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Advanced Quantum Technologies

An Efficient Method for Measuring Spin Relaxation Time Based on Electromagnetically Induced Transparency

Wanqing Li, Wei Li, Jiaxin Liu, Shuyi Zhang, Haoying Pang

Published Aug 10, 2025• 0 citations
Publisher

Abstract

Abstract Atomic precision measurement technologies based on the Spin‐Exchange Relaxation‐Free (SERF) regime play a vital role in advanced research and the relaxation characteristics of the vapor cell are key factors that determine the sensitivity and stability of measurements. As a result, the accurate measurement of relaxation time has become an important research focus. In this work, a novel and efficient method is proposed for measuring relaxation time based on the technique of Electromagnetically Induced Transparency (EIT), and validate it experimentally under both geomagnetic and magnetically shielded conditions. By utilizing the D1 transition line of 87 Rb atoms, a three‐level ‐type EIT system is constructed. An optical setup is developed, incorporating a specially designed light source, vapor cell, temperature‐controlled heating structure, and magnetic shielding. After turning off the coupling beam, the relaxation characteristics are obtained by measuring the transmission spectrum of the probe beam. Under geomagnetic conditions, the fitted fast and slow relaxation times are 0.56 and 105.67 ms, respectively; under magnetic shielding, the corresponding values are 47.88 and 100.32 ms. Furthermore, the influence of temperature and coupling beam power on the relaxation process is investigated, providing theoretical insight for improving the performance of atomic precision measurement technologies.

DOI
10.1002/qute.202500317

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Abstract Atomic precision measurement technologies based on the Spin‐Exchange Relaxation‐Free (SERF) regime play a vital role in advanced research and the relaxation characteristics of the vapor cell are key factors that determine the sensitivity and stability of measurements. As a result, the accurate measurement of relaxation time has become an important research focus. In this work, a novel and efficient method is proposed for measuring relaxation time based on the technique of Electromagnetically Induced Transparency (EIT), and validate it experimentally under both geomagnetic and magnetically shielded conditions. By utilizing the D1 transition line of 87 Rb atoms, a three‐level ‐type EIT system is constructed. An optical setup is developed, incorporating a specially designed light source, vapor cell, temperature‐controlled heating structure, and magnetic shielding. After turning off the coupling beam, the relaxation characteristics are obtained by measuring the transmission spectrum of the probe beam. Under geomagnetic conditions, the fitted fast and slow relaxation times are 0.56 and 105.67 ms, respectively; under magnetic shielding, the corresponding values are 47.88 and 100.32 ms. Furthermore, the influence of temperature and coupling beam power on the relaxation process is investigated, providing theoretical insight for improving the performance of atomic precision measurement technologies.
The spin transverse relaxation time ( T 2 ) of atoms is an important indicator for magnetic field precision measurement. Especially in optically-pumped atomic magnetometer, the linewidth of the magnetic resonance signal is one of the most important parameters of sensitivity, which is inversely correlated with T 2 of atoms. In this paper, we propose four methods, namely spin noise spectroscopy signal fitting, radio-frequency free induction decay (RF-FID) signal fitting, ω m (modulation frequency)-broadening fitting, and magnetic resonance broadening fitting, for in-situ measurement T 2 of atomic vapor cells based on light-atom interactions. Meanwhile, T 2 of three Rubidium (Rb) atomic vapor cells with different parameters are measured and discussed by using these four methods. A comparative analysis visualizes the characteristics of the different methods and the effects of buffer gas on T 2 of Rb atoms. Through theoretical and experimental analysis, we assess the applicability of each method and concluded that the RF-FID signal fitting method provides the most accurate measurements due to the timing sequence control system, which results in a cleaner measurement environment. Furthermore, we demonstrate and qualitatively analyze the relationship between temperature and T 2 of Rb atoms. This work may offer valuable insights into the selection of atomic vapor cells and it is also applicable for the spin-exchange relaxation-free region.
We have studied a time response of electromagnetically induced transparency (EIT) in a rubidium vapor to a rapid variation of optical phase. We have observed a very fast growth of the absorption when the phase of the optical field has been abruptly changed, followed by a slow return to the level of steady-state absorption. The recovery time decreases with increasing optical power. A simple theoretical analysis shows that under our experimental conditions the low power limit of the recovery time is determined by the ground relaxation time. In our case it is defined by a time-of-flight of rubidium atoms through laser beam. The obtained value of the ground state relaxation time is in a good agreement with result of direct measurements by ‘relaxation in the dark’ method. Our technique based on phase dynamics in EIT can be used for investigation of the ground state relaxation and the fast control of EIT.
The spin-exchange relaxation-free (SERF) atomic magnetometer (AM) has vital importance in advanced research and has been widely applied in different fields. To improve the sensitivity and robustness of SERF magnetometers, understanding the relaxation mechanism is a significant approach. By controlling the amplitude and direction of external magnetic fields, and switching statuses of the pump and probe laser beam, we measured both the transverse and longitudinal relaxation rate in two experimental methods, respectively, and obtained the Larmor precession frequency, the spin-exchange relaxation rate, the spin-destruction relaxation rate, and the relaxation rate due to collisions between atoms and vapor cell walls. All experimental results are verified with the theoretical calculations. The final longitudinal relaxation rate of our magnetometer is 53 Hz. Furthermore, we measured the sensitivity of our SERF magnetometer reaching 5 fT/Hz1/2 from 5 to 70 Hz after differential measurement with the 3-dB bandwidth of 70 Hz. Our work helps to improve the detection capability of SERF magnetometers, providing performance assurance for searching for exotic spin-dependent interactions and applications, including magnetoencephalograms and magnetocardiograms.
The behavior of multilevel atomic ensembles (e.g., alkali-metal atoms) can be influenced significantly by the intensity of a driving field (or controlling/coupling field). The phase coherence between two transition pathways driven by a probe light and a driving field can lead to the effect known as electromagnetically induced transparency (EIT). In EIT, the probe light can pass through a three-level alkali-metal atomic vapor without absorption or reflection when two coherent resonances (transition pathways driven by the driving and probe fields) are present and the linewidths of the transparency windows are sufficiently narrow. The optical characteristics of atomic systems can also be affected by the Doppler broadening of the absorption profile in a spectroscope. Our analysis indicates that both broadenings (related to the transitions excited by the driving and probe fields) can be expanded, leading to an increase in the transmittance and reflectance broadenings when a coupling field with adaptive strength is applied; the corresponding temperature would, thus, be implemented and readable. We show that the most suitable preparation for temperature sensing via an EIT vapor is to provide 80 times the spontaneous decay rate (SDR) of the excited atomic levels. This configuration results in reflectance and transmittance values that range between zero and one and cover a temperature range of 0 K to 600 K. As an example, we demonstrate the integration of specialized coating technologies with EIT ensembles for temperature sensing in the range of dozens of kelvins at and above room temperature. A key advantage of this temperature-sensing system is its ability to use adaptive resonant visible light as the probe field. This novel approach may find applications in providing unprecedented levels of precision and control in temperature sensing for coating processes and in the design of other photonic or optical devices. It can also be used to determine the temperature-dependent behavior of the specific heat of alkali-metal solids and gases (including the latent heats of vaporization or sublimation of alkali-metal solids) through the reflection and transmission spectra of the vaporized EIT atomic vapors.
We report characterization of electromagnetically induced transparency (EIT) resonances in the D1 line of (87)Rb under various experimental conditions. The dependence of the EIT linewidth on the power of the pump field was investigated at various temperatures for the ground states of the lambda system associated with different hyperfine levels of the atomic 5S(1/2) state as well as magnetic sublevels of the same hyperfine level. Strictly linear behavior was observed in all cases. A theoretical analysis of our results shows that dephasing in the ground state is the main source of decoherence, with population exchange playing a minor role.