Experimental realization of a 3D magnetic cloak and a magnetic wormhole

Abstract

Metamaterials have made possible many breakthroughs like electromagnetic cloaks or perfect lenses. In an interesting line of research, metamaterials have been used to simulate celestial objects that were thought to be impossible to replicate at the human scale. For example, the analogous of optical black holes has been theoretically and experimentally realized based on metamaterials. One of the most interesting celestial objects is a wormhole. In cosmology, a wormhole can be understood as a cosmic object that can connect two distant regions of the universe. Constructing such an object would require large amounts of negative gravitational energy, which obviously prevents its realization in a lab. A seminal work by Greenleaf et al. [1] presented a theoretical proposal for designing a wormhole that could work for for electromagnetic waves. Such an electromagnetic wormhole could allow electromagnetic wave propagation between two points in space through an invisible tunnel. However, the recipe given by transformation optics to construct such an object required the use of bulk metamaterials with complicated permeability and permittivity parameters. Here we present the design and experimental realization of a wormhole for the static magnetic field. It consist of a tunnel for transferring magnetic field lines from one point in space to another through a path that is magnetically undetectable. We make use of the unique properties that magnetostatic metamaterials offer in terms of magnetic field manipulation. First, we use a magnetic hose made of a ferromagnetic cylinder to guide to field of a magnetic source through a desired path to another location. And, second, we design a magnetic cloak that can surround the whole structure, thus rendering it magnetically undetectable. This latter requirement is an important feat in itself, since we need to construct an actual 3D spherical magnetic cloak to operate passively in free space. This is achieved by a careful combination of a superconducting shell and a ferromagnetic metasurface. The theory and numerical simulations are experimentally confirmed by constructing an actual device using soft ferromagnets and high-temperature superconductors, demonstrating all the required properties for the magnetic wormhole.