A superconductor is a material where electric current flows without loss, like water through a perfectly smooth pipe. But if vortices appear, the current starts to 'stumble' over these tiny whirlpools. In graphene twisted at a special angle, such vortices can be created and studied one by one.
Scientists built a device resembling a narrow bridge between two superconducting banks. When a vortex enters a bank, it changes the current through the bridge, and this change is detected like a doorbell. At temperatures above 100 millikelvin, the vortex enters by hopping over the energy barrier: like a ball that needs to be thrown over a fence. Below 90 millikelvin, it passes through the barrier like a ghost through a wall. This is called quantum tunneling.
Measurements showed that the barrier for vortex entry has a height of a few kelvin and a thickness of about 100 nanometers, roughly one-tenth of the device's width. It's as if a ball were flying over a fence as tall as a house but as thin as a sheet of paper.
Such precision opens the way to controlling superconductors at the level of individual vortices. It will be possible to switch superconductivity on and off at specific points, creating new types of quantum devices — from magnetic field sensors to elements of quantum computers.
A temperature of 100 millikelvin is just one-tenth of a degree above absolute zero. At such temperatures, even ordinary materials become as brittle as glass.