangular waveguides), the Kittel (or other magnon) mode can couple preferentially or exclusively to photons traveling in one direction. This is typically enforced by satisfying position-dependent criteria such as

$$\cot\left(\frac{\pi x_j}{a}\right) = -\sqrt{\frac{a^2\omega_l^2}{\pi^2 c^2} - 1}$$

where $x_j$ is the magnet coordinate and $a$ is the waveguide width (Yu et al., 2019).

• Non-Hermitian Effective Hamiltonians: Integrating out the photon degrees of freedom leads to asymmetric, complex (non-Hermitian) self-energies with chiral asymmetry encoded as different couplings in opposite directions,

$$\Sigma_{j\ell} = \begin{cases} -i(\Gamma_L+\Gamma_R)/2 & j=\ell \ -i\Gamma_R e^{ik_0(j-\ell)d} & j>l \ -i\Gamma_L e^{ik_0(\ell-j)d} & j<l \end{cases}$$

with $\Gamma_R\neq\Gamma_L$ indicating chirality (Yu et al., 2019, Yu et al., 2019).

• Synthetic Gauge Fields and Floquet Engineering: Temporal modulation of system parameters (Floquet engineering) can imprint phase bias and artificial flux, breaking time-reversal symmetry and realizing nontrivial chiral phase evolution among coupled magnonic modes (Qi et al., 2022).
• Breaking Microscope Symmetries: Chiral coupling arises via the breaking of time-reversal and inversion symmetry—using chiral resonators, applied magnetic fields, or symmetry-lowered molecular environments—resulting in directional nonreciprocity and polarization selectivity (Mita et al., 27 Jun 2024, Ullah et al., 16 May 2025).

2. Model Systems and Coupling Regimes

Chiral magnon-photon coupling has been realized across a variety of experimental and theoretical platforms, each displaying unique features.

Cavity QED and Microwave Structures

• Strong and Ultrastrong Regimes: In chiral cavity–magnon systems, the strong coupling regime is characterized by large cooperativity ($C \gg 1$):

$$C_i = \frac{g_i^2}{\gamma_m (\kappa_i + \kappa_i^{(c)})}$$

with normal-mode splitting observed at the magnon-photon resonance (Abdurakhimov et al., 2018).

• Torus and Helical Metamolecules: By integrating YIG-based magnetic meta-atoms with chiral (e.g., helical) resonators, coupling ratios $g/\omega$ up to 0.22 have been achieved at room temperature, reaching the ultrastrong coupling regime. The directional nature is observed as significant differences between $S_{21}$ and $S_{12}$ scattering parameters for forward and backward propagation (Mita et al., 27 Jun 2024).
• Nonreciprocal Cavity Structures: In torus-shaped (whispering-gallery mode) cavities, selective placement of magnetic elements can enable coupling only to one of two degenerate circulating photonic modes, producing nonreciprocal (unilateral) entanglement and enabling channel-multiplexed quantum teleportation (Fan et al., 4 Jan 2024).

Waveguide Arrays and Chains

• Edge Accumulation: Placing magnets at chiral positions in a waveguide (or engineering their cross-sectional placement to enforce chirality) leads to giant magnon accumulation at one edge of a chain, as only one direction is efficiently pumped by a phased antenna array (Yu et al., 2019).
• Non-Hermitian Skin Effect: The most radiative collective modes can become highly localized at a system boundary, manifesting topological-like features unique to chiral open systems (Yu et al., 2019).

Molecular and Topological Magnets

• Chiral Skyrmion-Magnon Coupling: In materials supporting skyrmion phases, strong coupling with propagating magnons is achieved only when the gyrotropic skyrmion mode has a quantum number $|l|>1$ and opposite chirality to the magnon. Hybridized polaragnonic band gaps appear as a direct signature (Liu et al., 2021).
• Molecular Magnets: Spin-vibronic coupling with chiral vibrational modes carrying angular momentum can lift degeneracy and, in the presence of broken $\mathcal{PT}$ symmetry, impart $\pi$-Berry phases to dressed states, yielding selective optical excitations and magneto-optical circular dichroism (Ullah et al., 16 May 2025).

3. Topology, Geometric Phases, and Edge States

Chiral magnon-photon coupling is intimately tied to topology in hybridized matter-light systems.

• Berry Curvature and Chern Numbers: Reformulating the coupled Landau-Lifshitz-Maxwell system as a Hermitian eigenproblem leads to hybrid magnon-photon modes acquiring Berry curvature:

$$\Omega_{z,n}(\boldsymbol{k}) = i\,\epsilon_{\alpha\beta} \partial_{k_\alpha} \boldsymbol{x}_{\boldsymbol{k}, n}^\dagger \gamma^{\perp} \partial_{k_\beta} \boldsymbol{x}_{\boldsymbol{k}, n}$$

which, integrated over momentum space, yields quantized Chern numbers for gapped bands—a haLLMark of topological matter (Okamoto et al., 2020).

• Chiral Edge Modes: According to bulk-boundary correspondence, when two magnetic domains with opposite Chern numbers meet, topological edge modes appear within the hybridization gap; these modes are manifest as dispersing, chiral edge excitations with combined magnon-photon character.
• Chiral Magnon-Polaron States: In spin-lattice-coupled magnets, circularly polarized phonon modes (chiral phonons) can hybridize with topological magnon edge states, resulting in new magnon-polaron edge modes that inherit polarization and topology from both sectors (Mella et al., 28 May 2024).
• Berry Phases and Optical Selection Rules: In molecular magnet systems, coupling to chiral vibrational modes imparts geometric phases to spin-vibronic states, resulting in selection rules and phase-dependent optical circular dichroism (Ullah et al., 16 May 2025).

4. Experimental Realizations and Observables

A diverse array of measurement techniques and configurations have confirmed and quantified chiral magnon-photon coupling:

• Anticrossing and Mode Splitting: Observation of normal-mode splitting and avoided level crossings in microwave spectroscopy directly evidences strong coupling, with couplings quantified by the magnitude of the splitting and the extracted cooperativity parameter (Abdurakhimov et al., 2018, Mita et al., 27 Jun 2024).
• Directional Nonreciprocity: S-parameter measurements ($S_{21}$ vs $S_{12}$) in chiral metamolecules reveal nonreciprocal transmission, the haLLMark of magnetochiral coupling (Mita et al., 27 Jun 2024).
• Magnon Accumulation Profiles: Magnetic resonance and antenna-based injection-detection reveal spatially resolved edge localization and imbalance in magnon number distribution, consistent with "skin effect" predictions in chiral waveguide systems (Yu et al., 2019).
• Spectroscopic Probes in 2D Magnets: Magneto-infrared and magneto-Raman spectroscopies reveal the formation and splitting of chiral magnon-polaron modes, including anomalous Raman circular polarization signatures (Cui et al., 2023).
• Floquet State Transfer: State transfer protocols with phase-modulated drives demonstrate chiral (directional) transfer and current in cavity magnonics, measurable by time-domain evolution and operator expectation values (Qi et al., 2022).

5. Applications and Quantum Information Processing

Chiral magnon-photon coupling underpins a wide range of technological and foundational advances:

• Quantum Networks and Channels: Directionally nonreciprocal chiral couplings enable robust quantum communication channels, quantum routers, and quantum state transfer schemes, with suppressed backaction and minimal cross talk (Ren et al., 2022, Fan et al., 4 Jan 2024).
• Quantum Entanglement and Squeezing: Chiral cavity-mediated protocols allow generation and stabilization of squeezed and entangled magnonic states, essential for quantum information processing and continuous-variable quantum computing. The dissipation of the cavity is harnessed to "cool" Bogoliubov magnon modes into strongly correlated states (Kang et al., 22 May 2025).
• Multiplexing and Teleportation: Selective channel addressing via chiral couplings enables multi-channel quantum teleportation, with robust entanglement that tolerates experimental imperfections (Fan et al., 4 Jan 2024).
• Topologically Protected Devices: The presence of Chern-number-induced edge modes promises the realization of topologically protected quantum interconnects and robust information transfer lines immune to scattering and defects (Okamoto et al., 2020, Mella et al., 28 May 2024).
• Nonlinear and Thresholdless Devices: Edge-enhanced magnon accumulation via chiral pumping allows low-power initiation of nonlinear effects and nonreciprocal logic elements (Yu et al., 2019).
• Hybrid Quantum Systems: Integrating superconducting qubits with magnetic structures through chiral photonic environments provides high-fidelity, directional interfaces for hybrid quantum processing (Ren et al., 2022).
• Magnon-Based Logic and Memory: The universal chiral locking relation between magnon flow and electron spin accumulation opens non-contact and tunable magnonic device architectures (Zhou et al., 18 Apr 2025).

6. Theoretical Extensions and Future Directions

Current research is expanding chiral magnon-photon coupling into new physical paradigms and architectures:

• Long-Distance Quantum Coherence: Multi-mode waveguide mediation, in combination with critical coupling, enables strong magnon-photon coupling over meter-scale distances, promising distributed and scalable quantum networks (Xiao, 3 Sep 2024).
• Chirality-Selective Coupling in Altermagnets: Dipole–dipole interactions in altermagnets induce strong, anisotropic coupling between magnons of opposite chirality, a phenomenon absent in conventional antiferromagnets and tunable for quantum routing applications (Jin et al., 24 May 2025).
• Chiral Vibrations and Spin-Orbitronics: Coupling of spins to chiral vibrational modes in molecules introduces geometric phases and new optical selection rules, highlighting the role of quantum geometry and breaking of discrete symmetries in selecting chiral interactions (Ullah et al., 16 May 2025).
• Polaritonic Chemistry and Synthetic Gauge Fields: Metamaterial architectures combining magnetic and chiral photonic elements facilitate deepstrong hybridization and synthetic gauge field realization, with implications for quantum simulation and photonic chemistry analogs (Mita et al., 27 Jun 2024).

Chiral magnon-photon coupling thus constitutes a central concept at the intersection of spintronics, quantum optics, topological matter, and hybrid quantum technology. Its future developments are anticipated to power new platforms for robust quantum state manipulation, nonreciprocal signal processing, and the topological protection of information flow in both fundamental and applied contexts.