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arXiv:1403.7796 · Review of Scientific Instruments

Shot-noise-limited magnetometer with sub-picotesla sensitivity at room temperature

Vito Giovanni Lucivero, Pawel Anielski, Wojciech Gawlik, Morgan W. Mitchell

Published Nov 1, 2014• 48 citations
arXivHTMLPDF

Abstract

We report a photon shot-noise-limited (SNL) optical magnetometer based on amplitude modulated optical rotation using a room-temperature (85)Rb vapor in a cell with anti-relaxation coating. The instrument achieves a room-temperature sensitivity of 70 fT / √Hz at 7.6 μT. Experimental scaling of noise with optical power, in agreement with theoretical predictions, confirms the SNL behaviour from 5 μT to 75 μT. The combination of best-in-class sensitivity and SNL operation makes the system a promising candidate for application of squeezed light to a state-of-the-art atomic sensor.

DOI
10.1063/1.4901588

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We report a photon shot-noise-limited (SNL) optical magnetometer based on amplitude modulated optical rotation using a room-temperature (85)Rb vapor in a cell with anti-relaxation coating. The instrument achieves a room-temperature sensitivity of 70 fT / √Hz at 7.6 μT. Experimental scaling of noise with optical power, in agreement with theoretical predictions, confirms the SNL behaviour from 5 μT to 75 μT. The combination of best-in-class sensitivity and SNL operation makes the system a promising candidate for application of squeezed light to a state-of-the-art atomic sensor.
We report on an optical magnetometer enhanced by vacuum-squeezed light, employing an Mx magnetometer based on 87^{87}87Rb vapor in a micrometer-scale cell (~100 μμμm). Using the well-established polarization self-rotation effect in a room-temperature 87^{87}87Rb vapor cell, we achieve -3 dB of vacuum squeezing within the noise spectral window of 100 Hz to several MHz, corresponding to 3.5 dB squeezing when accounting for optical losses. Leveraging this level of squeezing, we demonstrate a magnetic field sensitivity of approx. 1 pT///Hz\sqrt{Hz}Hz​. The combination of vacuum-squeezed light and micrometer-scale vapor cells paves the way for compact, low-power-consumption atomic sensors with enhanced performance.
We present a single-beam atomic magnetometer operating at room temperature for the measurement of ac magnetic fields. The magnetometer functions in the nonlinear regime of magneto-optical rotation of 85Rb{}^{85}\mathrm{Rb}85Rb atomic vapor without any buffer gases. We demonstrate a sensitivity of approximately 0.9pT/Hz0.9\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}0.9pT/Hz​ at 2 kHz and a large bandwidth of 24 kHz. The dynamic range of measurement is 106{10}^{6}106, making the sensor effective even in Earth's field. We present the signal-to-noise and bandwidth characteristics of the system for both shielded and unshielded modes of operation. Moreover, we perform theoretical analysis for the atom-light system for the single-laser-beam configuration. The effect of light intensity and detuning on the magnetometer is studied theoretically as well as experimentally to understand the strengths and limitations of the technique.
We describe a room-temperature alkali-metal atomic magnetometer for detection of small, high-frequency magnetic fields. The magnetometer operates by detecting optical rotation due to the precession of an aligned ground state in the presence of a small oscillating magnetic field. The resonance frequency of the magnetometer can be adjusted to any desired value by tuning the bias magnetic field. Based on experimentally measured signal-to-noise ratio, we demonstrate a sensitivity of 100pG∕Hz100\phantom{\rule{0.3em}{0ex}}\mathrm{pG}∕\sqrt{\mathrm{Hz}}100pG∕Hz​ (rms) in a $3.5\text{\ensuremath{-}}\mathrm{cm}$-diameter paraffin coated cell. Assuming detection at the photon shot-noise limit, we project a sensitivity as low as 25pG∕Hz25\phantom{\rule{0.3em}{0ex}}\mathrm{pG}∕\sqrt{\mathrm{Hz}}25pG∕Hz​ (rms).
We study the effect of optical squeezing on the performance of a sensitive, quantum-noise-limited optically-pumped magnetometer. We use Bell-Bloom optical pumping to excite a 87^{87}87Rb vapor and Faraday rotation to detect spin precession. The sub-pT/Hz\mathrm{pT}/\sqrt{\mathrm{Hz}}pT/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 signal bandwidth. The accompanying polarization anti-squeezing perturbs only an unmeasured spin component, so there is no loss of sensitivity at any frequency. The method is compatible with high-density and multi-pass techniques that reach extreme sensitivity.
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 consider the limitations due to noise (e.g., quantum projection noise and photon shot-noise) on the sensitivity of an idealized atomic magnetometer that utilizes spin squeezing induced by a continuous quantum nondemolition measurement. Such a magnetometer measures spin precession of N atomic spins by detecting optical rotation of far-detuned light. We show that for very short measurement times, the optimal sensitivity scales as N-3/4; if strongly squeezed probe light is used, the Heisenberg limit of N-1 scaling can be achieved. However, if the measurement time exceeds tau(rel)/N-1/2 in the former case, or tau(rel)/N in the latter, where tau(rel) is the spin relaxation time, the scaling becomes N-1/2, as for a standard shot-noise-limited magnetometer.