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Sixth Symposium on Novel Optoelectronic Detection Technology and Applications

Long-relaxation-time vapor cells for stability enhancement in SERF atomic spin co-magnetometers

Yi-an Xu, Yuan Hu, Haoran Xu, Yuxin Cong, Zhihong Wu, Haoying Pang, Zhuangsheng Zhu

Published May 11, 2026• 0 citations
Publisher

Abstract

Spin-exchange relaxation-free (SERF) co-magnetometers exhibit broad application prospects in navigation-grade gyroscopes and fundamental physics experiments. However, their performance is constrained by finite spin relaxation times. To address this challenge, long-relaxation-time vapor cells have been realized through the introduction of dry-film anti-relaxation coatings. Such coatings form a stable hydrophobic layer on the inner walls of the vapor cell, which reduces the adsorption energy of alkali-metal atoms and suppresses wall-induced spin-destruction processes, thereby enhancing polarization retention and extending the coherence times of both electronic and nuclear spins. Nevertheless, a clear quantitative relationship between the coating properties and the resulting anti-relaxation performance has not yet been established. To resolve this issue, the present study introduces the background potential field in the Hamiltonian as an intermediate variable, linking the surface adsorption energy of the coating to atomic spin relaxation processes. From the perspective of microscopic energy structure, a theoretical mapping between coating characteristics and anti-relaxation performance is constructed, thereby filling a critical gap in the theoretical understanding of anti-relaxation coating mechanisms.

DOI
10.1117/12.3109668

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Spin-exchange relaxation-free (SERF) co-magnetometers exhibit broad application prospects in navigation-grade gyroscopes and fundamental physics experiments. However, their performance is constrained by finite spin relaxation times. To address this challenge, long-relaxation-time vapor cells have been realized through the introduction of dry-film anti-relaxation coatings. Such coatings form a stable hydrophobic layer on the inner walls of the vapor cell, which reduces the adsorption energy of alkali-metal atoms and suppresses wall-induced spin-destruction processes, thereby enhancing polarization retention and extending the coherence times of both electronic and nuclear spins. Nevertheless, a clear quantitative relationship between the coating properties and the resulting anti-relaxation performance has not yet been established. To resolve this issue, the present study introduces the background potential field in the Hamiltonian as an intermediate variable, linking the surface adsorption energy of the coating to atomic spin relaxation processes. From the perspective of microscopic energy structure, a theoretical mapping between coating characteristics and anti-relaxation performance is constructed, thereby filling a critical gap in the theoretical understanding of anti-relaxation coating mechanisms.
Anti-relaxation coatings are commonly employed in the alkali metal vapor cells of spin-exchange relaxation-free (SERF) atomic magnetometers, as they can prevent spin depolarization of alkali metal atoms. However, the anti-relaxation mechanism and the interaction process between atoms and coatings remain unclear. This lack of understanding hinders the further improvement of the performance of SERF atomic magnetometers. In this study, we employed first-principles density functional theory to comparatively investigate the microscopic interactions between Rb atoms and SiO2 substrates as well as three commonly used organic coatings. The adsorption energies and valence electron loss of Rb atoms during their adsorption processes on these coated surfaces were systematically calculated. It was found that the Rb atoms exhibited the lowest adsorption energy and the highest charge transfer on the SiO2 surface, with values of –3.55eV and 0.90 |e|, respectively. In contrast, on the 1-nonadecene surface, the adsorption energy reached only –0.13eV, with a charge transfer of merely 0.14 |e|. Similar results were observed for other organic coatings and these findings are highly consistent with previous anti-relaxation performance tests, indicating that alkali metal atoms undergo less energy dissipation and charge transfer on the surface of organic coatings, which can better maintain the electron spin polarization state. A close relationship can be established between adsorption energy, charge transfer and the anti-relaxation performance of coatings, which provides effective guidance for future theoretical research and material selection, thus accelerating the development of SERF magnetometers.
Abstract Alkali‑metal vapor cells are critical for quantum precision measurement devices such as atomic gyroscopes, magnetometers and clocks. The anti‑relaxation coating on cell walls determines atomic spin polarization lifetime and measurement sensitivity. Conventional evaluations depend on short‑term relaxation tests and fail to characterize long‑term coating stability and microscopic degradation. This work proposes a novel evaluation method based on a cumulative relaxation damage factor. From long‑range kinetic perspectives, alkali‑metal surface dynamics are decomposed into three elementary processes quantified by adsorption energy, charge transfer and diffusion barrier. Drawing on Lusser’s reliability law, we construct this unified factor to interpret relaxation mechanisms and assess coating performance. A first‑principles computational framework with periodic slab models is established for alkali‑metal and organic/inorganic interfaces, validated using experimental interaction parameters of four typical coatings. The simulation results demonstrate that the proposed cumulative relaxation damage factor effectively distinguishes the anti-relaxation performance of different coatings and exhibits general applicability across various alkali-metal species. This study converts cumulative long‑range kinetic relaxation into microscopic coating design rules, offering a theoretical tool for wall‑collision relaxation analysis, coating screening and quantum sensing instruments optimization.
Alkali metal vapor cells are fundamental components in quantum sensors, and their relaxation characteristics critically influence both sensitivity and system stability. In Spin-Exchange Relaxation-Free (SERF) atomic magnetometers, although spin-exchange collisions are effectively suppressed, wall-collision-induced relaxation remains the primary limiting factor. A coating-enabled buffer-gas-free vapor cell (CBF-VC) is proposed as a novel sensor unit, realized through multilayer octadecyltrichlorosilane (OTS) selfassembled molecular coatings. The CBF-VC is integrated into a single-beam SERF atomic magnetometer, and experimental validation is conducted. The results indicate a significant extension of the wall collision relaxation time from 40.19 ms. This extended relaxation time directly enhances the magnetometer sensitivity by 48%, confirming the feasibility of CBF-VCs as a measurement-oriented vapor cell architecture. The findings demonstrate not only the effectiveness of anti-relaxation coatings, but also a new instrumentation pathway for advancing next-generation quantum sensing.
In the process of miniaturizing atomic spin gyroscopes, the relaxation due to the wall collisions of the alkali vapor cell leads to a shorter transverse relaxation time, which increases the key index of the atomic spin gyroscope, the angle random walk, and reduces the gyroscope’s accuracy. This paper theoretically analyzes the thermal stability of the interaction between the new anti-relaxation coating and molecules of the vapor cell, as well as the surface energy of the anti-relaxation coating molecular groups. Based on the results, a new anti-relaxation coating material and its preparation process were proposed. A novel anti-relaxation coating was experimentally prepared. Compositional analysis confirmed its successful covalent bonding with hydroxyl groups on the cell inner-surface, demonstrating superior thermal stability. Through comparative experiments including surface morphology analysis, contact angle measurement, and transverse relaxation time measurement, the new anti-relaxation coating proposed in this paper exhibits lower surface energy. It also has more excellent anti-relaxation performance compared to the traditional OTS anti-relaxation coating.
Abstract Anti-relaxation surface coatings can reduce collisions between alkali-metal atoms and cell walls, thereby prolonging the spin-polarization lifetime of alkali-metal atoms. This phenomenon can considerably improve the performance of quantum sensors. Researchers have focused on the surface topography and composition of anti-relaxation coatings in alkali-metal vapor cells and on methods that provide an increase in the relaxation time of alkali metals. We proposed a novel method to characterise the coating performance of alkali-metal cells by directly measuring the energy dissipation during collisions between the anti-relaxation coating surface and atoms in the cell. We found that the interactions between alkali-metal atoms and the anti-relaxation coating resemble elastic collisions, which can be indirectly characterised by the energy dissipation during interactions between an atomic force microscopy tip and atoms. In addition, we confirmed that the energy dissipation between collisions is positively correlated with the relaxation characteristics of alkali metals at various temperatures. This discovery is of great significance as it can be utilised to characterise the anti-relaxation coating performance of alkali-metal cells by directly analysing the energy dissipation of anti-relaxation coatings instead of measuring the relaxation time.
Vapor cells with antirelaxation coating are widely used in modern atomic physics experiments due to the coating's ability to maintain the atoms' spin polarization during wall collisions. We characterize the performance of vapor cells with different coating materials by measuring longitudinal spin relaxation and vapor density at temperatures up to 95 °C. We infer that the spin-projection-noise-limited sensitivity for atomic magnetometers with such cells improves with temperature, which demonstrates the potential of antirelaxation coated cells in applications of future high-sensitivity magnetometers.