Observation and Control of Spontaneous Magnon Emission from Spin Ensembles in 2D Hexagonal Boron Nitride

Abstract: Hybrid systems consisting of color centers and magnetic materials provide an appealing solid-state platform for advancing the burgeoning quantum technological revolution. Exploring novel coupling mechanisms between optically active spin defects and quantum degrees of freedom is directly relevant in this context. Here, we report observation and control of spontaneous magnon emission from boron-vacancy centers in 2D hexagonal boron nitride (hBN), an unconventional qubit-magnon dipole coupling channel that dominates in the near-zero temperature limit. The spontaneous magnon emission process starts to be overshadowed by thermal magnon effect as temperature increases, reflecting the crossover from an emission-dominated, effectively cold magnon reservoir to a thermally occupied spin bath where absorption and stimulated processes restore balance. By increasing the spin defect density, we further present that spontaneous magnon emission into a common spin bath could help establish quantum correlations in dense hBN spin ensembles. Our results are quantitatively captured by detailed theoretical modeling, bringing insights into understanding qubit-magnon coupling, correlated spin dynamics, and many-body physics of color centers in the quantum regime.

• The paper shows that spontaneous magnon emission from VB⁻ centers in 2D hBN dominates at millikelvin temperatures, confirming quantum emission in defect ensembles.
• It employs optical spin relaxometry with microwave manipulation and magnetic field tuning to control magnon spectral density and coupling strength.
• Density-dependent collective effects and the quantum-to-thermal crossover reveal potential for scalable quantum networks and advanced spintronic applications.

Observation and Control of Spontaneous Magnon Emission from Spin Ensembles in 2D Hexagonal Boron Nitride

Introduction and Motivation

Hybrid quantum platforms integrating color centers and collective bosonic excitations such as magnons underpin advances in quantum metrology, quantum optics, and quantum information science. The boron-vacancy (VB$^{-}$) center in two-dimensional (2D) hexagonal boron nitride (hBN) serves as a robust spin qubit with favorable optical and spin properties, and its compatibility with van der Waals materials enables precise proximity engineering with magnetic nanodevices. Despite prior progress in room- and cryogenic-temperature quantum sensing and coherent control with hBN color centers, the quantum spin relaxation and qubit-magnon interaction mechanism in the millikelvin regime remained elusive.

This work demonstrates direct observation and systematic control of spontaneous magnon emission from boron-vacancy centers in 2D hBN, interfaced with a yttrium iron garnet (YIG) thin-film magnon bath. A comprehensive theoretical and experimental analysis reveals the transition between spontaneous quantum emission and thermal magnon-dominated processes. The experimental platform pioneers the study of quantum magnonics at sub-Kelvin temperatures and provides new understanding on correlated spin relaxation dynamics in defect ensembles, relevant for the development of long-range entanglement and cooperative effects in quantum networks.

Experimental Platform and Methods

The system consists of exfoliated hBN nanoflakes containing controlled densities of VB$^{-}$ spin ensembles, placed on metallic Au striplines atop a high-quality 110-nm-thick YIG film. The spin defects, generated by neutron irradiation, exhibit S=1 ground-state spins addressed optically and microwave manipulated via on-chip striplines. Temperature is tuned from $\sim$3.5 K down to 58 mK, enabling access to regimes where the magnon bath resides near its quantum ground state. The magnon bath is characterized by Bose-Einstein statistics, with mode populations determined by the Zeeman-split spin resonance of the VB$^{-}$ center and the magnon band structure of YIG.

Spin relaxometry is conducted optically, quantifying photoluminescence decay of the defect ensemble under controlled optical and microwave pulse sequences. Magnetic field tuning and stripline thickness variation manipulate magnon spectral density, defect-magnon separation, and the resonance condition.

Results: Spontaneous Magnon Emission in the Near-Zero Temperature Limit

Systematic measurements reveal that in isolated hBN nanoflakes, the intrinsic spin relaxation rate $\Gamma$ of VB$^{-}$ centers decreases by over two orders of magnitude when cooling from several Kelvin to tens of millikelvin, consistent with the suppression of phonon-mediated relaxation. When hBN flakes are coupled to YIG, the magnon-mediated spin relaxation channel dominates in the low-temperature limit.

In the quantum regime ($k_BT \ll hf_{\text{ESR}}$), where the thermal magnon occupation is strongly suppressed, spontaneous emission of magnons analogous to vacuum spontaneous photon emission becomes the primary relaxation mechanism. This is evidenced by saturation of the spin relaxation rate to a nonzero floor at the lowest measured temperatures. The transition rate quantitatively matches the theoretical model predicting emission set by the qubit-magnon dipole coupling and the magnon spectral density, parameterized in terms of the physical separation, defect density, and magnon transfer function.

Increasing temperature produces a crossover to a regime where thermal magnon absorption and stimulated emission balance the spontaneous channel, with a clear linear-in-temperature dependence typical of Bose statistics at $k_BT \gg hf_{\text{ESR}}$. The data explicitly resolve the transition between quantum and classical magnonics, verifying theoretical predictions.

Control of Magnon Emission via Magnetic Field and Defect–Magnon Proximity

The coupling strength and spontaneous emission rate are modulated by two primary methods: (1) varying the Au stripline thickness, which adjusts the hBN–YIG separation, demonstrates the expected power-law enhancement in spontaneous emission with reduced qubit-bath distance; (2) sweeping the external magnetic field tunes the ESR transition across the YIG magnon band structure, mapping out the spin-wave dispersion and revealing the wavevector and density-of-states dependence of the emission process.

Measured spin relaxation rate is found to decrease monotonicly with increasing field due to k-space selection rules and the decrease of available low-k magnon modes for energy-matching emission. This behavior is robustly reproduced by the derived analytic model.

Emergence of Magnon-Mediated Quantum Correlations in Dense Spin Ensembles

The study extends to the collective regime by engineering hBN flakes with variable VB$^{-}$ center densities. In the dilute limit, spin relaxation is consistent with independent, uncorrelated defect decay. For high-density samples, however, the experimental data show an enhanced, density-dependent relaxation rate that exceeds the prediction for independent emitters. A model incorporating collective emission (correlated magnon output between spatially clustered centers) captures this superradiance-like enhancement, which is suppressed as temperature rises due to increased thermal magnon occupation.

This magnon-mediated cross-coupling establishes the essential mechanism for reservoir-induced collective spin phenomena (superradiant and subradiant relaxation, nonlocal interactions), with direct implications for scalable quantum information processing and quantum metrology. The theoretical framework developed provides a quantitative handle for predicting these effects in engineered hybrid quantum platforms.

Theoretical and Practical Implications

The demonstration of spontaneous magnon emission into a quantum magnon bath realizes a solid-state analog to paradigmatic quantum optical phenomena, but in the condensed matter context with tunable, long-range interactions and on-chip integrability. The results have far-reaching implications:

• Solid-state magnonics in the quantum regime: The system offers a platform for studying nontrivial many-body quantum dynamics, magnon Fock state engineering, and the interplay between localized and extended modes in artificial quantum materials.
• Quantum sensing and hybrid spintronic networks: The high-fidelity and tunability of qubit–magnon coupling at millikelvin temperatures open doors to high-sensitivity local quantum probes and long-range communication channels.
• Correlated emission and collective decoherence control: By harnessing the collective coupling to bosonic baths, the realization of subradiant and superradiant states, as well as bath engineering for protected quantum memories, becomes experimentally tractable.
• Scalability and integration: The compatibility of 2D hBN with other van der Waals materials and nanophotonic or magnonic structures supports the development of on-chip quantum processors and distributed quantum sensors.

Conclusion

This work establishes the observation and control of spontaneous magnon emission from VB$^{-}$ centers in 2D hBN in the quantum limit, with supporting theoretical and experimental analyses elucidating the crossover to thermal magnon-dominated regimes. The ability to control emission via magnetic field, defect density, and proximity sets the foundation for engineering magnon-mediated quantum networks with tunable locality and collectivity. The emergence of many-body quantum effects in dense spin ensembles signals new routes for solid-state quantum optics and quantum magnonics, with prospects for scalable quantum information platforms, advanced quantum metrology, and explorations of fundamental many-body physics in hybrid low-dimensional systems.

For extended theoretical context, see related discussions on cooperative quantum dynamics in magnonic platforms and entanglement mediated by collective spin waves (2606.20996).