Stellarators & Qubits in Garching

The Garching Research Center near Munich yesterday staged one of its infrequent open door nights, with all the various institutes showing off their work in lectures and demonstrations. The campus is big and I arrived rather late, but I did make it to the two most interesting places: the Max Planck Institute for Plasma Physics and the Walther Meißner Institute for Low Temperature Research.

The IPP had a ball lightning demonstration which I sadly couldn’t get a ticket for, so instead I admired the chunks of actual fusion (research) reactors they had on exhibition. While ITER and other projects with a Tokamak design get the most publicity, the IPP simultaneously pursues the distinctive Stellarator design. As always with fusion research, these are in the experimental stage with actual energy production likely still decades off.

In a stellarator the magnetic cage is produced with a single coil system – without a longitudinal net-current in the plasma and hence without a transformer. This makes stellarators suitable for continuous operation, whereas tokamaks without auxiliary facilities operate in pulsed mode. Dispensing with the ring-shaped plasma current means, however, abandoning the axial symmetry present in tokamaks. As the helical twisting of field lines is achieved solely with external coils, the later have to be twisted accordingly: the magnet coils and plasma have a complicated shape.

The ILTR had its obligatory liquid nitrogen show going on. Cooled to −195.79 °C (77 K), nitrogen becomes an easily handled liquid that produces a number of interesting effects. Pouring some into a copper spoon causes the water in the surrounding air to condense as ice – but only on the upper part of the spoon, away from the liquid nitrogen. The spoon’s bottom is coated by another liquid which is actually liquid air, i.e. the surrounding oxygen and nitrogen liquefying due to the extreme cold conducted by the copper. Moreover, liquid nitrogen enables high-temperature superconductivity for some amusing magnetic experiments, similar to the ones shown here.

The ILTR is also into superconducting quantum circuits and had a live demonstration of measuring the decaying state oscillations of a qubit system. This research takes place at the ILTR because of the extreme temperatures required to enable quantum computing based on superconduction. We heard that the technology is in principle ready for use, except for scale. Reliable error correction in particular requires a large overhead of circuits, about eighty for every qubit of information if I recall correctly. Still, it’s a safe bet that quantum computing will be industrially viable long before fusion power plants – unfortunately, as we need the latter much more urgently than the former.

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