Introducing the magnetic hose

Just as copper wires transport electric currents, so a new device designed and built by physicists in Spain and Germany can transmit magnetic fields over arbitrarily long distances. The "magnetic hose", consisting of a ferromagnet wrapped in a superconductor, could be used to create a variety of circuits and suggests a new way of addressing qubits inside a quantum computer, say the researchers.In our hi-tech world we make great use of the fact that electromagnetic waves can be transmitted vast distances, just as electricity can. But the same is not true for static electric and magnetic fields, the magnitudes of which decay rapidly with distance. The furthest that magnetic fields have been transmitted is a few metres, such as inside the cores of transformers.

To see if they could do any better, Alvaro Sanchez and two colleagues at the Autonomous University of Barcelona looked to transformation optics. This relatively new technique involves altering the trajectory of electromagnetic waves in unusual ways by mathematically transforming the waves' constituent electric and magnetic fields, as has famously been done to create "invisibility cloaks" that can shield objects at certain wavelengths. The aim of the Barcelona group, having got together with Ignacio Cirac and Oriol Romero-Isart at the Max Planck Institute for Quantum Optics in Garching near Munich, was to apply transformation optics to static fields in order to couple two quantum systems magnetically.

The researchers' first step was to model an infinitely wide slab of material with infinite magnetic permeability along its thicknes  and zero magnetic permeability along its width. They found, as they had hoped, that any magnetic field at the slab's lower surface would simply be shifted to its upper surface. Next they investigated whether or not the properties of this idealized system could be approximated in a real object. They found that a cylindrical piece of the same material, with a finite diameter, would do the job almost as well. But that still left the problem of mimicking the material's extreme anisotropy.

Looking ahead, they believe the new device could be used to manipulate quantum information at the nanoscale, for example via the isolated spins of defects in small crystals of diamond known as nitrogen vacancies. To function as bits inside a quantum computer, these spins should be independently addressable with magnetic fields, something that could be achieved with nano-sized magnetic hoses, they say. Such nano hoses, they add, might also be used to couple the spins.