Superconducting tube and cast iron reshape a simple coil's field into a nearly uniform one

The authors call the calculation exact. Fabian Resare, a Ph.D. student in quantum technology at Chalmers and a co-author, said: "Using combinations of coils, it is possible to approximate a desired magnetic field to high precision, but with magnetic omniconversion the field generated is mathematically exact." ^[9] The experiment reports a more ordinary result. Measurements inside the device closely matched the researchers' calculations, which the team describes as a proof of concept ^[7].

Witlef Wieczorek, professor of quantum technology at Chalmers and another co-author, said: "This is a universal method that removes previous limitations on how accurately an existing magnetic field can be transformed into another one." ^[12] The demonstration behind that claim is one conversion: a simple coil's nonuniform field into an almost perfectly uniform one ^[5]. Uniform and quadrupolar fields are the two shapes the source gives as examples of what applications need ^[11], and the summary describes a built device only for the uniform case.

The prototype was a superconducting aluminum tube with cast-iron components, made in a departmental mechanical workshop ^[6]. The measurement was taken with the device cooled below 1 kelvin ^[7]. The summary does not give the size of the remaining deviation from the calculation, or say how the method behaves above that temperature.

The assigning idea, that one simple magnet could replace a bespoke design for each application, goes further than the evidence. In our view the custom work moves rather than disappears. The source field can be an ordinary coil or permanent magnet ^[1], but the ferromagnetic and superconducting pieces have to be arranged in a carefully calculated geometry to fix how the field is distributed ^[4]. The old route is described as designing optimized magnets or large coil systems for each application ^[3], and as approximation ^[9]. The new route is a calculation followed by a build.

That could still be a real saving for groups that need an exact field. The team has filed a patent application ^[8]. Wieczorek said: "We hope that our magnetic omniconversion approach will contribute to the development of advanced instrumentation for quantum technologies and other areas, where precise spatial control of magnetic fields is essential." ^[10]