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Review of Scientific Instruments

An actively compensated 8 nT-level magnetic shielding system for 10-m atom interferometer

Yu-Hang Ji, Lin Zhou, Sitong Yan, Chuan He, Chao Zhou, Sachin Barthwal, Feng Yang, Wei-Tao Duan, Wendong Zhang, Rundong Xu, Qi Ming Wang, Dongxu Li, Jia‐Hong Gao, Xi Chen, Jin Wang, Mingsheng Zhan

Published Aug 1, 2021• 6 citations
Publisher

Abstract

We design and develop a high-performance magnetic shielding system for a long baseline fountain-type atom interferometer. The shielding system is achieved by a combination of passive shielding using permalloy and active compensation with coils. An 11.4 m-long three-layer cylindrical shield is completed by the process of welding, local annealing, and entire annealing. The active compensations compress the residual magnetic field to 8.0 nT max-to-min and the corresponding gradient below 30 nT/m over 10 m along the axial direction in which external compensation, internal compensation, and constant magnetic field (C-field) compensation reduce the inhomogeneities to 25.0, 12.6, and 1.7 nT (standard deviation) sequentially. We estimate that this system could reduce the systematic error of the quadratic Zeeman shift to the 10−13 level for the weak equivalence principle test with a simultaneous 85Rb–87Rb dual-species atom interferometer.

DOI
10.1063/5.0053971

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We design and develop a high-performance magnetic shielding system for a long baseline fountain-type atom interferometer. The shielding system is achieved by a combination of passive shielding using permalloy and active compensation with coils. An 11.4 m-long three-layer cylindrical shield is completed by the process of welding, local annealing, and entire annealing. The active compensations compress the residual magnetic field to 8.0 nT max-to-min and the corresponding gradient below 30 nT/m over 10 m along the axial direction in which external compensation, internal compensation, and constant magnetic field (C-field) compensation reduce the inhomogeneities to 25.0, 12.6, and 1.7 nT (standard deviation) sequentially. We estimate that this system could reduce the systematic error of the quadratic Zeeman shift to the 10−13 level for the weak equivalence principle test with a simultaneous 85Rb–87Rb dual-species atom interferometer.
Abstract Quantum sensors based on cold atoms are being developed which produce measurements of unprecedented accuracy. Due to shifts in atomic energy levels, quantum sensors often have stringent requirements on their internal magnetic field environment. Typically, background magnetic fields are attenuated using high permeability magnetic shielding, with the cancelling of residual and introduction of quantisation fields implemented with coils inside the shield. The high permeability shield, however, distorts all magnetic fields, including those generated inside the sensor. Here, we demonstrate a solution by designing multiple coils overlaid on a 3D-printed former to generate three uniform and three constant linear gradient magnetic fields inside the capped cylindrical magnetic shield of a cold atom interferometer. The fields are characterised in-situ and match their desired forms to high accuracy. For example, the uniform transverse field, Bx, deviates by less than 0.2% over more than 40% of the length of the shield. We also map the field directly using the cold atoms and investigate the potential of the coil system to reduce bias from the quadratic Zeeman effect. This coil design technology enables targeted field compensation over large spatial volumes and has the potential to reduce systematic shifts and noise in numerous cold atom systems.
We report on the design, construction, and characterization of a 10 m-long high-performance magnetic shield for very long baseline atom interferometry. We achieve residual fields below 4 nT and longitudinal inhomogeneities below 2.5 nT/m over 8 m along the longitudinal direction. Our modular design can be extended to longer baselines without compromising the shielding performance. Such a setup constrains biases associated with magnetic field gradients to the sub-pm/s2 level in atomic matterwave accelerometry with rubidium atoms and paves the way toward tests of the universality of free fall with atomic test masses beyond the 10−13 level.
The accurate control of magnetic fields is a cornerstone of multiple emerging quantum technologies. These technologies often require passive high permeability magnetic shielding and internal active field-generating coils to create their own bespoke magnetic field landscape. However, magnetic fields generated by coils are distorted by high permeability shielding, preventing the accurate and efficient generation of the desired field environment. Here, we design a cylindrical four-layer magnetic shield with an interior hybrid active-passive coil system that is explicitly optimised to include the electromagnetic coupling between the active and passive components. We use a combination of analytical methods and numerical simulations to determine the shield parameters - geometry, thickness, and access hole positions - to maximise the passive shielding efficiency and minimise the shield-induced Johnson noise and weight. Then, we apply an analytical formulation of the magnetic field, which accounts for the interaction with the magnetic shield, to design nine orthogonal hybrid active-passive field-generating coils inside the shield. The coils will be manufactured on thin low-via flex-PCBs near the shield's interior surface and generate three uniform fields and six gradient fields that deviate by less than 0.4% and 2%, respectively, over an internal cylindrical region extending over half the diameter and length of the innermost shield layer. These hybrid active-passive coils can accurately remove deviations in the background field or generate various complex magnetic field landscapes. Consequently, the hybrid shield provides an ideal platform for miniaturising and commercialising quantum technologies that require precisely-controlled magnetic fields within a low-noise environment.
The spin-exchange-relaxation-free (SERF) atomic magnetometer is an ultra-high sensitivity magnetometer, but it must be operated in a magnetic field with strength less than about 10 nT. Magnetic field compensation is an effective way to shield the magnetic field, and this paper demonstrates an in situ triaxial magnetic field compensation system for operating the SERF atomic magnetometer. The proposed hardware is based on optical pumping, which uses some part of the SERF atomic magnetometer itself, and the compensation method is implemented by analyzing the dynamics of the atomic spin. The experimental setup for this compensation system is described, and with this configuration, a residual magnetic field of strength less than 2 nT (±0.38 nT in the x axis, ±0.43 nT in the y axis, and ±1.62 nT in the z axis) has been achieved after compensation. The SERF atomic magnetometer was then used to verify that the residual triaxial magnetic fields were coincident with what were achieved by the compensation system.
Cold-atom quantum memories require a near-zero magnetic environment (residual field <1 microtesla) to mitigate Zeeman decoherence. Conventional three-axis active compensation suffers from inter-coil crosstalk and geomagnetic drift, which degrade the uniformity and long-term stability of the zero-field region. To address this, we propose a spatiotemporally cooperative feedback-controlled suppression architecture for magnetic interference. An integrally printed three-dimensional orthogonal coil frame mechanically suppresses inter-axis flux leakage. A spatial feedforward inverse matrix is introduced to algorithmically eliminate first-order cross-coupling. A dual-threshold hysteresis incremental-PI controller filters magnetometer quantization fluctuations and avoids frequent small jitter adjustments of coil driving currents. Hardware ablation experiments, long-duration stability tests and spatial magnetic-field mapping were conducted. Experimental results show that during a 714.4 s continuous steady-hold stage, the mean and maximum residual fields at the central point reach 0.112 microtesla and 0.224 microtesla, respectively. The overlapping Allan deviations of all three components remain below 5 nT for averaging times of 1.9-178.2 s. Based on differential-ferrofluid lattice-scan measurements combined with centre-constrained quadratic vector reconstruction and soft Maxwell regularization, a weak-field spatial distribution with an RMS variation of 5.65 microtesla is fitted within a spherical region of interest of radius 5 mm, where 67.5% of the model volume meets the sub-5 microtesla coverage criterion. For a prolate ellipsoidal region matching the atomic-cloud distribution with full axis lengths of 10, 5, and 5 mm, the upper field bound within 93.7% of the volume is 2.68 microtesla.
When the cold atom clock operates in microgravity around the near-earth orbit, its performance will be affected by the fluctuation of magnetic field. A strategy is proposed to suppress the fluctuation of magnetic field by additional coils, whose current is changed accordingly to compensate the magnetic fluctuation by the linear and incremental compensation. The flight model of the cold atom clock is tested in a simulated orbital magnetic environment and the magnetic field fluctuation in the Ramsey cavity is reduced from 17 nT to 2 nT, which implied the uncertainty due to the second order Zeeman shift is reduced to be less than 2 × 10 −16 . In addition, utilizing the compensation, the magnetic field in the trapping zone can be suppressed from 7.5 μT to less than 0.3 μT to meet the magnetic field requirement of polarization gradients cooling of atoms.