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arXiv:0907.2241 · Physical Review Letters

High Bandwidth Atomic Magnetometery with Continuous Quantum Nondemolition Measurements

Vishal Shah, Georgios Vasilakis, Michael V. Romalis

Published Jan 5, 2010• 150 citations
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Abstract

We describe an experimental study of spin-projection noise in a high sensitivity alkali-metal magnetometer. We demonstrate a fourfold improvement in the measurement bandwidth of the magnetometer using continuous quantum nondemolition measurements. Operating in the scalar mode with a measurement volume of 2 cm3 we achieve magnetic field sensitivity of 22 fT/Hz(1/2) and a bandwidth of 1.9 kHz with a spin polarization of only 1%. Our experimental arrangement is naturally backaction evading and can be used to realize sub-fT sensitivity with a highly polarized spin-squeezed atomic vapor.

DOI
10.1103/physrevlett.104.013601

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We study the effect of optical polarization squeezing on the performance of a sensitive, quantum-noise-limited optically pumped magnetometer. We use Bell-Bloom (BB) optical pumping to excite a ^{87}Rb vapor containing 8.2×10^{12} atoms/cm^{3} and Faraday rotation to detect spin precession. The sub-pT/sqrt[Hz] sensitivity is limited by spin projection noise (photon shot noise) at low (high) frequencies. Probe polarization squeezing both improves high-frequency sensitivity and increases measurement bandwidth, with no loss of sensitivity at any frequency, a direct demonstration of the evasion of measurement backaction noise. We provide a model for the quantum noise dynamics of the BB magnetometer, including spin projection noise, probe polarization noise, and measurement backaction effects. The theory shows how polarization squeezing reduces optical noise, while measurement backaction due to the accompanying ellipticity antisqueezing is shunted into the unmeasured spin component. The method is compatible with high-density and multipass techniques that reach extreme sensitivity.
Here we demonstrate an atomic magnetometer that has a bandwidth of more than 100 kHz and a sensitivity of $180\phantom{\rule{0.2em}{0ex}}\mathrm{fT/}\ensuremath{\surd}\mathrm{Hz}$ at a Fourier frequency of 8 Hz, and $0.7\phantom{\rule{0.2em}{0ex}}\mathrm{nT/}\ensuremath{\surd}\mathrm{Hz}$ at 100 kHz. These sensitivity measurements are achieved at geophysically useful magnetic field magnitudes (approximately $50\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{T}$ inside a three-layer $\ensuremath{\mu}$-metal shield) and are limited by the photon shot noise for frequencies above 8 Hz. This device is based on a nonlinear magneto-optical rotation sensor that is operated with an active feedback to the pump modulation frequency. We present a theoretical description of the response functions of the components in the magnetometer. We show that the range of operation of atomic magnetometers can thus be expanded beyond the conventional high-sensitivity, low-bandwidth domain, to provide a high-linearity, high-bandwidth, and high-sensitivity sensor.
We demonstrate a scalar Rb87 optical magnetometer that retains magnetic-field sensitivities below 10pT/√Hzover 3 dB bandwidths of 10 kHz in an ambient field Bo=11.4 μT and using a measurement volume of 1 mm3. The magnetometer operates at high atomic densities where both the sensitivity and the bandwidth are limited by spin-exchange collisions between the alkali atoms. By operating in this regime, our measurements show that the bandwidth of the magnetometer can be increased without a significant degradation in its sensitivity.
We present an experimental and theoretical study of a scalar atomic magnetometer using an oscillating field-driven Zeeman resonance in a high-density optically pumped potassium vapor. We describe an experimental implementation of an atomic gradiometer with a noise level below $10\text{ }\text{fT}\text{ }{\text{Hz}}^{\ensuremath{-}1/2}$, fractional field sensitivity below ${10}^{\ensuremath{-}9}\text{ }{\text{Hz}}^{\ensuremath{-}1/2}$, and an active measurement volume of about 1.5 cm31.5\text{ }{\text{cm}}^{3}1.5 cm3. We show that the fundamental field sensitivity of a scalar magnetometer is determined by the rate of alkali-metal spin-exchange collisions even though the resonance linewidth can be made much smaller than the spin-exchange rate by pumping most atoms into a stretched spin state.
Spin noise sets fundamental limits to the precision of measurements using spin-polarized atomic vapors, such as performed with sensitive atomic magnetometers. Spin squeezing offers the possibility to extend the measurement precision beyond the standard quantum limit of uncorrelated atoms. Contrary to current understanding, we show that, even in the presence of spin relaxation, spin squeezing can lead to a significant reduction of spin noise, and hence an increase in magnetometric sensitivity, for a long measurement time. This is the case when correlated spin relaxation due to binary alkali-atom collisions dominates independently acting decoherence processes, a situation realized in thermal high atom-density magnetometers and clocks.