ABSTRACT We propose a method for simultaneously measuring photon shot noise (PSN), spin projection noise (SPN), and technical noise in an atomic magnetometer using a spatial light modulator. A digital micromirror device (DMD) is placed behind the cell to modulate the area of the probe beam, thereby enabling the detected optical power and the number of polarized atoms to be varied while keeping the light intensity inside the cell constant—thus avoiding additional perturbations. Based on this experimental approach, we adapt the conventional models and fit the total equivalent magnetic‐field noise power spectral density as a function of the modulation area to obtain the key model parameters. Using the effective transverse relaxation time T 2 extracted from the free‐induction‐decay (FID) signal, the three noise components are then separated. The method is validated on a Bell‐Bloom magnetometer operated with a bias magnetic field of 2–3 µT. The results show that our model can simultaneously measure PSN, SPN, and technical noise with high accuracy, and outperforms conventional methods in terms of fitting robustness and goodness of fit. This work provides a practical approach for quantum‐noise diagnostics in operating atomic magnetometers without requiring ideal conditions such as weak perturbations or low technical noise.
ABSTRACT We propose a method for simultaneously measuring photon shot noise (PSN), spin projection noise (SPN), and technical noise in an atomic magnetometer using a spatial light modulator. A digital micromirror device (DMD) is placed behind the cell to modulate the area of the probe beam, thereby enabling the detected optical power and the number of polarized atoms to be varied while keeping the light intensity inside the cell constant—thus avoiding additional perturbations. Based on this experimental approach, we adapt the conventional models and fit the total equivalent magnetic‐field noise power spectral density as a function of the modulation area to obtain the key model parameters. Using the effective transverse relaxation time T 2 extracted from the free‐induction‐decay (FID) signal, the three noise components are then separated. The method is validated on a Bell‐Bloom magnetometer operated with a bias magnetic field of 2–3 µT. The results show that our model can simultaneously measure PSN, SPN, and technical noise with high accuracy, and outperforms conventional methods in terms of fitting robustness and goodness of fit. This work provides a practical approach for quantum‐noise diagnostics in operating atomic magnetometers without requiring ideal conditions such as weak perturbations or low technical noise.
ABSTRACT The Bell‐Bloom‐type optically‐pumped atomic magnetometers are well‐suited for weak geomagnetic field detection. However, conventional single‐beam pumping introduces an atomic spin polarization gradient, which limits the measurement accuracy and sensitivity. To address this issue, this paper proposes and experimentally demonstrates a Bell‐Bloom‐type rubidium FID magnetometer scheme integrating orthogonally polarized counter‐propagating pumping and multi‐pass probe detection. This design homogenizes the atomic spin polarization distribution and suppresses light shifts and power broadening. This uniform polarization, in turn, allows the five‐pass probe configuration to achieve a significantly larger signal amplitude. Experimental results reveal that, compared with the traditional single‐beam pumping and single‐pass detection scheme, the proposed magnetometer achieves a remarkable improvement in magnetic field measurement accuracy, and the magnetic field sensitivity is improved from 18.9 to 3.1 pT/. Notably, the two techniques work synergistically, yielding a 6.1‐fold sensitivity enhancement that far exceeds their individual gains. This work provides an effective technical approach and reference for optimizing the performance of atomic magnetometers and their applications in precision measurement fields.
Abstract In the last few decades, the atomic magnetometry [1–7] has experienced remarkable development and reached to an outstanding sensitivity. This exciting technology is finding applications in various of fields [8–18]. The last two obstacles to further improve the sensitivity of magnetometry are quantum fluctuations and ambient magnetic field. For magnetometry based on optical readout of atomic ensemble’s spin precession, the fundamental limitation of sensitivity is restricted by spin projection noise (SPN) and photon shot noise (PSN). Meanwhile, in practical applications, ambient magnetic field noise which is regarded as a common-mode magnetic-field (CMM) noise is also limiting the sensitivity. Several techniques for reducing PSN [19–25], SPN [26, 27], and CMM noise [7, 28–31] have been proposed, respectively. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. For this purpose, here we demonstrate a sub-shot-noise magnetic gradiometer utilizing entangled optical detection with quantum-correlated twin beams. This leads to the simultaneous suppression of PSN and CMM noise. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the standard quantum limit (SQL). The sensitivity of gradient magnetic field reaches 18 fT/cm√Hz at 20 Hz with these sophisticated techniques. Our study opens up new possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications with noisy and physically demanding environments.
An Atomic Magnetometer with Spin-Projection Noise Proportional to T2
There is a common sense view for atomic magnetometers that their spin-projection-noises (SPNs) are inversely proportional to , where T 2 is the transverse relaxation time. We analyze the current atomic magnetometer types and give a counter-example of this common sense, which is the all-optical spin precession modulated three-axis atomic magnetometer proposed by our group in 2015. Unlike the other atomic magnetometers, the SPN of this kind of atomic magnetometers is proportional to due to the fact that the scale factor between P x and B can be unrelated to the transverse relaxation time T 2 . We demonstrate this irrelevance experimentally and analyze the SPN theoretically. Using short-pulse ultra-high power laser to fully polarize the atoms, the phenomenon that SPN decreases with T 2 may also be demonstrated experimentally and a new tool for researching SPN in atomic magnetometers may be realized .
Ultra-sensitive multi-channel optically pumped atomic magnetometers based on the spin-exchange relaxation-free (SERF) effect are powerful tools for applications in the field of magnetic imaging. To simultaneously achieve ultra-high spatial resolution and ultra-high magnetic field sensitivity, we proposed a high-resolution multi-channel SERF atomic magnetometer for two-dimensional magnetic field measurements based on a digital micro-mirror device (DMD) as the spatial light modulator for a single vapor cell. Under the optimal experimental conditions obtained via spatial and temporal modulation of the probe light, we first demonstrated that the average sensitivity of the proposed 25-channel magnetometer was approximately 25fT/Hz 1/2 with a spatial resolution of 216µm. Then, we measured the magnetic field distribution generated by a gradient coil and compared the experimentally obtained distributions with those calculated via finite element simulation. The obtained g value of 99.2% indicated good agreement between our experimental results and the theoretical calculations, thereby confirming that our proposed multi-channel SERF magnetometer was effective at measuring magnetic field distributions with an ultra-high spatial resolution.
Traditional practice of suppressing low-frequency noise in atomic magnetometers is mainly through the application of magnetic field modulation, while magnetic fields of this kind introduce extra transverse relaxation, which results in a decrease of sensitivity. In this study, a novel method of measuring relaxation rate induced by spin-destruction, wall collisions and modulation magnetic fields is proposed. The optical pumping model is studied both in optical-thin and optical-thick vapor cell through the analysis of total relaxation, thus the optical pumping rate can be separated from total relaxation by analyzing the optical depth. Furthermore, based on the analysis of optical pumping rate and relaxation rate, the polarization of the electron ensemble was evaluated. A compact magnetometer with single-beam configuration is developed for the proof of concept, and the experimental results show good agreements with theoretical calculation. Our method provides a simple and feasible approach to in-situ measurement of transverse relaxation rate and optical pumping rate with physical structure of the device untouched, which is especially advantageous for designing and optimizing compact atomic magnetometers.
ABSTRACT Single‐beam optically pumped atomic magnetometers (OPAMs) are promising tools for biomagnetic measurements in nonzero‐field environments owing to their high sensitivity and compactness. However, the angle between the external bias magnetic field and the pump light affects the spin polarization of the atomic ensemble, thereby influencing the magnetometer performance and making the optimization of the magneto‐optical angle essential. In this study, a spatially resolved steady‐state spin polarization model incorporating the magneto‐optical angle is established for the first time, and an analytical expression of the spin precession signal under RF magnetic field excitation is derived. It is demonstrated that an optimal angle exists at which the magnetometer exhibits the maximum response and the highest sensitivity, which is experimentally determined to be 26° in our single‐beam OPAM. At this angle, the combined influence of the steady‐state spin polarization, spin precession projection, transmitted light intensity, and transverse spin relaxation reaches an optimal overall state, leading to a 14% enhancement in sensitivity and a 32% reduction in detection noise compared with the conventional 45° configuration. These findings provide guidance for optimizing the experimental parameters of single‐beam OPAMs for weak magnetic signal detection in nonzero‐field environments.