We heat the cells up to a temperature of 150 ☌, elevating the vapor pressure, such that the alkali equilibrium density is n = 10 14 cm −3. Each channel comprises a 1 cm 3 Pyrex vapor cell with Rubidium and approximately 200 Torr of nitrogen and neon buffer gas. 1) are spaced at a distance of L = 4 cm apart. In our experiments, the common-mode noise produced by these large sets of coils was measured to be approximately 5 fT / Hz at the location of the magnetometers. This reduces the current noise requirement by 20 dB. In practice, we null the field of one of the channels using the large set of coils and then minimize the residual fields in the other channel (≤5 nT) with its set of local coils. We compensate the majority of this DC field with large coils wound near the walls of the room, such that noise associated with these shimming currents is common-mode to a large degree. A 50 nT DC field, for example, requires a 150 dB Hz signal to noise ratio to reach 1 fT/ Hz. One potential source of magnetic noise is the noise on the currents creating the compensation fields. The MSR has a residual DC field on the order of 50 nT which we compensate for using active cancellation via a large ( D ≈ 3 m) set of 3-axis “common” coils wrapped on the perimeter of the MSR and sets of small ( D ≈ 4 cm) “local” coils wound around each individual magnetometer. 21 As a result, we work in a (two-layer mu-metal, 1-layer aluminum) magnetically shielded room (MSR). 19,20 SERF conditions require working in a near-zero field environment. We have developed a diffusion-mode, two-beam SERF magnetometer array configured as a gradiometer for use in fetal magnetocardiography, fMCG.
Some relevant questions to address are how best to configure the sensors, what CMRR is sufficient, and how to compare different gradiometer implementations. Our goal is to efficiently isolate the fMCG signal from the maternal background. IV, we consider the problem of detecting fetal magneto-cardiography (fMCG) in the presence of the large maternal MCG signal.
III, we report the development of a Spin Exchange Relaxation Free (SERF) gradiometer with performance metrics comparable to SQUIDs. This metric will be useful for comparing different implementations of gradiometers when applying the sensors to a particular measurement problem. It is a function of the CMRR, the gradiometer baseline, as well as the geometric scaling of the signal and the dominant background fields. II, we introduce a figure of merit for evaluating gradiometer performance. In this paper, we consider the challenge of performing such a subtraction and recommend procedures for calibrating and characterizing AM gradiometers. Most AM gradiometer implementations, this work included, involve subtracting magnetic field measurements from adjacent sensors in post processing. However, configuring AMs as gradiometers poses a number of challenges and is an active area of work. They are relatively inexpensive to operate and have significantly lower setup and operating costs due to their working temperatures and smaller shielding volume requirements. Unlike SQUIDs, atomic magnetometers (AMs) require no cryogens.
Typical values of SQUID gradiometer balance are about one part in 100, setting the upper bound of the common-mode suppression at that level. 6,7 The SQUID gradiometer CMRR is limited by the degree to which the areas and alignments of these (often hand-wound) coils are matched. Presently, the highest performing magnetic gradiometer implementations involve inductively coupling the magnetic field of interest to low Tc superconducting quantum interference devices, SQUIDs via counter-wound pickup coils. In practice, however, differences in sensors lead to a finite CMRR ξ.
For identical sensors in this situation ( M 1 = M 2, B 1 c = B 2 c), we expect a CMRR of infinity resulting in vanishing sensitivity to the difference signal. Where B ic is the field measured by sensor i and M i is the transfer function for each sensor.