Atomic Spin Precession Research Articles

Vector magnetometry employing a rotating RF field in a single-beam optically pumped magnetometer

Magnetic field vector information is crucial for many advanced applications, such as navigation and biomedical imaging. However, existing methods often lack high sensitivity or require complex setups. This study addresses these challenges by proposing a novel vector magnetometry method using a single-beam optically pumped magnetometer. A rotating radio-frequency field is innovatively utilized to excite atomic spin precession, enabling accurate measurement of the magnetic field direction based on scalar measurement. The method is tested through physical experiments with different magnetic field configurations to validate its performance. The experimental results demonstrate high accuracy, and achieve a magnetic field amplitude sensitivity of 800 fT/Hz1/2, an azimuth sensitivity of 100 μrad/Hz1/2, and a polar angle sensitivity of 13 μrad/Hz1/2. The proposed method facilitates sensor miniaturization and is suitable for applications in high magnetic field environments, such as geomagnetic field.

In-situ suppression of high-frequency magnetic field generated by the electric heater for atomic magnetometers

In this study, an in-situ suppression for the high-frequency magnetic field (HFMF) generated by the electric heater based on the detection of the atomic magnetometer signal is proposed. Specifically, by modeling the response of atomic spin precession signals from an optically pumped atomic magnetometer to the HFMF, the in-situ suppression is achieved by the radio frequency fields produced by coils to compensate the HFMF. The suppression of the HFMF generated by the electric heater is judged based on the amplitude response of the spin precession signal detected at the atomic resonance. Our proposed method is validated through both simulation and experimental results. Before and after in-situ suppression of the HFMF generated by the electric heater, the sensitivity of 13000 nT and 15000 nT static magnetic field within the heating frequency band is measured, resulting in approximately 50% and 60% increases in sensitivity after suppression, respectively.

On-Chip Mach-Zehnder-Like Interferometer for Atomic Spin Precession Detection

At present, most atomic spin precession detection schemes use discrete optical elements, which leads to bulky detection systems. However, chip-based spin precession detection schemes lack modulation, resulting in lower detection sensitivity. In this paper, we propose and simulatively demonstrate an integrated atomic spin precession detection scheme using an on-chip Mach-Zehnder-like interferometer. An on-chip polarization sorter is designed, with contrast ratio of 24dB and coupling efficiency of 16.8% at 795 nm. With this device, linearly polarized probe light that experienced optical rotation can be split and coupled into two waveguide arms of the interferometer. To avoid the effect of low frequency noise, our scheme uses a micro-heater to modulate the phase difference signal, allowing for high sensitivity detection. The whole detection system can reach micron size, which provides a practical new technique for high precision atomic sensors that can be integrated into chips.

Ultrasensitive Optical Rotation Detection With Closed-Loop Suppression of Spin Polarization Error

Optical rotation detection system (ORDS) utilizing quantum nondemolition (QND) measurement methods is widely applied in the field of quantum metrology and quantum information. However, the sensitivity of ORDS is limited by the uncertainty from optical-couple noise during the measurement of the atomic spin ensemble. In this study, we specifically analyze the mechanism of optical-couple noise caused by the fluctuations of probe light’s polarization in the modulated ORDS with a new model established to describe atomic spin precession in this particular condition. It is discovered that transverse electron-spin polarization errors are generated by the residual probe photon spin polarization in the ORDS, which results in extra coupling magnetic noise. In order to suppress this noise, a novel in situ method is proposed that the resultant electron-spin errors are reduced by a specifically designed closed-loop system. The results are verified through the ORDS in a co-magnetometer. After zeroing the extra probe photon spin polarization, an angular sensitivity better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1 \times 10 ^{\mathrm {-8}}$ </tex-math></inline-formula> rad/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\mathrm {1/2}}$ </tex-math></inline-formula> is achieved for frequencies higher than 5 Hz, demonstrating a probe background noise of 0.26 fT/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\mathrm {1/2}}$ </tex-math></inline-formula> @14.5 Hz, approaching electronic noise and photon shot noise (PSN). With closed-loop control, the optical rotation bias instability is promoted by 4.2 times (from 107.9 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$25.7 \mu $ </tex-math></inline-formula> rad/h), and the angular noise of the ORDS is reduced by 2.4 times at 1–100 Hz. The measurement uncertainty of the realized ORDS nears the standard quantum limit, paving the road for long-term ultrasensitive measurements for new physics explorations.

Atomic spin precession detection method based on the Mach-Zehnder interferometer in an atomic comagnetometer.

A new method for the detection of atomic spin precession based on the Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. Different from the conventional polarization detection methods which obtain the atomic spin precession signal by measuring the change of the probe laser power, the proposed method uses the laser modulated by an electro-optic phase modulator (EOM) as the source of the interferometer, and obtains the atomic spin precession signal by measuring the phase difference between the two arms of the MZI. The output of interferometer is independent of the probe laser power, which avoids the system error caused by the fluctuation of the probe laser power, and the long-term stability of the system is effectively improved. At the same time, the method adopts high-frequency electro-optic modulation, which can effectively suppress low-frequency noise, such as 1/f noise, and can significantly improve the detection sensitivity. The rotation sensitivity and long-term stability of the atomic comagnetometer were tested using the MZI detection method and a typical detection method, respectively. The comparison results show that the proposed method has the highest low frequency sensitivity and the potential to improve the long-term stability of the system.

Design of a Fiber Alkali Vapor Cell for Atomic Magnetometer for Magnetoencephalography Applications

Spin exchange relaxation free (SERF) atomic magnetometer (AM), based on the Larmor precession of alkali atoms, is considered a promising candidate for magnetoencephalography (MEG) systems with the advantages of high sensitivity and no need for cryogenic devices. The footprint of the sensor header contains alkali vapor cell and bulk optical elements determining the spatial resolution of the MEG system. Optical fiber could separate the vapor cell far from other parts of the sensor header to improve the spatial resolution. However, coupling between glass cell and fibers limits the coupling loss of the light. Here, we describe the design of a fiber-based alkali vapor cell that could alleviate these issues. A pair of fiber cables combining a polarization maintaining fiber (PMF) and hollow-core photonic crystal fibers (HC-PCFs) are enclosed in a vacuum-sealed T-shape glass tube filled with alkali atoms. The fiber cell ensures a flexible integration with most fiber systems. The fiber structure, with an air gap between HC-PCFs, provides a large interaction volume between light and atoms. The vapor of the alkali atoms diffuses into the air core of the HC-PCF from the glass tube by heating. The alkali atoms still contained in SERF regime are within the wall relaxation rates of 12,764 s−1 in the coating fiber cell. The insertion loss due to fiber coupling is analyzed. The coupling efficiency could be 91%, with the fiber structure consisting of a 40 μm diameter HC-PCF and a 1 mm air gap. The limit sensitivity under this condition is simulated at 14.7 fT/Hz1/2. The fabrication technique and the light insertion loss are discussed. The fiber alkali vapor cell is of compact size and has flexible integration with the fiber atomic spin precession detection system.

Two-stage digital differential atomic spin precession detection method

We propose and experimentally demonstrate a two-stage digital differential method for atomic spin precession detection. The first differential operation is carried out with a polarimeter module and subsequent digital differential. The second differential operation is achieved by orthogonally modulating the polarization direction of a linearly polarized probe light with a LiNbO3 electro-optic modulation module and by digitally demodulating the difference in the outputs corresponding to the positive and negative half periods of the modulation square-waves. This method is insensitive to the error of modulator and double sensitivity coefficient was obtained. The built detection system with the digital circuit was applied to a spin-exchange relaxation-free magnetometer, and the sensitivity coefficient, sensitivity, and bias instability were tested and compared with two other typical detection methods. The highest sensitivity and minimum bias instability and noise were achieved with the proposed method.

Search for topological defect of axionlike model with cesium atomic comagnetometer* *Project supported by the National Natural Science Foundation of China (Grant No. 62071012), the National Science Fund for Distinguished Young Scholars of China (Grant No. 61225003), and National Hi-Tech Research and Development Program of China.

Many terrestrial experiments have been designed to detect domain walls composed of axions or axionlike particles (ALPs), which are promising candidates of dark matter. When the domain wall crosses over the Earth, the pseudoscalar field of ALPs could couple to the atomic spins. Such exotic spin-dependent couplings can be searched for by monitoring the transient-in-time change of the atomic spin precession frequency in the presence of a magnetic field. We propose here a single-species cesium atomic comagnetometer, which measures the spin precession frequencies of atoms in different ground-state hyperfine levels, to eliminate the common-mode magnetic-field variations and search for the exotic non-magnetic couplings solely between protons and ALPs. With the single-species atomic comagnetometer, we experimentally rule out the possibility that the decay constant of the linear pseudoscalar couplings of ALPs to protons is f p ≲ 3.71 × 107 GeV. The advanced system has the potential to constrain the constant to be f p ≲ 10.7 × 109 GeV, promising to improve astrophysical constraint level by at least one order of magnitude. Our system could provide a sensitive detection method for the global network of optical magnetometers to search for exotic physics.

Improved temperature stability of a fiber Sagnac-like detection system for atomic magnetometers.

A novel fiber Sagnac-like detection system has unique competitive advantages for detecting atomic spin precession in atomic magnetometers. Unfortunately, its operating stability is severely limited by temperature fluctuations. In this paper, we describe a new approach to improve the temperature stability by using the ratio signal as the output instead of the conventional fundamental component. This method can effectively counteract the temperature-caused fluctuations in both light intensity and scale factor of photodetector. For a temperature range from 20°C to 40°C, a relative fluctuation of the ratio output signal of 0.97% was achieved, which was 17.4 times better than the fundamental component output. Moreover, no additional equipment and complex compensation algorithms are required during this process. It is a generic method that can also be applied to improve the stability of other detection schemes used in atomic magnetometers.

Position and Direction Tracking of a Magnetic Object Based on an Mx-Atomic Magnetometer

Remote and non-invasive tracking of a moving magnetic object based on an atomic magnetometer has been developed recently. The sensitivity of atomic magnetometers is limited by mechanisms that relax the spin precession of alkali atoms. Meanwhile, some of these mechanisms such as magnetic field gradient are applicable in magnetic object tracking. Correspondingly, we have illustrated a way of operating an Mx atomic magnetometer to measure the magnetic field and its gradient simultaneously for a moving magnetic microwire, which resulted in recording a spike-like signal. We described the dependency of the signal on the position, velocity, and direction of the microwire. According to the results, the measurement of the inhomogeneous local magnetic field gradient opens new ways for obtaining the direction of the velocity of magnetic objects accessible in cells with large sizes. Furthermore, the accuracy of the velocimetry was found as 40 µm/s which could be an important means for assessing the microvascular blood flow.

基于原子自由进动的光缩效应实验研究

基于原子自由进动的光缩效应实验研究

Miniaturized optical rotation detection system based on liquid crystal variable retarder in a K-Rb-21Ne gyroscope.

A novel method to control the light intensity stability and modulate the probe light polarization using a liquid crystal variable retarder (LCVR) to detect atomic spin precession simultaneously in a K-Rb-21Ne gyroscope is reported. A sinusoidal driving voltage is applied to drive the LCVR and is skillfully used to produce a high-frequency modulation for the probe light. The modulation helps to avoid electronic detection noise appearing at low frequencies and allows for phase-sensitive detection. The coefficient of rate ramp can be reduced from 1.31 (deg/h)/h to 0.05 (deg/h)/h (Allan deviation), and the bias instability of about 0.08 deg/h at the averaging time of 200 s is achieved. Therefore, the long-term stability of the angular velocity measurement can be improved and other optical modulators can be replaced to facilitate the miniaturization of the gyroscope by using this intensity modulation detection method. This optical rotation detection method also can be applied to other miniaturized atomic sensors, such as atomic magnetometers.

Subpicotesla scalar atomic magnetometer with a microfabricated cell

We demonstrated a scalar atomic magnetometer using a microfabricated Cs vapor cell. The atomic spin precession is driven by an amplitude-modulated circularly-polarized pump laser resonant on the D1 transition of Cs atoms and detected by an off-resonant linearly-polarized probe laser using a balanced polarimeter setup. Under a magnetic field with amplitude in the Earth's magnetic field range, the magnetometer in the gradiometer mode can reach sensitivities below 150fT/Hz, which shows that the magnetometer by itself can achieve sub-100fT/Hz sensitivities. In addition to its high sensitivity, the magnetometer has a bandwidth close to 1 kHz due to the broad magnetic resonance inside the small vapor cell. Our experiment suggests the feasibility of a portable low-power and high-performance magnetometer which can be operated in the Earth's magnetic field. Such a device will greatly reduce the restrictions on the operating environment and expand the range of applications for atomic magnetometers, such as detection of nuclear magnetic resonance in low magnetic fields.

A τ−1 measurement across three orders of magnitude based on an atomic spin precession magnetometer in self-sustaining mode

The absolute measurement of magnetic fields can be realized by monitoring the Larmor precession of atomic spins. Yet, this spin-precession magnetometer can only be put into use in weak magnetic fields and the sensitivity is limited by the coherence time beyond which, the uncertainty decreases as τ−1/2 rate. Now we demonstrate that the dynamic range of an atomic spin magnetometer can be extended to geomagnetic field magnitude with τ−1 property maintained based on the the self-sustaining method. A mean sensitivity of 20 pT/Hz and a frequency response bandwidth of 5 kHz are realized in a magnetic field of 10000 nT. More important applications in large magnetic fields can be enabled in virtue of the superiority using this self-sustaining magnetometer.

Optical Rotation Detection for Atomic Spin Precession Using a Superluminescent Diode

A superluminescent diode (SLD) as an alternative of laser is used to detect optical rotation for atomic spin precession. A more uniform Gauss configuration without additional beam shaping and a relatively high power of the SLD have a potential for atomic magnetometers, which is demonstrated in theory and experiments. In addition, the robustness and compactness enable a more practical way for optical rotation detections, especially for applications in magnetoencephalography systems.

Single-Fiber Sagnac-Like Interferometer for Optical Rotation Measurement in Atomic Spin Precession Detection

In this paper, a single-fiber Sagnac-like interferometer is proposed and experimentally demonstrated for detecting atomic spin precession in an atomic magnetometer. The single-fiber Sagnac-like interferometer has a reciprocal optical structure, which is insensitive to environmental disturbances. The fiber system is more flexible for implementation in miniature atomic magnetometers, which has potential for application in magnetoencephalography systems. The sensitivity and the bandwidth of the system are also discussed. The thermal and bending disturbances to fiber system are observed, whereas the long-term stability is analyzed and compared with the Faraday modulation scheme.

基于圆偏振探测光的光纤原子自旋进动检测技术

基于圆偏振探测光的光纤原子自旋进动检测技术

Investigation on Rotation Response of Spin-Exchange Relaxation-Free Atomic Spin Gyroscope

Spin-exchange relaxation-free atomic spin gyroscope (SERF gyro) has ultrahigh theoretical sensitivity and is regarded as one of the most potential atomic gyroscopes. To investigate its rotation response, a high linear atomic spin precession detection module was developed firstly by modifying the fiber reflective Sagnac interferometer for atomic spin precession detection and operating it in closed-loop mode. The SERF gyro based on K-Rb- 21 Ne comagnetometer was built and tested and appeared obvious nonlinear response feature. The nonlinear rotation response model was deduced by assuming the rotation-induced nuclei spin procession detecting as an in-situ magnetometer inside the SERF gyro and the fitting curves are in good agreement with the experimental results. The optimized pump power for maximum sensitivity and measurable dynamic range were analyzed theoretically and demonstrated experimentally. The investigation results show that the nonlinear rotation response is an intrinsic property of SERF gyro and its measurable dynamic range is limited due to the saturation characteristic of the sum of pumping and relaxation rate. This indicates that SERF gyro is propitious to a position gyroscope with high sensitivity.

Polarization-driven spin precession of mesospheric sodium atoms.

We report experimental on-sky observations of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. A magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 and 100km altitude. The experiment was performed at La Palma, and the uncertainty (0.2kHz) in the measured Larmor frequency (≈260 kHz) corresponded to an error in the geomagnetic field of 0.3mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere.

An atomic spin precession detection method based on electro-optic modulation in an all-optical K–Rb hybrid atomic magnetometer

We present an ultrahigh-sensitivity electro-optic modulator (EOM) detection method for detecting the atomic Larmor precession in an all-optical K–Rb hybrid atomic magnetometer operating in the spin-exchange relaxation-free regime. A magnetic field sensitivity of ~10 f T Hz−1/2 has been achieved by optimizing the probe laser parameters and the EOM modulation conditions, which is comparable to that with the Faraday modulation method and has a better performance than the balanced polarimetry method in the low frequency range. The EOM detection method in the atomic magnetometer presents several advantages, such as simple structure, no extra magnetic noise, moderate thermal effect, high measurement sensitivity and reliable stability. It is demonstrated to be feasible for the improved compactness and simplicity of atomic magnetic field measurement devices in the future.