arXiv:2008.13234 · Optics Express

All-optical intrinsic atomic gradiometer with sub-20 fT/cm/√Hz sensitivity in a 22 µT earth-scale magnetic field

Abigail R. Perry, Michael Bulatowicz, M. Larsen, Thad G Walker, Robert Wyllie

Published Nov 10, 2020• 38 citations

Abstract

In this work we demonstrate a high sensitivity atomic gradiometer capable of operation in earth-field level environments. We apply a light-pulse sequence at four times the Larmor frequency to achieve gradiometer sensitivity <20 fT/cm/ H z at the finite field strength of 22 µT. The experimental timing sequence can be tuned to the field magnitude of interest. Our all-optical scalar gradiometer performs a differential measurement between two regions of a single vapor cell on a 4 cm baseline. Our results pave the way for extensions to operation in higher dimensions, vector sensitivity, and more advanced gradiometers.

DOI: 10.1364/oe.408486

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arXiv:2008.13234 · Optics Express

All-optical intrinsic atomic gradiometer with sub-20 fT/cm/√Hz sensitivity in a 22 µT earth-scale magnetic field

Abigail R. Perry, Michael Bulatowicz, M. Larsen, Thad G Walker, Robert Wyllie

Published Nov 10, 2020• 38 citations

arXiv HTML PDF

Abstract

DOI: 10.1364/oe.408486

Related papers

Pulsed vector atomic magnetometer using an alternating fast-rotating field

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{{{\rm{fT}}}}/\sqrt{{{{\rm{Hz}}}}}

fT / Hz (0.7 parts per billion) and angular resolutions of 6

{{{\rm{nrad}}}}/\sqrt{{{{\rm{Hz}}}}}

nrad / Hz at a 50 μT Earth field strength. The noise spectra remain flat down to 1 Hz and 0.1 Hz, respectively. Here we show that this method overcomes several metrological challenges commonly faced by vector magnetometers and gradiometers. We propose a unique peak-altering modulation technique to mitigate systematic effects, including a newly identified dynamic heading error. Additionally, we establish the fundamental sensitivity limits of the sensor, demonstrating that its vector sensitivity approaches scalar sensitivity while preserving the inherent accuracy and calibration benefits of scalar sensors. This high-dynamic-range, ultrahigh-resolution magnetometer offers exceptional versatility for diverse applications.

We investigate an all-optical scheme for high-sensitivity measurements of the Earth's scale magnetic field (

B

). The scheme is based on the Bell-Bloom technique with frequency-modulated light. A single elliptically polarized light beam is used both for pumping the alkali-metal atoms and probing Larmor precession of their spins under the external magnetic field. In contrast to many other high-sensitivity magnetometry schemes based on nonlinear polarization rotation of the light, the proposed approach enables the observation of magnetic resonance via changes in the light-beam ellipticity parameter. The approach has been validated in experiments with a vertical-cavity surface-emitting laser, irradiating a

0.125

^3

cesium vapor cell filled with a buffer gas. In the current experimental conditions, the achieved sensitivity is estimated at

220

fT/

\surd

Hz under

B

\,\approx\,

50

μ

T, while the shot-noise floor corresponds to

\approx\,

20

fT/

\surd

Hz. The bandwidth is estimated at

\approx\,

1

kHz. The proposed simple and robust single-beam scheme is well suited for miniaturization and is therefore particularly promising for the development of compact, highly sensitive, low-power-consumption atomic magnetometers for a wide range of applications in the geomagnetic field.

^{87}

Rb atoms in a coherent superposition of magnetically sensitive hyperfine ground states. Interference between the sidebands generates a low-frequency beat note whose frequency is determined by the magnetic field gradient between the two vapor cells. In contrast to traditional magnetic gradiometers, our approach provides a direct readout of the gradient field without the intermediate step of subtracting the outputs of two spacially separated magnetometers. The technique is expected to provide effective common-mode magnetic field cancellation at frequencies far greater than the bandwidth of the gradiometer. Using this technique, we developed a compact magnetic gradiometer sensor head with integrated optics with a sensitivity of

25 \ fT/cm/\sqrt{Hz}

with a

4.4

cm baseline, while operating in a noisy laboratory environment unshielded from Earth's field. We also outline a theoretical framework that accurately models sideband generation using a density matrix formalism.

We present a realization of a magnetic sensor based on electromagnetically induced transparency (EIT) resonances observed in hot Rb vapor using lin

\parallel

lin polarized dichromatic light and evaluate scalar and vector capabilities of the sensor for measuring Earth-like magnetic fields. We demonstrate scalar measurement sensitivity of 2 pT/Hz^1/2 in the 1-100~Hz spectral frequency band using a cm^3 Rb vapor cell, significantly improving the performance for such a configuration if compared with earlier measurements of large magnetic fields. By using a single linearly polarized dichromatic optical field, we are also able to determine the direction of the magnetic field with respect to the light propagation direction and polarization, taking advantage of the symmetries of the interaction scheme. We accomplish that by combining the polarization-sensitive transmission measurements and sparse sensing machine learning techniques. A path for further improvement of the sensitivity and elimination of systematic effects, such as heading error, is discussed.