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arXiv:1303.2312 · Applied Physics Letters

Real-time vector field tracking with a cold-atom magnetometer

N. Behbood, Ferran Martin Ciurana, Giorgio Colangelo, M. Napolitano, M. W. Mitchell, R. J. Sewell

Published Apr 29, 2013• 64 citations
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Abstract

We demonstrate a fast three-axis optical magnetometer using cold, optically trapped 87Rb gas as a sensor. By near-resonant Faraday rotation we record the free-induction decay of a spin polarized ensemble following optical to obtain the three field components and one gradient component. A single measurement achieves shot-noise limited sub-nT sensitivity in 1 ms, with transverse spatial resolution of ∼20 μm. We make a detailed analysis of the shot-noise-limited sensitivity.

DOI
10.1063/1.4803684

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arXiv:1008.1721•Physical Review Letters•Jul 29, 2010•240 citations
We demonstrate a fast three-axis optical magnetometer using cold, optically trapped 87Rb gas as a sensor. By near-resonant Faraday rotation we record the free-induction decay of a spin polarized ensemble following optical to obtain the three field components and one gradient component. A single measurement achieves shot-noise limited sub-nT sensitivity in 1 ms, with transverse spatial resolution of ∼20 μm. We make a detailed analysis of the shot-noise-limited sensitivity.
This paper reports on a fast, nondestructive and sensitive measurement technique that uses atomic spin precession and non-destructive Faraday rotation probing to measure all three components of the magnetic field with kHz bandwidth. In contrast to other vector magnetometer techniques that modulate an applied magnetic field, here the initial atomic polarization is modulated and the magnetic field components are extracted from the resulting free-induction decay (FID) signal, monitored via paramagnetic Faraday rotation. An ensemble of up to 106cold87Rb in the F = 1 hyperfine ground state was used and held in an optical trap as the field sensor.
A miniature wide-range three-axis vector atomic magnetometer under spin-exchange relaxation-free regime has been developed. With a cylindrical vapor cell of size 10 mm in diameter and length, we have constructed a single-beam configuration through a polarization-maintaining fiber carrying the laser beam to the sensor and a multimode fiber guiding the transmitted light field to the photodiode. A rotating field on the plane perpendicular to the direction of propagation of the laser and another modulation field in the pump direction, which are generated in the field programmable gate array using direct digital synthesis to measure the three components of the magnetic field simultaneously. The spin-exchange relaxation is suppressed in zero field controlled by a feedback system, thus a wide range and excellent accuracy of three-axis vector atomic magnetometer response is obtained. Consequently, the transverse magnetic field sensitivity of 350 fT/Hz1/2and longitudinal field sensitivity of 3pT/Hz1/2are achieved with a dynamic range of 60000 nT. This sensor with compact size, wide range and high sensitivity is particularly suitable for geomagnetic observatory and space exploration.
We demonstrate a three-axis atomic magnetometer with one intensity-modulated pump beam and one orthogonal probe beam. The main field component is measured using the resonance of the pumping light, while the transverse field components are measured simultaneously using the optical rotation of the probe beam modulated by the spin precession. It is an all-optical magnetometer without using any modulation field or radio frequency field. Magnetic field sensitivity of 0.8 pT/Hz1∕2 is achieved under a bias field of 2 μT.
(English) This thesis describes the generation of macroscopic spin singlets in a cold atomic ensemble by performing quantum non-demolition measurement. Toward this goal we have implemented a realtime shot-nose limited detection system, incoherent feedback and spin state preparation via optical pumping, an upgraded absorption imaging system and coherent rotation of atomic spin via magnetic field control. Working with a magnetically sensitive atomic system triggered the development of a vector field magnetometry technique. We demonstrate a fast three-axis optical magnetometer using cold, optically trapped 87Rb gas as a sensor. By near-resonant Faraday rotation we record the free-induction decay following optical pumping to obtain the three field components and one gradient component. A single measurement achieves shot-noise limited sub-nT sensitivity in 1 ms, with transverse spatial resolution of about 20 micrometer. We make a detailed analysis of the shot-noise-limited sensitivity. We apply entropy removal by measurement and feedback to a cold atomic spin ensemble. Using quantum non-demolition probing by Faraday rotation measurement, and feedback by weak optical pumping, we drive the initially random collective spin variable F toward the origin F = 0. We use inputoutput relations and ensemble quantum noise models to describe this quantum control process and identify an optimal two-round control procedure. We observe 12 dB of spin noise reduction, or a factor of 63 reduction in phase-space volume. The method offers a non-thermal route to generation of exotic entangled states in ultra-cold gases, including macroscopic singlet states and strongly correlated states of quantum lattice gases. We generate approximate singlet states using the tools of measurementinduced spin squeezing: quantum non-demolition measurement and coherent magnetic rotations . By squeezing all three spin components, we approach the zero spin noise. Using a cold Rubidium atomic ensemble and nearresonant Faraday rotation probing, we have observed up to 3 dB of squeezing relative to the standard quantum limit, and a violation of the generalized spin squeezing inequality by more than 5 standard deviations. (Español) Esta tesis describe la generación del estado singlete de espín macroscópico formado por conjunto de átomos fríos mediante medidas cuánticas no destructivas. Para tal objetivo hemos implementado un sistema de detección en tiempo real limitado por propiedades fundamentales de la luz, "shot noise", así como un sistema de retroalimentación (feedback) incoherente, una preparación del estado de espín mediante bombeo óptico, una mejora del sistema de absorción de imágenes y un control del campo magnético para rotar de forma coherente el espín de los átomos. Trabajar con átomos sensibles al campo magnético desencadenó la necesidad de desarrollar una técnica pare medir el vector del campo magnético. Hemos demostrado un magnetómetro rápido, óptico, tri-axial que utiliza átomos de 87Rb atrapados ópticamente como sensor. Midiendo el decaimiento libre de la inducción (FID) a través de la rotación de Faraday con pulsos casi-resonantes obtenemos las tres componentes del campo así como una componente del gradiente. Una única medida está limitada por shot-noise y sensible en el rango sub-nT en 1 mili segundo, con una resolución espacial transversa de 20 micrómetros. En el cuerpo de la tesis estudiamos con detalle la sensibilidad en el límite shot-noise. Reducimos la entropía en un conjunto de átomos fríos con medidas y feedback. Utilizando medidas cuánticas no destructivas con la rotación de Faraday, y aplicando un leve feedback por bombeo óptico, llevamos la distribución inicial aleatoria de la variable colectiva de espín F hacia el origen de coordenadas F = 0. Utilizando relaciones de entrada-salida y modelos colectivos de ruido cuántico para describir la interacción identificamos como rutina óptima repetir el proceso de medir y feedback dos veces. Experimentalmente observamos 12 dB de reducción del ruido del espín, o una reducción en el espacio de fases de un factor 63. Éste método ofrece un procedimiento no térmico para la generación de estados entrelazados exóticos en gases ultrafríos, incluyendo un estado de singlete macroscópico y estados fuertemente correlacionados en quantum lattice gases. Generamos aproximadamente estados singlete utilizando los recursos que inducen spin squeezing: medidas no destructivas y rotaciones magnéticas coherentes. Al reducir el ruido en las tres componentes del espín, nos acercamos al cero ruido de espín. Utilizando un conjunto de átomos fríos de Rubidio y rotaciones de Faraday casi-resonantes, hemos observado hasta 3 dB de squeezing relativos al límite cuántico estándar, y una violación de la generalización de la desigualdad para spin squeezing de más de 5 desviaciones estándar.
Abstract We introduce a vector atomic magnetometer that employs a fast-rotating magnetic field applied to a pulsed 87Rb scalar atomic magnetometer. This approach enables simultaneous measurements of the total magnetic field and its two polar angles relative to the rotation plane. Operating in gradiometer mode, the magnetometer achieves a total field gradient sensitivity of 35 fT/Hz{{{\rm{fT}}}}/\sqrt{{{{\rm{Hz}}}}}fT/Hz​ fT / Hz (0.7 parts per billion) and angular resolutions of 6 nrad/Hz{{{\rm{nrad}}}}/\sqrt{{{{\rm{Hz}}}}}nrad/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.