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International Conference on Photonics and Optical Engineering

An atomic magnetic gradiometer with 1/τ uncertainty property based on the self-sustaining method

Qin Zhao, Boling Fan, Shiguang Wang, Lijun Wang

Published Jan 24, 2019• 0 citations
Publisher

Abstract

The magnetic field is one of the most important physical observables with about all electromagnetic information. Larmor precession atomic magnetometers based on optical pumping experienced considerable attention recently. However, the sensitivity of this kind of atomic magnetometers is limited by their spin coherence time and the systematic noise. Here, we propose a self-sustaining atomic magnetic gradiometer that shows a superior 1/τ behavior in the magnetic field uncertainty measurement over time. The gradiometer is implemented by mixing and filtering the different frequency signals from two adjacent spin self-sustaining magnetometers with a baseline equal to 1 cm. The common-mode noise is suppressed and a gradient sensitivity of 200 fT/(√Hz=cm) is realized, being close to the shot noise limit.

DOI
10.1117/12.2521032

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The magnetic field is one of the most important physical observables with about all electromagnetic information. Larmor precession atomic magnetometers based on optical pumping experienced considerable attention recently. However, the sensitivity of this kind of atomic magnetometers is limited by their spin coherence time and the systematic noise. Here, we propose a self-sustaining atomic magnetic gradiometer that shows a superior 1/τ behavior in the magnetic field uncertainty measurement over time. The gradiometer is implemented by mixing and filtering the different frequency signals from two adjacent spin self-sustaining magnetometers with a baseline equal to 1 cm. The common-mode noise is suppressed and a gradient sensitivity of 200 fT/(√Hz=cm) is realized, being close to the shot noise limit.
We demonstrate an atomic magnetic gradiometer based on self-sustaining Larmor precession. By coherent optical pumping, we measure the phase of the Larmor precession directly and observe that the gradiometer shows a 1/τ improvement in magnetic field gradient uncertainty over time τ. Since the measurement gives frequency signals, the gradiometer can be easily implemented by mixing and filtering the different frequency signals from two adjacent magnetometers. A gradient sensitivity of 186 fT/Hz/cm−1) is realized, which is close to the shot-noise limit. In a noisy environment, the gradiometer can still maintain its 1/τ behavior by suppressing 90% of the common-mode noise. This method should be widely applicable to the measurement of magnetic field gradients owing to its simplicity and outstanding performance.
Quantum measurement using coherent superposition of intrinsic atomic states has the advantage of being absolute measurement and can form metrological standards. One example is the absolute measurement of magnetic field by monitoring the Larmor precession of atomic spins whilst another being the Ramsey type atomic clock. Yet, in almost all coherent quantum measurement, the precision is limited by the coherence time beyond which, the uncertainty decreases only as τ(-1/2). Here we show that by non-destructively measuring the phase of the Larmor precession and regenerating the coherence via optical pumping, the self-sustaining Larmor precession signal can persist indefinitely. Consequently, the precision of the magnetometer increases with time following a much faster τ(-1) rule. A mean sensitivity of 240 from 1 Hz to 10 Hz is realized, being close to the shot noise level. This method of coherence regeneration may also find important applications in improving the performance of atomic clocks.
The spin self-sustaining atomic magnetometer has the advantage of 1/τ measurement and great development potential in many applications. In this paper, we investigated the main elements that affect the stability and accuracy of the self-sustaining magnetometer and proposed the methods to improve its performance based on the measurement results. The correlation coefficient between fluctuations of the magnetic field generated by coils and the spin Larmor precession frequency is 0.97, which mainly dominates the stability in a short term. The accuracy of the magnetometer is affected by the power and frequency of the pump light. The Larmor precession frequency coefficient related to the pump light power is 26 mHz/mW, and the effect on the Larmor precession frequency is minimized when the pump light frequency is red detuned by 200 MHz from the 85Rb transition D1 line F = 3 to F′ = 3. The 1/τ measurement time after these corrections can be extended to 10 s, and the sensitivity achieved is 149 fT/Hz, which is close to the quantum projection noise limit of the system.
We report on the development of a microfabricated atomic magnetic gradiometer based on optical spectroscopy of alkali atoms in the vapor phase. The gradiometer, which operates in the spin-exchange relaxation free regime, has a length of 60 mm and cross sectional diameter of 12 mm, and consists of two chip-scale atomic magnetometers which are interrogated by a common laser light. The sensor can measure differences in magnetic fields, over a 20 mm baseline, of 10 fT/[Formula: see text] at frequencies above 20 Hz. The maximum rejection of magnetic field noise is 1000 at 10 Hz. By use of a set of compensation coils wrapped around the sensor, we also measure the sensor sensitivity at several external bias field strengths up to 150 mG. This device is useful for applications that require both sensitive gradient field information and high common-mode noise cancellation.
Abstract In the last few decades, the atomic magnetometry [1–7] has experienced remarkable development and reached to an outstanding sensitivity. This exciting technology is finding applications in various of fields [8–18]. The last two obstacles to further improve the sensitivity of magnetometry are quantum fluctuations and ambient magnetic field. For magnetometry based on optical readout of atomic ensemble’s spin precession, the fundamental limitation of sensitivity is restricted by spin projection noise (SPN) and photon shot noise (PSN). Meanwhile, in practical applications, ambient magnetic field noise which is regarded as a common-mode magnetic-field (CMM) noise is also limiting the sensitivity. Several techniques for reducing PSN [19–25], SPN [26, 27], and CMM noise [7, 28–31] have been proposed, respectively. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. For this purpose, here we demonstrate a sub-shot-noise magnetic gradiometer utilizing entangled optical detection with quantum-correlated twin beams. This leads to the simultaneous suppression of PSN and CMM noise. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the standard quantum limit (SQL). The sensitivity of gradient magnetic field reaches 18 fT/cm√Hz at 20 Hz with these sophisticated techniques. Our study opens up new possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications with noisy and physically demanding environments.
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.