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IEEE Transactions on Instrumentation and Measurement

All-Optical Broadband Atomic Magnetometer Utilizing Raman-Driven Spin Noise Spectroscopy

Sayari Majumder, Saptarishi Chaudhuri

Published Jan 1, 2025• 6 citations
Publisher

Abstract

We report the development and performance of an atomic magnetometer (AM) utilizing the Raman-driven spin noise spectroscopy (RDSNS) technique, which measures the intrinsic spin fluctuations of an atomic medium subjected to a coherent two-photon optical drive. The RDSNS signal is noninvasively detected using a far-off resonant weak probe light utilizing a polarimetric detection scheme. This overcomes the limitations of the small dynamic range and low bandwidth of traditional optically pumped magnetometers (OPMs) or AMs operating in the spin exchange relaxation-free (SERF) regime. We experimentally demonstrate, using both paraffin-coated and buffer-gas-filled vapor cell containing neutral rubidium atoms, that the dynamic range of such magnetometers can reach beyond 150 G while maintaining a dc magnetometer resolution of around 30 pT and a magnetic noise sensitivity of around$30~\text {pT}/\sqrt {\text {Hz}}$at a bandwidth of 100 Hz and a magnetic field of about 0.6 mT. Being an all-optical setup, this instrument is largely immune to interference with stray radio frequency fields and has the potential to be miniaturized and field deployable. One of the practical applications would be to measure variations in geomagnetic fields precisely without having to shield the magnetometer.

DOI
10.1109/tim.2025.3554278

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We report the development and performance of an atomic magnetometer (AM) utilizing the Raman-driven spin noise spectroscopy (RDSNS) technique, which measures the intrinsic spin fluctuations of an atomic medium subjected to a coherent two-photon optical drive. The RDSNS signal is noninvasively detected using a far-off resonant weak probe light utilizing a polarimetric detection scheme. This overcomes the limitations of the small dynamic range and low bandwidth of traditional optically pumped magnetometers (OPMs) or AMs operating in the spin exchange relaxation-free (SERF) regime. We experimentally demonstrate, using both paraffin-coated and buffer-gas-filled vapor cell containing neutral rubidium atoms, that the dynamic range of such magnetometers can reach beyond 150 G while maintaining a dc magnetometer resolution of around 30 pT and a magnetic noise sensitivity of around30 pT/Hz30~\text {pT}/\sqrt {\text {Hz}}30 pT/Hz​at a bandwidth of 100 Hz and a magnetic field of about 0.6 mT. Being an all-optical setup, this instrument is largely immune to interference with stray radio frequency fields and has the potential to be miniaturized and field deployable. One of the practical applications would be to measure variations in geomagnetic fields precisely without having to shield the magnetometer.
Atomic magnetometry was performed at Earth’s magnetic field over a free-space distance of ten meters. Two laser beams aimed at a distant alkali-vapor cell excited and detected the 87Rb magnetic resonance, allowing the magnetic field within the cell to be interrogated remotely. Operated as a driven oscillator, the magnetometer measured the geomagnetic field with ≲3.5 pT precision in a ∼2 s data acquisition; this precision was likely limited by ambient field fluctuations. The sensor was also operated in self-oscillating mode with a 5.3 pT/Hz noise floor. Further optimization will yield a high-bandwidth, fully remote magnetometer with sub-pT sensitivity.
A high-sensitivity optically pumped atomic magnetometer working in the geomagnetic range utilizing the light-narrowing effect and in-phase excitation is described. The setup is based on a simple pump–probe arrangement built around a Cs vapor cell whose active volume is 64 mm3. The transverse oscillating field is applied parallel to the probe beam to drive Zeeman resonance, and the in-phase component of the resonance signal is measured to determine the field. The sensitivity of the magnetometer is improved by pumping most atoms into the stretched state. Consequently, spin-exchange relaxation is suppressed, and a sensitivity of 0.1 pT/Hz1/2 in the range of 10 μT is achieved. This magnetometer has the advantages of large dynamic range, high performance of low-frequency stabilization, high response speed, and compact size. It can be used for many cutting-edge applications such as detection of magnetic anomalies.
Present protocols for obtaining the ultimate magnetic sensitivity of optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on uncorrelated atoms in stretched states. A new approach for calculating the spin projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic sensitivity of OPMs is proposed. Our model is based solely on the mean-field density matrix dynamics and in contrast to previous models, it applies to both low and high field regimes, it takes into account the degree of spin polarization, the intra- and interhyperfine correlations, the decoherence processes, the atom-light coupling and the effects of the spin dynamics on the spin-noise spectra. Fine tuning of the probe frequency allow us to explore different hyperfine states and ground-state correlations. Especially in the spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance narrowing and the increased number density, hallmarks of SERF magnetometers, we report on a new SERF feature; the reduction of spin-projection noise at the spin precession frequency as a consequence of strongly-correlated hyperfine spins that attenuate and redistribute SPN when properly probed.
We demonstrate a minimally perturbative detection method—Raman driven spin noise spectroscopy (RDSNS)—to directly probe the local density of a cloud of cold 39K atoms in real time. The technique relies on detecting the polarization fluctuations of a far-off resonant focused probe light passing through the atoms, driven by two coherent Raman lasers connecting adjacent Zeeman spin states. This is an easily implementable scheme with a wide-ranging applicability in detecting cold atoms. Unlike traditional fluorescence and absorption imaging techniques, this approach does not require stochastic photon scattering from trapped atoms, thereby leaving the cold cloud unperturbed. The data can be collected in MHz refresh rate, allowing for near real-time detection. We validated this technique by monitoring the strength of the RDSNS signal by varying the key parameters of the three-dimensional magneto-optical trap (MOT) and the cold atom source (two-dimensional MOT). We performed experiments to probe local densities across the cold atom cloud using our technique and compared it with the inverse Abel-transformed density profiles extracted from the fluorescence of a spherically symmetric atomic cloud and found excellent agreement. This detection protocol is particularly advantageous for probing systems that lack inherent symmetry, where the Abel transformation is inapplicable. Moreover, RDSNS imaging is suitable for extracting the dynamical properties of systems in real time.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.
We describe our approach to atomic magnetometry based on the push-pull optical pumping technique. Cesium vapor is pumped and probed by a resonant laser beam whose circular polarization is modulated synchronously with the spin evolution dynamics induced by a static magnetic field. The magnetometer is operated in a phase-locked loop, and it has an intrinsic sensitivity below 20fT/Hz, using a room temperature paraffin-coated cell. We use the magnetometer to monitor magnetic field fluctuations with a sensitivity of 300fT/Hz.