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Tiny batteries for superconductivity

The current in any lightbulb flows due to a difference in voltage, to overcome the electrical resistance. But not in superconductors, where the current doesn’t experience any resistance. Superconductive currents require a phase difference to flow, which so far required energy to create. Scientist from QuTech at Delft University built a so called Phi0 Josephson Junction, that has a phase difference at default. It can therefor function as a tiny battery to store superconducting currents. The scientist published their work on 2 May in Nature Physics.

Their research might help to operate large arrays of superconducting quantum bits, potential candidates to use in a future quantum computer. It is also another step towards the definite proof of Majorana fermions, an elusive particle for which the first signs where seen in QuTech in 2012, that form another potential qubit candidate. The conditions required for a phi0 Josephson Junction are identical to those needed for Majorana’s to appear.

“A Josephson Junction consists of two superconductors, connected by a bridge, for instance a nanowire made of a non-superconducting material such as a semiconductors”, PhD student Daniel Szombati explains. Already since the ‘60s of last century, scientists know that Josephson Junctions can be used to manipulate superconducting currents. “They have the special property that the phase drop, which controls the current flow (a feature of the superconducting material) can be adjusted between the ends: just like for a hydro plant where the dam controls the water level drop or a galvanic battery with a potential difference between its electrodes, the Josephson junction serves as a barrier for superconducting phase. This phase can be manipulated with strong magnetic fields, but this requires a lot of energy. Our φ0-Josephson Junction is special, because it has a default phase difference. To stop the current, you need an electrical field”.

Effectively, Szombati and his colleagues in the group of prof. Leo Kouwenhoven, have created a small superconducting battery, that can store a tiny amount of superconductive current.

The physics of the φ0-Josephson Junction is intimately linked to symmetry breaking.

Symmetries play an important role in physics as it imposes rules on a system’s behavior: for example, a ball sitting on a flat surface has rotational symmetry. From the ball’s perspective, all directions look the same, therefore the ball will not roll anywhere. This symmetry can be broken: as soon as the surface is tilted, the ball will roll down the angle of steepest incline. Standard Josephson junctions all possess a certain symmetry, called chiral (Greek work for ‘handedness’) symmetry: when unperturbed, they cannot distinguish between left or right, implying no current can flow, as a flowing current would have to pick a preferred direction. “The φ0-Josephson Junction is different”, Szombati says. “Prof. Erik Bakkers and his team from TU Eindhoven have provided us with a novel semiconductor, in which we found that superconductivity behaves quite differently from usual when an external magnetic field is applied: the symmetry between left and right is lifted, and supercurrent flows without a phase difference: much like if we had a river flowing in a certain direction even though the water bed is completely flat. This is a fundamental breakthrough”.

φ0-Josephon Junctions are also a step towards quantum bits based on Majorana fermions. Signs of this elusive particle, predicted in the 1930’s by Ettore Majorana, where first seen in the lab of Leo Kouwenhoven. Research is now focusing on creating Majorana fermions and performing operations with them that can only be explained with so called ‘non-abelian statistics’. “If you exchange a property of two particles, and then change them back, you are back at the beginning.

Except with Majorana’s which retain a memory of the interaction. A read-out from this memory would be the firm and definitive proof that Majorana fermions not only exist, but also could be used as quantum bit“, Kouwenhoven explains. “They are promising, because as Majorana’s hardly interact with their environment they should be much better shielded from decoherence. On the downside: there weak interactions also make them a lot harder to detect “.

The condition met in φ0-Josephson Junctions are identical to those predicted for Majorana’s to appear. To be able to detect them, scientists are now working on finer measurements. The research is performed at QuTech with support from FOM/NWO and Microsoft Project Q.

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