Benchtop Magnetic Shielding for Benchmarking Atomic Magnetometers
Peter J. Hobson, Niall Holmes, Prashant Patel, Ben Styles, J. Alan Chalmers, C. T. Morley, Alister Davis, Michael Packer, Thomas X. Smith, Sintija Raudonyte, Darragh Holmes, Robert Harrison, David Woolger, Dominic Sims, Matthew J. Brookes, Richard Bowtell, T. M. Fromhold
Abstract
Here, a benchtop hybrid magnetic shield containing four mumetal cylinders and nine internal flexible printed circuit boards is designed, constructed, tested, and operated. The shield is designed specifically as a test-bed for building and operating ultra-sensitive quantum magnetometers. The geometry and spacing of the mumetal cylinders are optimized to maximize shielding efficiency while maintaining Johnson noise < 15 fT/ √ Hz. Experimental measurements at the shield’s center show passive shielding efficiency of (1.0 ± 0.1)×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for a 0.2 Hz oscillating field applied along the shield’s axis. The nine flexible printed circuit boards generate three uniform fields, which all deviate from perfect uniformity by ≤ 0.5% along 50% of the inner shield axis, and five linear field gradients and one second-order gradient, which all deviate by ≤ 4% from perfect linearity and curvature, respectively, over measured target regions. Together, the target field amplitudes are adjusted to minimize the remnant static field along 40% of the inner shield axis, as mapped using an atomic magnetometer. In this region, the active null reduces the norm of the magnitudes of the three uniform fields and six gradients by factors of 19.5 and 19.8, respectively, thereby reducing the total static field from 1.68 nT to 0.23 nT.