High-sensitivity total-field atomic magnetometer enables brain-activity measurements in unshielded Earth’s field.

The comagnetometer has been one of the most sensitive devices with which to test new physics related to spin-dependent interactions, but the comagnetometers based on overlapping ensembles of multiple spin species usually suffer from systematic errors due to magnetic field gradients. Here, we propose a comagnetometer based on the Zeeman transitions of the dual hyperfine levels in ground-state ${}^{87}\mathrm{Rb}$ atoms, which shows nearly negligible sensitivity to variations of laser power and frequency, magnetic field, and magnetic field gradients. We measured the hypothetical spin-dependent gravitational energy of the proton with the comagnetometer, which is smaller than $4\times {10}^{-18}\mathrm{eV}$, comparable to the most stringent existing constraint. Through optimizing the system parameters such as cell temperature, laser power, amplitude of driving magnetic field, as well as choosing better current source, it is possible to improve the sensitivity of the comagnetometer further.

We demonstrate an alignment-based ${}^{87}\mathrm{Rb}$ magnetometer that is immune to nonlinear Zeeman (NLZ) splitting, addressing an important problem in alkali-metal atomic magnetometry. In our scheme, there is a single magnetic resonance peak and well-separated hyperfine transition frequencies, making the magnetometer insensitive or even immune to NLZ-related heading errors. It is shown that the magnetometer can be implemented for practical measurements in geomagnetic environments, and the photon-shot-noise-limited sensitivity reaches $9\mathrm{fT}/\sqrt{\mathrm{Hz}}$ at $5\mathrm{\mu }\mathrm{T}$ and remains at tens of $\mathrm{fT}/\sqrt{\mathrm{Hz}}$ at $50\mathrm{\mu }\mathrm{T}$ at room temperature.

A magnetically-quiet environment is important for detecting faint magnetic-field signals or nonmagnetic spin-dependent interactions. Passive magnetic shielding using layers of large magnetic-permeability materials is widely used to reduce the magnetic-field noise. The magnetic-field noise can also be actively monitored with magnetometers and then compensated, acting as a complementary method to the passive shielding. We present here a general model to quantitatively depict and optimize the performance of active magnetic-field stabilization and experimentally verify our model using optically-pumped atomic magnetometers. We experimentally demonstrate a magnetic-field noise rejection ratio of larger than ∼800 at low frequencies and an environment with a magnetic-field noise floor of ∼40 ${\mathrm{fT}/\mathrm{Hz}}^{1/2}$ in unshielded Earth’s field. The proposed model provides a general guidance on analyzing and improving the performance of active magnetic-field stabilization with magnetometers. This work offers the possibility of sensitive detections of magnetic-field signals in a variety of unshielded natural environments.

Stand-off magnetometry allows measuring magnetic field at a distance, and can be employed in geophysical research, hazardous environment monitoring, and security applications. Stand-off magnetometry based on resonant scattering from atoms or molecules is often limited by the scarce amounts of detected light. The situation would be dramatically improved if the light emitted by excited atoms were to propagate towards the excitation light source in a directional manner. Here, we demonstrate that this is possible by means of mirrorless lasing. In a tabletop experiment, we detect free-precession signals of ground-state sodium spins under the influence of an external magnetic field by measuring backward-directed light. This method enables scalar magnetometry in the Earth field range, that can be extended to long-range remote sensing.

This paper introduces a method for using an atomic magnetometer to suppress current source noise. In this method, the current noise is (i) suppressed with a lamp-pumped cesium atomic magnetometer and (ii) measured with a higher-sensitivity laser-pumped cesium atomic magnetometer. The current noise is compared before and after noise suppression at an output current of 50 mA, and the noise-suppression ratio is ∼27 dB. The noise-suppression performance can be enhanced by improving the sensitivity of the atomic magnetometer and the ability to restrain the ambient magnetic noise. This method has practical applications in metrology and fundamental physics research.