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arXiv:1809.08978

Gradiometry-free noise suppression in RF atomic magnetometers

Vladislav P. Gerginov

Published Sep 24, 2018• 1 citation
arXivHTMLPDF

Abstract

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.

DOI
10.48550/arxiv.1809.08978

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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.
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We report a laser-pumped 4He atomic magnetometer with light shift suppression through the atomic sensor itself. A linearly polarized light is used to optically align the 4He metastable atoms and we monitor the magneto-optical double resonance (MODR) signals produced by the left- and right-circularly orthogonal components. It is shown that light shift leads to the atomic alignment to orientation conversion effect, and thus, the difference between the two MODR signals. One of these two MODR signals is locked at the Larmor frequency and is used to measure the ambient magnetic field, while the differential signal is, simultaneously, fed back to suppress the light shift. The scheme could be of the advantage to the design of compact magnetometers by reducing the systematic errors due to light shift.
Under negative feedback, the quality factor QQQ of a radio-frequency atomic magnetometer can be decreased by more than two orders of magnitude, so that any initial perturbation of the polarized spin system can be rapidly damped, preparing the magnetometer for detection of the desired signal. We find that noise is also suppressed under such spin damping, with a characteristic spectral response corresponding to the type of noise; therefore magnetic, photon shot, and spin-projection noise can be measured distinctly. While the suppression of resonant photon shot noise implies the closed-loop production of polarization-squeezed light, the suppression of resonant spin-projection noise does not imply spin squeezing, rather simply the broadening of the noise spectrum with QQQ. Furthermore, the application of spin damping during phase-sensitive detection suppresses both signal and noise in such a way as to increase the sensitivity bandwidth. We demonstrate a threefold increase in the magnetometer's bandwidth while maintaining 0.3 fT/Hz\sqrt{\mathrm{Hz}}Hz​ sensitivity.