Vector measurements using all optical scalar atomic magnetometers

A little more than a decade ago spin-exchange relaxation free (SERF) magnetometers set a new record of magnetic field sensitivity surpassing cryogenic SQUIDs. Since then a lot of progress has been made in design, commercialization, and development of novel applications of the SERF magnetometers. In addition, the operation of the SERF magnetometer was extended beyond the SERF regime resulting in the discovery of ultra-high sensitivity high frequency and scalar magnetometers. This chapter will cover some basic principles of SERF and high-density SERF-like magnetometers in the regimes when spin-exchange collisions affect the line-width of the magnetometers. Various topics will be covered: the SERF operation, the role of spin-exchange collisions, fundamental and technical noises in SERF and other high-density magnetometers, light shifts, optical pumping. The formalism of density matrix equations will be briefly described with some illustrations. At some conditions, Bloch equations can also provide adequate treatment of spin dynamics, so this topic is also briefly covered. Some applications, such as magnetoencephalography and magnetic resonance imaging (MRI), of SERF, high-frequency, and scalar magnetometers will be discussed. The number of applications will grow in the future, especially when high-sensitivity SERF magnetometers become commercially available and their operation becomes simple and user-friendly. Finally, it is anticipated that in the near future many applications developed with SQUIDs will be gradually replaced with those based on SERF and other ultra-sensitive atomic magnetometers.

In inter-satellite laser communication systems, angular deviations between the boresights of terminals exert an impact on system performance, thereby imposing higher requirements on performance parameters such as the angle measurement accuracy and range of the system. Conventional coherent angle measurement systems feature a fixed baseline, which results in insufficient dynamic adjustment capability for accuracy and range. To address the aforementioned issues, this paper proposes a variable-baseline coherent angle measurement method based on nutation scanning. By altering the baseline length through a nutation scanning strategy, this method achieves dynamic adjustment of angle measurement accuracy and range, enhancing the adaptability of the angle measurement system in different scenarios. For long-distance communication, a small nutation scanning angle (short baseline) is used to adapt to significant satellite relative position changes via a large angle measurement range, while a large nutation scanning angle (long baseline) is adopted for short-distance communication to ensure stable tracking with high angle measurement accuracy. Firstly, a mathematical model of the system is established, and the relationships between the nutation scanning angle, angle measurement accuracy, and angle measurement range are derived. The Monte Carlo method is employed to construct a variable-baseline angle measurement simulation system for data verification, and a tabletop experimental system is built for performance analysis. Experimental results show that when the nutation scanning angle γ increases from 10 μrad to 100 μrad, the baseline length increases from 0.05 μm to 0.5 μm, the angle measurement accuracy improves from 56.9 μrad to 6.35 μrad, and the angle measurement range narrows from 880 μrad to 560 μrad. These results are consistent with the trend of theoretical derivation and correspond to the simulation results. This study verifies the feasibility of dynamically regulating angle measurement accuracy and range via the nutation scanning angle and provides a reference for the design of space laser communication systems.

Static weak magnetic field measurements based on low-field nuclear magnetic resonance.

We propose an operating condition for an optically pumped scalar atomic magnetometer with a unique band-pass frequency response. Under this operating condition, the signal response of the magnetometer is increased threefold around a nonzero resonance frequency. The resonance frequency is equal to the detuning between the Larmor frequency and the frequency of the oscillating magnetic field used for excitation. The resonance is explained in terms of the characteristics of the first-order perturbation expansion of the Bloch equation for a modulated bias magnetic field. The scalar magnetometer provides an enhanced magnetic response around its resonance frequency even beyond the cutoff frequency, which is defined as the inverse of the phase relaxation time of the spin polarization.

The polarization of X-ray emission is a unique tool used to investigate the magnetic field structure around astrophysical objects. In this paper, we study the linear polarization of X-ray emissions from gamma-ray binary systems based on pulsar scenarios. We discuss synchrotron emission from pulsar wind particles accelerated by a standing shock. We explore three kinds of axisymmetric magnetic field structures: (i) toroidal magnetic fields, (ii) poloidal magnetic fields, and (iii) tangled magnetic fields. Because of the axisymmetric structure, the polarization angle of integrated emission is oriented along or perpendicular to the shock-cone axis projected on the sky and swings around 360° in one orbit. For the toroidal case, the polarization angle is always directed along the shock-cone axis and smoothly changes along the orbital phase. For the poloidal/tangled magnetic field, the direction of the polarization angle depends on the system parameters and orbital phase. In one orbit, the polarization degree for the toroidal case can reach the maximum value of the synchrotron radiation (∼70%), while the maximum polarization degree for poloidal/tangled field cases is several 10%. We apply our model to bright gamma-ray binary LS 5039 and make predictions for future observations. With the expected sensitivity of the Imaging X-ray Polarimetry Explorer, linear polarization can be detected by an observation of several days if the magnetic field is dominated by the toroidal magnetic field. If the magnetic field is dominated by the poloidal/tangled field, significant detection is expected with an observation longer than 10 days.

LiteBIRD is JAXA Strategic Large Mission for the late 2020s that aims to observe the large-scale B-mode polarization pattern of the cosmic microwave background. One of its telescopes, the Low Frequency Telescope (LFT), has a crossed-Dragone design and observes at 34-161 GHz with a field of view of 18° x 9°. Because a miscalibration of the polarization angles mixes E- and B-mode polarization, we have measured the variation of the polarization angles in the field of view of a 1/4-scaled LFT antenna at 140-220 GHz, which corresponds to 35-55 GHz for the full-scale LFT, considering a scaling of the wavelength. We placed a collimated-wave source near the scaled-LFT aperture and rotated the polarization angle of the LFT feed. The measurements were explained well with a simple Jones matrix calculation, and the fitting errors of the polarization angles were less than 0.1’. We also measured the polarization angles by rotating the polarization direction in the scaled-LFT aperture, and the results were consistent with the angles measured by rotating the feed polarization at the ±10” level, except at the lowest frequencies. The polarization angle at the edges of the focal plane varied from that at the center by up to around a degree, with larger variation at lower frequencies. We evaluated the polarization angles for both Pol-X and Pol-Y feeds, and the results with Pol-Y showed a trend consistent with ray-tracing simulations. The results for Pol-X showed the opposite trend of the polarization rotation direction and larger angle variations.

Planck observations at 353GHz provide the first fully-sampled maps of the polarized dust emission towards interstellar filaments and their backgrounds. The polarization data provide insight on the structure of their magnetic field (B). We present the polarization maps of three nearby star forming filament of moderate column density (NH~10^22cm^-2): Musca, B211, and L1506. We use the spatial information to separate Stokes I, Q, and U of the filaments from those of their backgrounds, an essential step to measure the intrinsic polarization fraction (p) and angle (psi) of each emission component. We find that the polarization angles in the three filaments (psi_fil) are coherent along their lengths and not the same as in their backgrounds (psi_bg). The differences between psi_fil and psi_bg are 12deg, 6deg, and 54deg for Musca, B211, and L1506, respectively. These differences for Musca and L1506 are larger than the dispersions of psi, both along the filaments and in their backgrounds. The observed changes of psi are direct evidence for variations of the orientation of the plane of the sky (POS) projection of the B-field. As in previous studies, we find a decrease of several percent of p with NH. We show that the drop in p cannot be explained by random fluctuations of the orientation of B within the filaments because they are too small (sigma_psi<10deg). We recognize the degeneracy between dust alignment efficiency and the structure of B in causing variations in p, but we argue that the decrease of p from the backgrounds to the filaments results in part from depolarization associated with the 3D structure of B: both its orientation in the POS and with respect to the POS. We do not resolve the inner structure of the filaments, but at the smallest scales accessible with Planck (~0.2pc), the observed changes of psi and p hold information on the B-field structure within filaments.

In order to balance a rotor, the standard rotor balancing methods involve measurement of vibration amplitude and vibration phase angle. But accurate phase angle measurement is generally difficult because vibration phase is sensitive to speed variations and it is difficult to maintain a constant speed in operating condition. Also, phase angle measurement requires very high end instrumentation for accurate phase angle measurement. Number of trial runs during balancing process also plays an important criterion in deciding the effectiveness of that method. A Method is considered to be more effective if it balances a rotor in less number of trial runs. This paper presents a novel rotor balancing method called amplitude subtraction method in which vibration amplitudes are measured only. Performance of this method is analysed by a comparative study with phase angle based rotor balancing method (trim balancing method). Trim balancing and amplitude subtraction method are compared on the basis of its accuracy and number of trial runs during balancing process. The results of experimentation demonstrate successful implementation of proposed method.

The optically pumped magnetometer (OPM) operating in spin exchange relaxation-free (SERF) regime is a kind of magnetic sensor that has ultra-high sensitivity. SERF OPMs need to operate in a near zero magnetic field environment. In many applications, multi-channel OPMs are utilized to measure the magnetic field at different observation points, where the magnetic fields generated by the OPMs interfere with each other. To suppress the magnetic field interference, this paper proposes a magnetic field compensation method based on an artificial neural network (ANN). The transfer function between the compensation currents and the observed signals is derived, based on which a nonlinear multiparameter optimization problem is derived. Experimental data with different initial magnetic field conditions is collected. Then, an ANN model is employed to optimize the compensation currents of the OPMs to minimize the magnetic field experienced by each sensor. It is demonstrated that this method can effectively reduce the magnetic field crosstalk of multi-channel OPMs.

We investigate the effect of the Milky Way's magnetic field in star forming regions using archived 350 micron polarization data on 52 Galactic star formation regions from the Hertz polarimeter module. The polarization angles and percentages for individual telescope beams were combined in order to produce a large-scale average for each source and for complexes of sources. In more than 80% of the sources, we find a meaningful mean magnetic field direction, implying the existence of an ordered magnetic field component at the scale of these sources. The average polarization angles were analyzed with respect to the Galactic coordinates in order to test for correlations between polarization percentage, polarization angle, intensity, and Galactic location. No correlation was found, which suggests that the magnetic field in dense molecular clouds is decoupled from the large-scale Galactic magnetic field. Finally, we show that the magnetic field directions in the complexes are consistent with a random distribution on the sky.

We propose an experimental search for an axion-induced oscillating electric dipole moment (OEDM) for electrons using state-of-the-art alkali vapor-cell atomic magnetometers. The axion is a hypothesized new fundamental particle which can resolve the strong charge-parity problem and be a prominent dark matter candidate. This experiment utilizes an atomic magnetometer as both a source of optically polarized electron spins and a magnetic-field sensor. The interaction of the axion field, oscillating at a frequency equal to the axion mass, with an electron spin induces a sizable OEDM of the electron at the same frequency as the axion field. When the alkali vapor is subjected to an electric field and a magnetic field, the electron OEDM interacts with the electric field, resulting in an electron spin precession at the spin’s Larmor frequency in the magnetic field. The resulting precession signal can be sensitively detected with a probe laser beam of the atomic magnetometer. We estimate that the experiment is sensitive to the axion-photon interaction in ultralight axion masses from 10^-15 to 10^-10 eV. It is able to improve the current experimental limit up to 5 orders of magnitude, exploring new axion parameter spaces.

Atomic magnetometers serve as a type of quantum sensing instrument, offering extensive applications in the field of extremely weak magnetic field measurement. Miniaturized zero‐field atomic magnetometers typically use continuous high‐frequency magnetic fields to modulate atomic spin precession for multi‐axis measurements. However, this modulation induces additional spin‐exchange relaxation, which significantly broadens the resonance linewidth. In this study, the pulsed magnetic field modulation is introduced instead of continuous modulation to reduce the effective duration of the magnetic field. This operation significantly diminishes perturbations of steady‐state spin distribution in Zeeman sublevels caused by Larmor precession during each modulation cycle, thus effectively mitigating decoherence effects. The relationship between spin‐exchange relaxation rate and pulsed field parameters is studied based on the generalized hyperfine Bloch equation, and a spin dynamics model under the pulsed modulation is developed. Agreement between experimental and theoretical results validates the suppression effect of spin‐exchange relaxation and the accuracy of this model. Experimental results indicate that the pulsed modulation scheme results in a 30% reduction in the spin‐exchange relaxation rate and achieves dual‐axis sensitivities of 1.5 and 2.5 fT Hz −1/2 , respectively, representing improvements of 25% and 20% over conventional continuous modulation methods.

The magnetic field is one of the most fundamental and ubiquitous physical observables, carrying information about all electromagnetic phenomena. For the past 30 years, superconducting quantum interference devices (SQUIDs) operating at 4 K have been unchallenged as ultrahigh-sensitivity magnetic field detectors, with a sensitivity reaching down to 1 fT Hz(-1/2) (1 fT = 10(-15) T). They have enabled, for example, mapping of the magnetic fields produced by the brain, and localization of the underlying electrical activity (magnetoencephalography). Atomic magnetometers, based on detection of Larmor spin precession of optically pumped atoms, have approached similar levels of sensitivity using large measurement volumes, but have much lower sensitivity in the more compact designs required for magnetic imaging applications. Higher sensitivity and spatial resolution combined with non-cryogenic operation of atomic magnetometers would enable new applications, including the possibility of mapping non-invasively the cortical modules in the brain. Here we describe a new spin-exchange relaxation-free (SERF) atomic magnetometer, and demonstrate magnetic field sensitivity of 0.54 fT Hz(-1/2) with a measurement volume of only 0.3 cm3. Theoretical analysis shows that fundamental sensitivity limits of this device are below 0.01 fT Hz(-1/2). We also demonstrate simple multichannel operation of the magnetometer, and localization of magnetic field sources with a resolution of 2 mm.

Spin is a kind of complicated, dangerous and abnormal flight status when airplane stalls in the air. Spin and recovery research is usually done in vertical wind tunnel test. This method has the advantages of low-cost, short cycle and so on. For the measurement of spin attitude angles, a set of stereo vision measurement system using two CCD cameras and other equipments is built. By the research of coordinate system transformation and digital image processing algorithms, 3D parameters of man-made markers of the model surface are obtained, and the measurement of spin attitude angles is realized. The test results show that the technology has the features of non-contact, high recognition rate and flexibility, the measurement accuracy of attitude angles reaches 0.1 °.

The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.