Strongly anisotropic non-Kramers electron spin as a quantum coherence probe of angular fluctuations
Abstract
Strongly anisotropic non-Kramers rare-earth ions combine giant longitudinal g-factors with a vanishing transverse component imposed by time-reversal symmetry, a combination that makes their spin transitions exquisitely sensitive to the orientation of the applied magnetic field. We show that this sensitivity carries a dual identity: it is simultaneously an overlooked decoherence channel and the basis for a spin-coherence-based angular probe. Using pulsed electron paramagnetic resonance at X-band, we report the first measurements of the quantum coherence of Tb3+ in a native-doped CaWO4 crystal (15 ppb) and map the Hahn-echo coherence time T2 as a function of temperature (2 to 10 K) and resonant field (103 to 104 G). A parameter-free model combining spin-lattice relaxation, instantaneous diffusion and spectral diffusion from all independently quantified impurities overestimates T2 by an order of magnitude at low temperature and wrongly predicts the field dependence of T2, inconsistent with the observed monotonic decrease of T2 with Br. A two-parameter extension, including dynamical angular fluctuations of the crystal axis, reproduces the full dataset across multiple setups and laboratories. Two controlled experiments nominally identical except for different mechanical configuration of the setup establish the mechanical origin of the dominant contribution. The two-parameter extension corresponds to an angular amplitude noise spectral density of overall order 36 n°/√(Hz) from global external vibrations (ranging from 10 to 66 n°/√(Hz) depending on the exact setup mechanical configuration) estimated at ∼ 2.5 kHz plus a temperature-dependent contribution assumed to come from local phonon-driven angular jitter. It identifies and highlights a decoherence pathway of practical relevance to any anisotropic solid-state spin system.
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BibTeX
@misc{kumar2026strongly,
title = {{Strongly anisotropic non-Kramers electron spin as a quantum coherence probe of angular fluctuations}},
author = {Achuthan Manoj Kumar and Remy Dassonneville and Guillaume Gerbaud and Nolwenn Le Breton and Athanassios K. Boudalis and Patrice Bertet and Philippe Goldner and Sylvain Bertaina},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.03989},
eprint = {2609.03989},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05763842v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Strongly anisotropic non-Kramers electron spin as a quantum coherence probe of angular fluctuations
AU - Achuthan Manoj Kumar
AU - Remy Dassonneville
AU - Guillaume Gerbaud
AU - Nolwenn Le Breton
AU - Athanassios K. Boudalis
AU - Patrice Bertet
AU - Philippe Goldner
AU - Sylvain Bertaina
PY - 2026
DA - 2026/09/03
PB - HAL
DO - 10.48550/arxiv.2609.03989
UR - https://hal.science/hal-05763842v1
AB - Strongly anisotropic non-Kramers rare-earth ions combine giant longitudinal g-factors with a vanishing transverse component imposed by time-reversal symmetry, a combination that makes their spin transitions exquisitely sensitive to the orientation of the applied magnetic field. We show that this sensitivity carries a dual identity: it is simultaneously an overlooked decoherence channel and the basis for a spin-coherence-based angular probe. Using pulsed electron paramagnetic resonance at X-band, we report the first measurements of the quantum coherence of Tb^3+ in a native-doped CaWO_4 crystal (15 ppb) and map the Hahn-echo coherence time T_2 as a function of temperature (2 to 10 K) and resonant field (10^3 to 10^4 G). A parameter-free model combining spin-lattice relaxation, instantaneous diffusion and spectral diffusion from all independently quantified impurities overestimates T_2 by an order of magnitude at low temperature and wrongly predicts the field dependence of T_2, inconsistent with the observed monotonic decrease of T_2 with B_r. A two-parameter extension, including dynamical angular fluctuations of the crystal axis, reproduces the full dataset across multiple setups and laboratories. Two controlled experiments nominally identical except for different mechanical configuration of the setup establish the mechanical origin of the dominant contribution. The two-parameter extension corresponds to an angular amplitude noise spectral density of overall order 36 n°/√(Hz) from global external vibrations (ranging from 10 to 66 n°/√(Hz) depending on the exact setup mechanical configuration) estimated at ∼ 2.5 kHz plus a temperature-dependent contribution assumed to come from local phonon-driven angular jitter. It identifies and highlights a decoherence pathway of practical relevance to any anisotropic solid-state spin system.
ER -