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

The gist: The study reports observation and control of spontaneous magnon emission from boron-vacancy centers in 2D hexagonal boron nitride, revealing an unconventional qubit-magnon dipole coupling channel that dominates in the near-zero temperature limit<ref:2606.20996#pg2>

Spin Relaxation Dynamics at Low Temperatures

The researchers investigated the spin relaxation dynamics of hBN defects in the near zero-temperature limit using a millikelvin quantum sensing platform to study intrinsic spin relaxation rates<ref:2606.20996#pg4> The intrinsic spin relaxation rate shows a dramatic decrease when temperature is lowered from 5 K to 60 mK, with the rate dropping from 140 Hz to 2 Hz<ref:2606.20996#pg5> This temperature dependence of the intrinsic spin relaxation rate can be fitted to a physically motivated model suggesting a combination of one-phonon and two-phonon scattering-driven spin relaxation<ref:2606.20996#pg6> In the zero-temperature limit when coupled with a magnetic material, thermal magnon absorption and stimulated processes are effectively frozen out because the relevant modes of the spin bath are nearly unoccupied<ref:2606.20996#pg8> Instead, an excited center relaxes by emitting a magnon into the quantum vacuum of the magnetic reservoir, revealing an unconventional spin-defect–magnon coupling channel driven purely by quantum fluctuations of the bath<ref:2606.20996#pg9>

Observation and Control via YIG Bath

Experimental observation was conducted on an exfoliated hBN nanoflake containing VB ensembles transferred onto an Au microwave stripline patterned on a 110-nm-thick Y3Fe5O12 (YIG) film, which serves as a solid-state magnon bath proximal to the spin defects<ref:2606.20996#pg10> Systematic spin relaxometry measurements were performed in the sub-Kelvin regime from 58 mK to 3.5 K, where magnon-induced spin relaxation significantly enhances the intrinsic one The measured magnon-induced spin relaxation rate Γ is found to be enhanced compared with the intrinsic rate The spontaneous emission describes flipping of a VB center from the ms = −1 to ms = 0 state that emits a magnon to the YIG spin bath at a rate of Γ0 at zero temperature The measured population relaxation rate is given by Γ = Γ0 (1 + 2Γ+2) = Γ0 (1 + 2Γ+2) In the near zero-temperature limit, the spontaneous emission effect is more pronounced than or dominates over the thermal effect

Field Control and Wavevector Dependence

The spontaneous magnon emission rate Γ0 can be effectively controlled by tuning the spin transition frequency to show that it can be controlled by the wavevector of the YIG magnon bath The measured Γ0 shows a monotonical decrease with Bext for three hBN/YIG samples surveyed and nicely follows the theoretical prediction A slower spin relaxation rate observed in the higher magnetic field regime is attributed to the k dependent transfer function that enters in the calculation of VB spin relaxation rate In the limit of k < 1/t, a high-wavevector YIG magnon is energetically more favorable to be emitted from a VB center, thus a higher magnon energy at a lower magnetic field will give rise to an enhanced magnon emission rate

Magnon-Mediated Quantum Correlations

The study explores the engineering of magnon-mediated quantum correlations in VB spin ensembles by varying the spin defect density, which is estimated to be 1.7 × 1017 cm−3 up to 1.7 × 1018 cm−3 In dense ensembles, magnon-mediated interference contributions can add constructively, potentially resulting in collective quantum spin dynamics where spin relaxation evolves to a correlated state with an enhanced relaxation rate The correlated VB spin relaxation rate can be approximately described by the expression (Γ4 - Γ5) = Γ (3) where s = λΓ0λ2ߩ represents the correlated magnon emission contribution When normalized by the uncorrelated relaxation value, ₭(1+2σ) features significant enhancement in the sub-Kelvin regime as shown in Fig. 4E This enhancement is suppressed with increasing temperature, and signatures of magnon-mediated quantum correlation fully disappear when T > 1 K due to the dominant thermal magnon effect

Discussion and Implications

The work demonstrates millikelvin quantum sensing using boron-vacancy spin defects in 2D hBN nanoflakes by observing orders of magnitude decrease of the intrinsic spin relaxation rate due to suppression of phonon scattering-induced spin depolarization By introducing a shared YIG magnon bath, the researchers present clear evidence of spontaneous magnon emission from VB centers in the near zero-temperature limit The results bring new insights into understanding collective spin dynamics, magnon-qubit coupling and many-body physics of color centers in the quantum regime The hybrid systems consisting of solid-state magnons and hBN color centers with tailored quantum mechanical properties open new avenues for developing cutting-edge technological innovations from sensing, metrology, and potentially to computing at the forefront of quantum revolution

Acknowledgements

The quantum sensing measurements are supported by the Office of Naval Research (ONR) under grant No. N00014-23-1-2146 Development of millikelvin quantum microscopy is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-25-1-0082 Device fabrications are supported by U.S. National Science Foundation under award No. DMR-2437294, No. ECCS-2445826 and No. ECCS-2525800 B. F. and J. M. P. N acknowledge support from DOE under Award No. DE-SC0024090 J. H. E and T.-X thank the National Science Foundation for supporting the h10B15N crystal growth under award No 2413808 Neutron irradiation of the h10B15N is supported by the U.S. Department of Energy, Office of Nuclear Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517 as part of Nuclear Science User Facilities award No 24-4911 S.-R. and Z. J. thank the NASA Solar System Exploration Research Virtual Institute (SSERVI), under cooperative agreement number NNH22ZDA020C for synthesis of 10B-enriched monoisotopic hBN crystals

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The gist: The study reports observation and control of spontaneous magnon emission from boron-vacancy centers in 2D hexagonal boron nitride, revealing an unconventional qubit-magnon dipole coupling channel that dominates in the near-zero temperature limit<ref:2606.20996#pg2>

Spin Relaxation Dynamics at Low Temperatures

The intrinsic spin relaxation rate shows a dramatic decrease when temperature is lowered from 5 K to 60 mK, with the rate dropping from 140 Hz to 2 Hz<ref:2606.20996#pg5> This temperature dependence of the intrinsic spin relaxation rate can be fitted to a physically motivated model suggesting a combination of one-phonon and two-phonon scattering-driven spin relaxation<ref:2606.20996#pg6> In the zero-temperature limit when coupled with a magnetic material, thermal magnon absorption and stimulated processes are effectively frozen out because the relevant modes of the spin bath are nearly unoccupied<ref:2606.20996#pg8> Instead, an excited center relaxes by emitting a magnon into the quantum vacuum of the magnetic reservoir, revealing an unconventional spin-defect–magnon coupling channel driven purely by quantum fluctuations of the bath<ref:2606.

1. Improved AI for Quantum Materials Simulation: The AI can now quantitatively model many-body physics of color centers in the quantum regime by incorporating the correlated spin defect model described as (3) (a + b)nγ = Γ(3)n4 - σ/ω. This allows the system to predict density-dependent deviations from the independent-defect decay for various hBN flake densities, moving beyond simple single-defect approximations.

2. Enhanced Quantum Sensing Algorithms: The AI can design algorithms that exploit magnon-mediated quantum correlations in spin ensembles, enabling it to distinguish between uncorrelated spin relaxation behavior in the dilute regime and the enhanced relaxation rate observed at higher defect densities, which is crucial for interpreting experimental data from high defect density samples.

3. Spontaneous Magnon Emission Control: The system can develop control strategies based on magnetic field tuning, leveraging the finding that spontaneous magnon emission rate Γ଴ can be effectively controlled by the wavevector of the YIG magnon bath. This capability allows for active manipulation of spin relaxation rates by adjusting external fields to favor emission into specific magnon modes.