# Electromagnetic Induction Imaging with Atomic Magnetometers: Progress and Perspectives
## 4 Electromagnetic Induction Imaging with Atomic Magnetometers
### Development of EMI with AMs and the Quest for Ultimate Sensitivity  
Progress of EMI with atomic magnetometer beyond the proof-of-concept first involved the identification of the AM configurations best suited for the task. While the first proof of concept used a self-oscillating AM [26], further parallel development by the UCL [29] and the Mainz groups [30] was based on the so-called radio-frequency atomic magnetometer [31], a configuration specially devised to measure rf fields, which was found more suitable for the implementation of EMI. In this configuration, shown in Figure 3, atoms contained in a glass cell are polarized via optical pumping with a circularly polarised laser beam propagating collinearly to an applied bias field. Spin precession is driven by an applied rf field, and probed by a linearly polarized laser field propagating perpendicularly to the bias field. The probe beam polarization direction oscillates at the frequency of the rf driving field, with the amplitude and phase of the oscillating polarization measured by a lock-in amplifier. The RF-AM does not require any modification to operate in electromagnetic imaging modality. The sample of interest is introduced in the vicinity of the atomic magnetometer. The coil driving the atomic spin precession in the magnetometer plays now the additional role of inducing eddy currents in the sample, with the amplitude and phase of the resulting secondary field read by the atomic magnetometer. The demonstration of EMI with RF-AMs opened up a series of investigations targeting specific applications, leading also to further technical developments. In the period 2015-2017, the potential of the EMI-AM system for through-barrier imaging was investigated [32; 33]. An important requirement for security is the ability to penetrate conductive barriers which may also be ferromagnetic. In [33], detrimental effects due to the ferromagnetic barriers, including unpredictable magnetic signatures from ferromagnetic screens and variations in the magnetic background, were automatically compensated by active compensation coils controlled by servo loops. Imaging of multiple conductive targets concealed by a 2.5 mm ferromagnetic steel shield and/or a 2.0 mm aluminium shield, in a single scan was reported, thus validating the technique for security applications. In 2018, a proof-of-concept of active underwater detection and tracking was reported with an array of four RF-AMs operating in EMI modality [34]. This demonstrated a detection principle different from the conventional measurement of residual spontaneous magnetisation of the object of interest, and finds direct application in underwater surveillance. In this context, it has the significant advantage over conventional detection systems that it relies on the object conductivity only, and thus degaussing procedures are not an effective countermeasure. Of course, one may also argue that being an active  
Figure 3: Configuration of a radio-frequency atomic magnetometer. system has the disadvantage of potentially revealing the presence of the observer, although this is mitigated by the weakness and narrow band of the rf field used as exciter. The application area which led to the most significant progress in EMI-AM technology is the biomedical one, with mapping of the heart conductivity, identified in [21] as a new approach to the diagnostics of atrial fibrillation, a specific goal. Mapping the conductivity of biological tissue requires extreme sensitivity, so to be able to image structure with volumes of a few ml and conductivity of the order of 1-10 S/m. This stimulated a continuous development of the performances of EMI-AM systems [35; 36]. This rapidly led to the demonstration [37] of imaging of a few mL of calibrated saline solution with conductivity below 1 S/m, as required for biomedical imaging.