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Atomic Magnetometry for the Detection of Cardio-magnetic Fields

Alexander Fay

Published Jan 1, 2020• 0 citations
OpenAlex

Abstract

We demonstrate a method of measuring small constant gradients on top of a large constant background magnetic field using Electromagnetically Induced Transparency (EIT). The Earth provides a constant magnetic field of 25-50 μT, and as such, measuring much smaller magnetic fields as well as smaller gradients presents a challenge often requiring special shielding. We show that by making use of common mode noise subtraction from a dual rail setup, our measurement is insensitive to these large fields, and in theory our method does not require shielding. Our dual rail setup allows us to measure small magnetic field gradients by utilizing EIT resonances in 87Rb. In order to detect a cardio-magnetic field gradient, a magnetometer must be sensitive to gradients on the order of 10-100 pT. Our setup has been found to be sensitive to magnetic field gradients as low as 400 pT, and has the necessary bandwidth to acquire gradient fields at the rate necessary to detect features of the heart's magnetic field. We also discuss potential future work that could be done to remove the necessity of shielding and produce greater sensitivity.

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•Jun 14, 2024•0 citations
We demonstrate a method of measuring small constant gradients on top of a large constant background magnetic field using Electromagnetically Induced Transparency (EIT). The Earth provides a constant magnetic field of 25-50 μT, and as such, measuring much smaller magnetic fields as well as smaller gradients presents a challenge often requiring special shielding. We show that by making use of common mode noise subtraction from a dual rail setup, our measurement is insensitive to these large fields, and in theory our method does not require shielding. Our dual rail setup allows us to measure small magnetic field gradients by utilizing EIT resonances in 87Rb. In order to detect a cardio-magnetic field gradient, a magnetometer must be sensitive to gradients on the order of 10-100 pT. Our setup has been found to be sensitive to magnetic field gradients as low as 400 pT, and has the necessary bandwidth to acquire gradient fields at the rate necessary to detect features of the heart's magnetic field. We also discuss potential future work that could be done to remove the necessity of shielding and produce greater sensitivity.
We demonstrate a new method for measuring magnetic gradients in the presence of large constant magnetic fields using electromagnetically induced transparency in rubidium vapor. As the Earth provides a near-constant magnetic field of 25-50 µT, most measurements of small magnetic fields involve subtracting or shielding the Earth's field. Our method can measure small gradients in the presence of large magnetic fields using Electromagnetically Induced Transparency in 87Rb. By comparing the resonances of two laser beams in a dual-rail arrangement propagating through rubidium vapor in a spatially changing magnetic field, we are able to measure a constant gradient of ≥7 nT/cm and a time-varying gradient of ≥50 pT/cm, over a constant magnetic field of 35 µT.
Magnetic sensors are widely used in many applications including medical diagnostics, geomagnetic survey, etc. Scalar atomic magnetometers that have demonstrated sensitivity at and below fT/ Hz^1/2 level can be adapted to include measurement of magnetic field orientation but usually at the cost of accuracy and with limitation of dynamic range. Magnetometry using electromagnetically induced transparency (EIT) instead was used to provide both scalar accuracy and vector information with angle resolution at a level of 10-3 deg/Hz^1/2. We present a scheme for EIT vector magnetic field measurement with the common-mode drift suppression by probing multiple EIT resonances. The demonstrated sensitivity is stable to < 200 pT/ Hz^1/2 at 1 mHz.
A new design of compact zero-field atomic gradiometer was proposed and integrated using alkali vapor cell and customized optical components. This gradiometer used two parallel elliptically polarized lights, whose output intensity was measured to perform gradient magnetic field detection, respectively. To improve the gradiometer’s sensitivity of the magnetic field detection, the gradiometer was operated in the spin-exchange relaxation-free (SERF) regime. For the sensitivity study, the gradiometer was placed in a person-sized four-layer μ-metal magnetic shielding. With the magnetic shield closed up, the sensitivity of each channel was near 34 fT/ √ Hz, and the corresponding gradient sensitivity could reach 14 fT/√ Hz/cm on a 1cm baseline. When the cover of magnetic shielding was removed, the magnetometer sensitivity of single channel was about 90 fT/√ Hz, and the corresponding gradient sensitivity could reach 34 fT/√ Hz/cm. The experimental results implied that in a poor magnetic shielding environment, the performance of magnetometer was limited due to the fluctuations of the environmental magnetic field, while the gradiometer could work well.
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.
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.
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.