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Introducing the magnetic hose

01 May 2013
Pipeline for magnetic fields

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.

Coupling quantum systems

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 thickness (height) 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.

Magnet and superconductor

The answer, they found, was to build up cylinders using concentric rings of a ferromagnet and a superconductor. They calculated that 20 such layers would transmit more than 90% of a magnetic field from the cylinder’s base to its top, and showed that even with just two layers – a ferromagnetic core and a superconducting shell – such a device could transport as much of 75% of the field. The researchers then went on to show how such a cylinder could be extended and shaped to create closed circuits.

John Pendry of Imperial College in London, who pioneered transformation optics, describes the scheme as “a rather novel idea”. He points out that ferromagnets conduct magnetic fields much worse than most metals conduct electricity, but says that the latest work shows they can become efficient magnetic conductors when enclosed inside a superconductor. “A superconductor expels all magnetic fields and can therefore be regarded as a perfect magnetic insulator, keeping the fields bottled up and preventing them from spilling all over the place,” he explains.

Theorists in the lab

Such is the simplicity of the bilayer device that the Spanish/German collaboration, made up entirely of theorists, built one in the laboratory. But being theorists, they did not have fantastic equipment at their disposal, and had to make do with a bismuth-based high-temperature superconductor that was shorter than the cobalt–iron ferromagnet that they used. To get round this problem they measured the magnetic field strength, because of the presence of a direct-current coil at one end, at various points along the length of the cylinder rather than at the cylinder’s opposite end.

The researchers found that, as expected, the field was, on average, smaller than it was with either no device at all or with just the ferromagnet. Crucially, they also observed that with the full magnetic hose in place, the field strength spiked about halfway along the cylinder, and they noticed that there was a barely visible crack in the superconductor at that point. They took this as convincing evidence that the field was being efficiently guided by the device.

‘Very important work’

Tie Jun Cui, an electrical engineer at Southeast University in China, is enthusiastic about the latest research. He describes it as “very important work”, agreeing that the magnetic hose would be analogous to a metal wire. “Rather than electric circuits, this proposal would give us magnetic circuits,” he says.

Sanchez and colleagues explain that the static field transmitted by the hose can be regarded as having an infinite wavelength and that as a result the field can be used to carry out tasks on any length scale – be it 20 m or 20 nm. 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.

A preprint describing the research appears on the arXiv server.

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