A narrow corridor forces people to walk in rows. In graphene, a similar corridor was made for electrons: two gates with a gap of 75 nanometers create a constriction, and a third gate changes its width. Electrons in this corridor fly without scattering, and the number of rows, quantum modes, can be changed like the width of a door. Each new row adds a step to the conductance.

Aluminum, which becomes a superconductor at very low temperatures, was attached to the corridor. At the interface, Andreev reflection occurs: an electron approaching the superconductor turns into a hole that flies back. This amplifies the current: the conductance at each step increases by about a quarter. But as the voltage increases, superconductivity is destroyed, and the switching current also changes in steps, following the number of open modes. Each new mode is like a switch: it turns on the amplification but brings the superconducting switch-off threshold closer. Previously, it was unknown how these rows affect superconductivity, but now it's clear: they control both amplification and switch-off.