Active radio-frequency imaging in the super-low and ultra-low frequency (SLF/ULF) bands makes complex demands of the transceiver system used. Conventional antennae trade size for sensitivity at low frequencies, which is incompatible with high-resolution imaging. Penetrative imaging at these frequencies is an important enabling technique for nuclear threat reduction, treaty monitoring and border security.

Quantum magnetic sensors break the size-weight-sensitivity constraint for magnetic detection in the SLF/ULF bands, as the magnetic signal is transduced by resonant detection on polarised alkali ground state Zeeman transitions, generating magneto-optical rotation. Signal generation by this process is free of the inverse-frequency scaling which degrades inductive measurement at low frequencies.

To realise these benefits in penetrative imaging, it is essential to separate with very high discrimination (part-per-million or better) the excitation field from the signal response. Magnetic gradiometry, utilising alkali spin maser techniques in a unique microfabricated caesium cell, is under development at the University of Strathclyde. The technique under development targets very high common-mode noise rejection (CMNR) by cancellation of common-mode systematics at source, ensuring the bandwidth, uniformity and linearity required for high CMNR.
By developing this device for penetrative SLF/ULF imaging, the requirements of AWE’s applications will be embedded from the outset, maximising impact in this important set of end uses. It is also important to note the value of high CMNR, high-sensitivity magnetometry in a range of healthcare and biomedical applications.