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IEEE Transactions on Instrumentation and Measurement

High-Sensitivity Operation of Unshielded Radio Frequency Atomic Magnetometers Using Phase-Lock Techniques

Han Yao, Ferruccio Renzoni

Published Jan 1, 2025• 3 citations
Publisher

Abstract

High sensitivity operation of radio-frequency atomic magnetometers in unshielded environment requires compensation of low-frequency fluctuations of the ambient magnetic field. Here we demonstrate the use of phase-lock techniques to stabilise the magnetic environment and achieve high sensitivity at high frequencies. This is achieved by using the output of the atomic magnetometer both for stabilisation and for measurement purposes. The approach is validated by a proof-of-concept in unshielded environment. The phase-lock approach is also compared to the standard approach where the magnetic environment is stabilised with the help of a set of fluxgate magnetometers, and it is shown that the phase-lock approach features superior performances in signal detection.

DOI
10.1109/tim.2025.3608340

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High sensitivity operation of radio-frequency atomic magnetometers in unshielded environment requires compensation of low-frequency fluctuations of the ambient magnetic field. Here we demonstrate the use of phase-lock techniques to stabilise the magnetic environment and achieve high sensitivity at high frequencies. This is achieved by using the output of the atomic magnetometer both for stabilisation and for measurement purposes. The approach is validated by a proof-of-concept in unshielded environment. The phase-lock approach is also compared to the standard approach where the magnetic environment is stabilised with the help of a set of fluxgate magnetometers, and it is shown that the phase-lock approach features superior performances in signal detection.
Real-world applications of atomic magnetometers require the ability to operate them with high-sensitivity in the presence of magnetic noise. In the present work, high-sensitivity operation of unshielded atomic magnetometers in a magnetically noisy environment is demonstrated. The distinguishing feature of the demonstrated approach is the implementation of active in-situ bias field stabilization using multiple fluxgate magnetometers. This is combined with the use of a counter-propagating pump and triple-pass probe configuration, to maximize the atomic polarization and the probe rotation respectively, so to reach high-sensitivity. The improvement in sensitivity of the unshielded system with respect to previous realizations is fully characterized, with the contributions of the different modifications of the apparatus individually quantified. The presented set-up is suitable for the detection of long-range magnetic fields, where shielding or differential measurements using multi-sensor set-ups do not constitute viable options.
The sensitive detection of either static or radio frequency (RF) magnetic fields is essential to many fundamental studies and applications. Here, we demonstrate the operation of a cold-atom-based RF magnetometer in performing 1D and 2D imaging of small metallic objects. It is based on a cold 85Rb atomic sample and operates in an unshielded environment with no active field stabilization. It shows a sensitivity up to 200 pT/Hz in the 5−35 kHz bandwidth range and can resolve a 0.4 mm wide cut in a 0.8 mm thick metallic foil. The characteristics of our system make it a good candidate for applications in civil and industrial surveillance.
We present a dual-frequency operation technique for a radio-frequency atomic magnetometer utilizing a single natural-abundance rubidium (Rb) vapor cell for the first time, and demonstrate a proof-of-concept for magnetic field communication using binary frequency-shift keying (BFSK) under unshielded conditions. The two operational frequencies correspond to the Larmor frequencies of 85 Rb and 87 Rb, as determined by their distinct gyromagnetic ratios in the same magnetic field environment. A rate of 400 symbols/s is adeptly demonstrated with an ambient noise floor of < 20 pT/√Hz, attaining a signal-to-noise ratio of 20 dB. These findings highlight the efficacy of dual-frequency atomic magnetometry as a sophisticated and resilient technique for magnetic field communication in unshielded environments.
We report on a single-channel rubidium radio-frequency atomic magnetometer operating in unshielded environments and near room temperature with a measured sensitivity of 130 fT/Hz. We demonstrate consistent, narrow-bandwidth operation across the kHz–MHz band, corresponding to three orders of magnitude of the magnetic field amplitude. A compensation coil system controlled by a feedback loop actively and automatically stabilizes the magnetic field around the sensor. We measure a reduction in the 50 Hz noise contribution by an order of magnitude. The small effective sensor volume, 57 mm3, increases the spatial resolution of the measurements. Low temperature operation, without any magnetic shielding, coupled with the broad tunability, and low beam power, dramatically extends the range of potential field applications for our device.
High-sensitivity operation of a radio-frequency atomic magnetometer (RF-AM) requires careful setting of the system parameters, including the lasers intensity and detuning, and the vapour cell temperature. The identification of the optimal operating parameters, which ensures high sensitivity, is typically performed empirically and is often a lengthy process, which is especially labour intensive if frequent retuning of the magnetometer is required to perform different tasks. This paper demonstrates an efficient approach to RF-AM performance optimisation which relies on an open-loop optimisation technique based on Uniform Design (UD). This paper specifically describes the optimisation of an unshielded RF-AM based on a 4-factor-12-level UD of the experimental parameters space. The proposed procedure is shown to lead to the efficient optimisation of the atomic magnetometer at different frequencies, and is applicable to both AC and DC sensitivity optimisation. The procedure does not require any detailed knowledge of the model underlying the operation of the RF-AM and is effective in reducing the number of experimental runs required for the optimisation. It is ideally suited to self-calibration of devices without human supervision.
The magnetic field is one of the most fundamental and ubiquitous physical observables, often conveying rich information about its generation source. Atomic magnetometers have been proven to be high-potential techniques for weak magnetic field measurement in various applications. However, due to the lack of excellent frequency-locking ability against abrupt magnetic field changes, atomic magnetometers are limited to use in application scenarios, which require both wide-range and self-locking characteristics. In this paper, theoretical and experimental research for a wide-range and self-locking rubidium atomic magnetometer based on free spin precession is presented. The rubidium atomic magnetometer can track a magnetic field jump of 10,000 nT in the measurement range from 500 nT to 100,500 nT. We also investigate the bandwidth and dead zone of the atomic magnetometer. An atomic magnetometer with excellent self-locking ability in a wide range can be developed into new highly sensitive magnetic measuring devices, enabling geomagnetic measurement in real environments and monitoring the magnetic phase transitions or coercive fields of soft magnetic materials in very magnetic-shielding environments.