Physical bounds on electromagnetic invisibility and the potential of superconducting cloaks

The possibility of suppressing the scattering cross section of an object is subject to fundamental physical bounds imposed by causality and passivity. Global cloaking limitations have been recently derived, which imply that any linear, causal and passive cloak necessarily increases the global scattering, integrated over the whole electromagnetic spectrum, compared to the uncloaked object. Here, we expand on this topic, discussing in detail an interesting exception to this limit represented by cloaks with static diamagnetism. In this context, we explore the potential of superconducting materials to realize global and local reduction of the scattering cross section. The concepts of plasmonic and mantle cloaking are extended to superconductors, realizing strong and tunable invisibility, with some unique properties stemming from the peculiar electrodynamics of superconductors. We conclude by qualitatively discussing a possible method to derive more stringent local bounds on cloaking.