We show, theoretically and experimentally, how the absolute orientation and polarization state of radio-frequency (rf) magnetic fields in a transverse two-dimensional (2D) plane can be uniquely determined using two optically pumped atomic magnetometers. In addition, the output signals from these quantum sensors can be readily expressed as a magnetic Jones vector. This composite device can complement electric field detection in finding rf directions or it can be used in applications where the electric field is negligible. The latter is found in a myriad of applications where the source can be modeled as radiating magnetic dipoles in the near-field limit. This tool could be used to characterize the response of a material to rf excitation or to distinguish signal from noise.
We show, theoretically and experimentally, how the absolute orientation and polarization state of radio-frequency (rf) magnetic fields in a transverse two-dimensional (2D) plane can be uniquely determined using two optically pumped atomic magnetometers. In addition, the output signals from these quantum sensors can be readily expressed as a magnetic Jones vector. This composite device can complement electric field detection in finding rf directions or it can be used in applications where the electric field is negligible. The latter is found in a myriad of applications where the source can be modeled as radiating magnetic dipoles in the near-field limit. This tool could be used to characterize the response of a material to rf excitation or to distinguish signal from noise.
We demonstrate the sensitivity of a sensor based on an optically-pumped radio-frequency (RF) atomic magnetometer to the polarization state of the detected RF magnetic field, and measure $>$36 dB difference in amplitude sensitivity for opposite circular field polarizations. This sensitivity could be used to create novel sensors that would allow signal detection while suppressing the ambient noise, regardless of the distance between the sources of signal and noise, in contrast to traditional gradiometry configurations. Additionally, such sensor will be orientation-sensitive, as the phase of the detected signal is shown to depend on the angle between the sensor's detection axis and the direction to the transmitter.
Optically pumped atomic magnetometers are capable of measuring radio-frequency magnetic fields with high sensitivity, but their narrow bandwidth, of the order of 1 kHz, restricts their applications. Further, their full potential is often not realized, with noisy environments or quantum noise curtailing their sensitivity. This study presents a two-chamber atomic magnetometer, with independent control over the tuning field and pumping helicity in each chamber, which addresses both bandwidth and sensitivity limitations. It demonstrates a practical instance of bandwidth expansion, by simultaneous measurement of signals spaced more than 100 kHz apart, at frequencies corresponding to nuclear quadrupole resonance of ammonium nitrate and potassium chlorate, two materials commonly found in improvised explosive devices. It further demonstrates: the intrinsic reduction of common-mode noise, including environmental and probe light-shift noise, down to 2.4 fT/Hz, with the use of resonant, but oppositely pumped, atomic chambers; the reduction of photon shot noise down to 28 aT/Hz with the use of 44 probe passes through the two chambers; and the reduction of atom noise down to 34 aT/Hz, in agreement with prediction, using a large combined volume, 128 cm3, of 87Rb atoms. This work underlines the potential sensitivity of these quantum sensors, while exploring the technical limitation, namely the high pump power required, that stands as a barrier to that sensitivity. Further, it marks a significant step towards utilizing atomic magnetometers in applications demanding broader bandwidth or multiplexing, notably simultaneous measurements at far-spaced radio frequencies.
When internal states of atoms are manipulated using coherent optical or radio-frequency (rf) radiation, it is essential to know the polarization of the radiation with respect to the quantization axis of the atom. We first present a measurement of the two-dimensional spatial distribution of the electric-field amplitude of a linearly polarized pulsed rf electric field at $\ensuremath{\sim}25.6\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$ and its angle with respect to a static electric field. The measurements exploit coherent population transfer between the 35s and 35p Rydberg states of helium atoms in a pulsed supersonic beam. Based on this experimental result, we develop a general framework in the form of a set of equations relating the five independent polarization parameters of a coherently oscillating field in a fixed laboratory frame to Rabi rates of transitions between a ground and three excited states of an atom with arbitrary quantization axis. We then explain how these equations can be used to fully characterize the polarization in a minimum of five Rabi-rate measurements by rotation of an external bias field, or, knowing the polarization of the driving field, to determine the orientation of the static field using two measurements. The presented technique is not limited to Rydberg atoms and rf fields but can also be applied to characterize optical fields. The technique has the potential for sensing the spatiotemporal properties of electromagnetic fields, e.g., in metrology devices or in hybrid experiments involving atoms close to surfaces.
We propose and demonstrate a radio-frequency (rf) atomic magnetometer based on parametric resonances. Previously, most rf atomic magnetometers are based on magnetic resonances and their sensitivities are often limited by spin-exchange relaxation. Here, we introduce a novel scheme for an rf magnetometer where the rf magnetic field is measured by exciting the parametric resonances instead of magnetic resonances using parametric modulation fields. In this way, the spin-exchange relaxation is almost eliminated. Benefiting from the low spin relaxation rate, the parametric resonance scheme exhibits a narrower linewidth and stronger signal, which results in a higher sensitivity. With a 6×6×3 mm^{3} Rb atomic vapor cell, we developed an rf atomic magnetometer with a noise floor of 2 fT/Hz^{1/2}, which is about one order of magnitude higher than the sensitivity achieved in the magnetic-resonance-based scheme. The presented rf detection scheme holds promise in advancing rf atomic magnetometers and brings new insight into their various applications.
We demonstrate an all-optical magnetometer capable of measuring the magnitude and direction of a magnetic field using nonlinear magneto-optical rotation in cesium vapor. Vector capability is added by effective modulation of the field along orthogonal axes and subsequent demodulation of the magnetic-resonance frequency. This modulation is provided by the ac Stark shift induced by circularly polarized laser beams. The sensor exhibits a demonstrated rms noise floor of $\ensuremath{\sim}65\text{ }\text{ }\mathrm{fT}/\sqrt{\mathrm{Hz}}$ in measurement of the field magnitude and 0.5mrad/Hz in the field direction; elimination of technical noise would improve these sensitivities to 12fT/Hz and $10\text{ }\text{ }\ensuremath{\mu}\mathrm{rad}/\sqrt{\mathrm{Hz}}$, respectively. Applications for this all-optical vector magnetometer would include magnetically sensitive fundamental physics experiments, such as the search for a permanent electric dipole moment of the neutron.
Accurate magnetic field direction sensing in compact platforms is critical in applications spanning magnetic navigation, space science, and biomedical imaging. We demonstrate a single-optical-axis vector optically pumped magnetometer based on Rabi oscillations between Zeeman sublevels driven by a series of resonant radiofrequency (RF) polarization ellipses (PEs). A calibration protocol based on controlled rotations of the DC magnetic field determines the spatial orientation of each PE. We develop a detailed theoretical model describing the angular dependence of the Rabi frequencies, incorporating key systematics including RF Stark shifts and Bloch-Siegert shifts. We also account for an RF-based heading-error systematic affecting Rabi-frequency measurements arising from the nonlinear Zeeman effect. Simultaneous Larmor measurements yield the magnitude of the magnetic field, enabling integrated vector-scalar measurements. The magnetometer achieves deadzone-free vector operation with 80 μrad mean angular accuracy and angular noise densities as low as 8 μrad/Hz, offering a pathway towards miniaturized sensors without requiring 3D optical access or sensor rotations.