Six Electrons Tunnel as One
Many quantum technologies rely on devices called Josephson junctions, in which electrons tunnel without resistance through a barrier separating superconducting terminals. If there are two terminals, the electrons must be transmitted as individual pairs, known as Cooper pairs. But if the barrier separates three or more terminals, scientists predict that multiple Cooper pairs can tunnel at once. Now Miriam Rike Ebert at the University of Konstanz in Germany and her colleagues have analyzed a hypothetical scenario in which three Cooper pairs tunnel simultaneously in a four-terminal junction [1]. Such a process is important because it could facilitate new functionalities in quantum technologies.
In the considered scenario, three Cooper pairs enter the barrier of a four-terminal junction from the same terminal and then separate so that each leaves through a different terminal. Using theoretical methods, the researchers studied this junction’s so-called current–phase relation, which describes how the current flowing across the barrier depends on the phase difference between the wave functions of a given pair of terminals. The team found that the tunneling process would leave an observable imprint in this current–phase relation, assuming that the barrier were sufficiently transparent to Cooper pairs.
Ebert and her colleagues also showed that their process could induce interactions between so-called Andreev bound states―localized electronic states that form at the boundary between a terminal and the barrier. Andreev bound states have potential uses in quantum information processing, particularly when their interactions cause them to become “hybridized,” as Ebert and her colleagues predict for this multiterminal system. Lastly, the researchers argue that their process might have already been indirectly observed in recent experiments on four-terminal junctions realized in hybrid superconductor–semiconductor structures.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- M. R. Ebert et al., “Sextets in four-terminal Josephson junctions,” Phys. Rev. B 112, 195430 (2025).