Atomic Magnetometer Research Articles

An integrated and scalable experimental system for nitrogen-vacancy ensemble magnetometry.

Nitrogen-vacancy (NV) centers in diamond are extremely promising solid-state spin quantum sensors for magnetic field in recent years. The rapid development of NV-ensemble magnetometry has put forward higher requirements for high-speed data acquisition, real-time signal processing and analyzing, etc. However, the existing commercial instruments are bulky and expensive, which brings extra complexity to the weak magnetic field detection experiment and hinders the practicality and miniaturization of NV-ensemble magnetometry. Here, we report on an integrated and scalable experimental system based on a field-programmable-gate-array (FPGA) chip assisted with high-speed peripherals for NV-ensemble magnetometry, which presents a compact and compatible design containing high-speed data acquisition, oscilloscopes, signal generator, spectrum analyzer, lock-in amplifier, proportional-integral-derivative feedback controller, etc. To verify its applicability and reliability in experiments, various applications, such as optical magnetic resonance detection, optical cavity locking, and lock-in NV magnetometry, are conducted. We further realize the pump-enhanced magnetometry based on NV center ensembles using the optical cavity. Through the flexible FPGA design approach, this self-developed device can also be conveniently extended into atomic magnetometer and other quantum systems.

Rapid Electromagnetic Induction Imaging With an Optically Raster-Scanned Atomic Magnetometer

We present an apparatus to overcome the limitations of mechanical raster-scanning in electromagnetic induction imaging (EMI) techniques by instead performing a 2D optical raster-scan within the vapour cell of a radio-frequency atomic magnetometer (RF-AM). A large cuboidal <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup> Rb vapour cell is employed to act as the medium of an RF-AM with the pump and probe beams translated in the cell via acousto-optics. The technique is shown to give robust and repeatable magnetic measurements over the cell volume and successfully resolves conductive targets with EMI. Optical raster-scanning removes the limitation of slow mechanical actuation and a fast imaging procedure is enacted resolving conductive targets at a rate of 40 ms/pixel.

Precise Measurement of Magnetic Shielding Residual Field and Light Shift Based on 87Rb Atomic Magnetometer

Precise Measurement of Magnetic Shielding Residual Field and Light Shift Based on ⁸⁷Rb Atomic Magnetometer

A neural network assisted 171Yb+ quantum magnetometer

A versatile magnetometer must deliver a readable response when exposed to target fields in a wide range of parameters. In this work, we experimentally demonstrate that the combination of171Yb+ atomic sensors with adequately trained neural networks enables us to investigate target fields in distinct challenging scenarios. In particular, we characterize radio frequency (RF) fields in the presence of large shot noise, including the limit case of continuous data acquisition via single-shot measurements. Furthermore, by incorporating neural networks we significantly extend the working regime of atomic magnetometers into scenarios in which the RF driving induces responses beyond their standard harmonic behavior. Our results indicate the benefits to integrate neural networks at the data processing stage of general quantum sensing tasks to decipher the information contained in the sensor responses.

High-sensitivity operation of an unshielded single cell radio-frequency atomic magnetometer: erratum.

An erratum is presented to include the right vertical scale and label in Fig.2 and Fig.7, which were omitted in our published manuscript ["High-sensitivity operation of an unshielded single cell radio-frequency atomic magnetometer" Opt. Express30 (23), 42015 (2022) 10.1364/OE.476016].

High-sensitivity pump–probe atomic magnetometer based on single fiber-coupled

In this paper, we demonstrate an all-optical spin-exchange relaxation-free atomic magnetometer based on optical-fiber coupling, which simultaneously transmits orthogonally coupled pump and probe beam to an alkali vapor cell through a single polarization-maintaining fiber. The extinction ratios of the two pump–probe beams propagating along the fast- and slow-axis of the polarization-maintaining fiber were measured experimentally. To achieve the best sensitivity of the magnetometer, we theoretically analyze and experimentally optimize the triaxial magnetic compensation and optical parameters. Based on this scheme, it is proved that the compensation of the background magnetic field and the acquisition of three-axis magnetic field information can be achieved using only a pair of pump–probe beams. By optimizing the bias magnetic field Bx, Bz and optical parameters, we achieved bandwidths of 85 Hz, 42 Hz, and 48 Hz and magnetometer sensitivities of 18 fT/Hz1/2, 12 fT/Hz1/2, and 26 fT/Hz1/2 in the x-, y-, and z-axis, respectively. The proposed miniaturized configuration will be beneficial to developing magnetocardiography and magnetoencephalography instrumentation based on atomic magnetometers.

Three-axis isotropic-sensitivity He4 magnetometer with alignment-based longitudinal parametric resonance

For measurements in near-zero magnetic fields, vector atomic magnetometers are promising alternatives to scalar versions, but generally suffer from inconsistent performance over three axes. We present a single-beam vector magnetometer with three-axis isotropic sensitivity based on $^{4}\mathrm{He}$ atomic alignment. Longitudinal parametric resonance with atoms pumped by linearly polarized light is proposed for the $z$-axis measurement, while the Hanle effect in transverse geometry is used for $x$- and $y$-axis measurements. The evolution of atomic alignment multipole components and expressions of the three-axis magnetic responses are analyzed in detail. We describe the experimental operation of the proposed magnetometer with a magnetic-field noise floor of $150\phantom{\rule{0.16em}{0ex}}\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ for the three axes, which is attractive for biomagnetism requiring vector characterization and source reconstruction.

Different Configurations of Radio-Frequency Atomic Magnetometers-A Comparative Study.

We comprehensively explore different optical configurations of a radio-frequency atomic magnetometer in the context of sensor miniaturisation. Similarities and differences in operation principles of the magnetometer arrangements are discussed. Through analysis of the radio-frequency and noise spectra, we demonstrate that all configurations provide the same level of atomic polarisation and signal-to-noise ratio, but the optimum performance is achieved for significantly different laser powers and frequencies. We conclude with possible strategies for system miniaturisation.

Correlating the microstructure of Mn–Zn ferrite with magnetic noise for magnetic shield applications

Many sensitive atomic devices require magnetic shield to reduce external magnetic field interference. Mn–Zn ferrites are optimistic candidates for shielding because they provide a high shielding factor and a low magnetic noise. This study evaluated the magnetic noise of Mn–Zn ferrite magnetic shield based on its grain size. The morphologies of Mn–Zn ferrite samples were characterized to establish a correlation between their grain sizes and magnetic permeability, which can be used to calculate magnetic noises. The correlation was then used to evaluate the shielding performance of another Mn–Zn ferrite, where the magnetic noise of the magnetic shield made from the ferrite was calculated to be 2.53 f−1/2fT. The magnetic noise of the magnetic shield was also measured experimentally using a spin-exchange relaxation-free atomic magnetometer. The measured magnetic noise of ferrite shield was 2.61 f−1/2fT, while the white noise of the atomic magnetometer was 0.71 fT/Hz1/2 below 100 Hz. The consistency between the experimental results and predicted values suggests the viability of the proposed approach, providing a convenient, efficient, and precise method for developing ferrite ceramics materials for low-noise magnetic shields.

Triaxial closed-loop measurement based on a single-beam zero-field optically pumped magnetometer

In this study, we propose an approach for the simultaneous measurement of triaxial magnetic fields using a single-beam zero-field optically pumped atomic magnetometer, in which a rotational high-frequency (ω1) and another high-frequency (ω2) modulated magnetic field magnetic fields are applied along the transverse directions and the longitudinal direction, respectively. Theoretical analysis, numerical simulation, and experiments are conducted to demonstrate this method. Experimental sensitivities of 18 fT/Hz1/2 along the two transverse directions and 140 fT/Hz1/2 along the longitudinal direction are simultaneously achieved. On this basis, we operate the magnetometer in closed-loop mode to expand the bandwidth and dynamic range, and to keep the triaxial magnetic field sensed by the magnetometer at zero. The triaxial bandwidths are increased from below 100 Hz to over 1.6 kHz. The triaxial dynamic ranges are all extended to ±150 nT. Plus, we verify the ±1,000 nT dynamic range of the triaxial magnetometer through increasing the triaxial coil constants. The synchronization of triaxial closed-loop measurement, simplicity of magnetometer structure, and closed-loop detection with high sensitivities make it applicable and attractive for biomagnetism imaging in challenging environments.

Demonstration of a high-density alkali-metal atomic magnetometer based on the frequency-symmetrical detuning effect of two pumping lights.

The existence of an approximately uniform and unsaturated electron spin polarization distribution within a high-density alkali-metal vapor is considered of great importance for significantly improving the response amplitude and sensitivity properties of an atomic magnetometer. However, when a high-density alkali-metal vapor is formed, the optical depth is much larger than the value of one, resulting in the electron spin polarization gradient. In this work, it was demonstrated from both numerical simulations and experimental points of view, that by replacing the resonant pumping light with two off-resonant pumping light sources, the signal amplitude of the magnetometer can be doubled. By using this approach, the electron spin polarization gradient can be significantly suppressed and the sensitivity can be improved by more than 10%. The proposed scheme is generally applicable to various optical pumping high-density alkali-metal vapor systems, where a uniform electron spin polarization distribution is required, such as optical pumping co-magnetometers and atomic gyroscopes.

A miniaturized spin-exchange relaxation-free atomic magnetometer based on uniform light field

We experimentally study the dynamic characteristics of a miniaturized spin-exchange relaxation-free (SERF) magnetometer based on uniform light field. The ceramic ferrule is used to expand the Gaussian beam to improve light intensity uniformity, while the volume of the sensor is also reduced. This scheme makes the magnetometer have better sensitivity when the detected light intensity is less than 3.16 mW/cm2 at 120 °C. When the temperature rises to 150 °C the sensitivity under the action of uniform light field is 18.5 fT/Hz1/2. The bandwidth of the sensor remains at the original level and meets application needs. The proposed structure improves transverse polarization uniformity within the miniaturized sensor, which is ideal for the magnetoencephalography and magnetocardiography imaging systems.

Application of VCSEL in Bio-Sensing Atomic Magnetometers.

Recent years have seen rapid development of chip-scale atomic devices due to their great potential in the field of biomedical imaging, namely chip-scale atomic magnetometers that enable high resolution magnetocardiography (MCG) and magnetoencephalography (MEG). For atomic devices of this kind, vertical cavity surface emitting lasers (VCSELs) have become the most crucial components as integrated pumping sources, which are attracting growing interest. In this paper, the application of VCSELs in chip-scale atomic devices are reviewed, where VCSELs are integrated in various atomic bio-sensing devices with different operating environments. Secondly, the mode and polarization control of VCSELs in the specific applications are reviewed with their pros and cons discussed. In addition, various packaging of VCSEL based on different atomic devices in pursuit of miniaturization and precision measurement are reviewed and discussed. Finally, the VCSEL-based chip-scale atomic magnetometers utilized for cardiac and brain magnetometry are reviewed in detail. Nowadays, biosensors with chip integration, low power consumption, and high sensitivity are undergoing rapid industrialization, due to the growing market of medical instrumentation and portable health monitoring. It is promising that VCSEL-integrated chip-scale atomic biosensors as featured applications of this kind may experience extensive development in the near future.

Frequency shift compensation for single and dual laser beam pass sensors of a coherent population trapping resonance based coupled dark state magnetometer

The Coupled Dark State Magnetometer is an atomic magnetometer designed for magnetic field measurements in space. Its measurement principle is based on the excitation of coupled dark state resonances prepared via the coherent population trapping effect. The coupled dark state resonances enable the compensation of disturbing frequency shifts which degrade the measurement accuracy. Results with a sensor design based on a single laser beam pass through the sensor unit reveal a reduced ability to compensate these disturbing effects compared to theoretical predictions. Therefore, a new sensor design is presented which enables a dual laser beam pass through the sensor unit. For almost all angles between the external magnetic field and the light propagation direction, the dual pass sensor design enables a better compensation of disturbing frequency shifts than the single pass sensor design - up to a factor of nine.

Intrinsic radio-frequency gradiometer

The authors develop two optically pumped atomic magnetometers with sub-fT sensitivity operating at radio frequencies and compare their magnetic-field sensitivity and interference rejection. They identify ways to improve the sensitivity of unshielded magnetic detectors, which could advance the development of portable magnetometers for field uses.

Micro-fabricated caesium vapor cell with 5 mm optical path length

Micro-fabricated vapor cells have applications in a number of emerging quantum technology-based devices including miniaturized atomic magnetometers, atomic clocks, and frequency references for laser systems. Increasing the cell optical path length (OPL) and smallest cell dimension are normally desirable to increase the signal to noise ratio (SNR) and minimize the de-polarization rate due to collisions between atomic or molecular species and the cell walls. This paper presents a fully wafer-level scalable fabrication process to manufacture vapor cells with dimensions approaching those of glass-blown cells. The fabrication process is described, and spectroscopic measurements (optical absorption and magnetic resonance) are reported. A magnetic resonance linewidth of 350 Hz is demonstrated, and this is the smallest linewidth reported to date for a micro-fabricated vapor cell.

Study on the Magnetic Noise Characteristics of Amorphous and Nanocrystalline Inner Magnetic Shield Layers of SERF Co-Magnetometer.

With the widespread use of magneto-sensitive elements, magnetic shields are an important part of electronic equipment, ultra-sensitive atomic sensors, and in basic physics experiments. Particularly in Spin-exchange relaxation-free (SERF) co-magnetometers, the magnetic shield is an important component for maintaining the SERF state. However, the inherent noise of magnetic shield materials is an important factor limiting the measurement sensitivity and accuracy of SERF co-magnetometers. In this paper, both amorphous and nanocrystalline materials were designed and applied as the innermost magnetic shield of an SERF co-magnetometer. Magnetic noise characteristics of different amorphous and nanocrystalline materials used as the internal magnetic shielding layer of the magnetic shielding system were analyzed. In addition, the effects on magnetic noise due to adding aluminum to amorphous and nanocrystalline materials were studied. The experimental results show that compared with an amorphous material, a nanocrystalline material as the inner magnetic shield layer can effectively reduce the magnetic noise and improve the sensitivity and precision of the rotation measurement. Nanocrystalline material is very promising for inner shield composition in SERF co-magnetometers. Furthermore, its ultra-thin structure and low cost have significant application value in the miniaturization of SERF co-magnetometers.

In-situ measurement and cancellation of the light-shift in fiber-coupled atomic magnetometers.

In optical atomic magnetometers (AMs), the light-shift caused by the circularly polarized pumping beam have a significant impact on the response and is also one of the non-negligible sources of the noise. In this paper, we develop a novel method whereby utilizing the symmetry of the frequency response in an AM to measure and cancel the light-shift. Furthermore, we theoretically analyze and experimentally verify a rapid method of magnetic field compensation and the approach is convenient to measure and cancel of the light-shift. Moreover, the influence of intensity and frequency of the pumping beam is also investigated. The proposed method of in - situ measurement and cancellation of light-shift will be particularly profitable to other optical systems based on AMs.

Theory of colliding-probe atomic magnetometry: breaking the symmetry-enforced magneto-optical rotation blockade.

We show theoretically the presence of an optical field polarization rotation blocking mechanism in single-probe-based magnetic field sensing schemes, revealing the root cause for extremely small nonlinear magneto-optical rotation (NMOR) signal in single-probe-based atomic magnetometers. We present a colliding-probe atomic magnetometer theory, analytically describing the principle of the first nonlinear-optical atomic magnetometer. This new atomic magnetometry technique breaks the NMOR blockade in single-probe atomic magnetometers, enabling an energy circulation that results in larger than 20-dB enhancement in NMOR signal as well as better than 6-dB improvement of magnetic field detection sensitivity. Remarkably, all experimental observations reported to date can be qualitatively well-explained using this colliding-probe atomic magnetometry theory without numerical computations. This colliding-probe atomic magnetometry technique may have broad applications in scientific and technological fields ranging from micro-Tesla magnetic resonance imaging to cosmic particle detection.

A review of fiber‐coupled atomic magnetometer

AbstractFiber‐coupled atomic magnetometer (AM) based on a spin‐exchange relaxation‐free (SERF) regime has been a promising candidate for cryogenic devices for traditional magnetoencephalography (MEG) systems, as it has a flexible structure and a small lateral section for biomedical applications. The principles, key technologies, and applications of fiber‐coupled AM based on the atomic spin effect are reviewed in this paper. The principle of fiber‐coupled AM is introduced, the characteristics of conventional and fiber‐coupled AMs are compared, and the development history and current situation are reviewed. In addition, according to the demands for MEG applications, the key technologies of fiber‐coupled AM are summarized, and the bottleneck problems and future development directions for weak magnetic measurement are analyzed.