Electrons in superconductors form Cooper pairs that are theoretically predicted to break (depair) at a certain current. However it's hard to observe because the other effects break superconductivity before you reach that current. They managed to circumvent this by sending the current in picosecond pulses, which is the novel part, and confirm the prediction. They did it for two types of superconductors with different mechanisms of action: classic low-temperature superconductors and modern high-temperature (YBCO).