To spot their quarry, Kouwenhoven's group created specially designed transistors. In standard transistors, applying a voltage to a metal electrode called a gate turns on the flow of current through a semiconductor between two other metal electrodes. Previous theoretical predictions suggested that if one of the secondary electrodes was a superconductor, and the current was allowed to flow through a special semiconductor nanowire under a magnetic field, the combination would force electrons in the nanowire to behave collectively as if Majorana fermions were present at opposite ends of the wire. Theory further offered that if researchers tried to send an electric current from the normal electrode to the superconducting electrode without the magnetic field turned on, the electrons trying to make the journey would essentially bounce off the superconductor, so no current would be detected at the superconducting electrode. But if the magnetic field is turned on, this would trigger the presence of Majorana fermions, which would enable electrons to enter the superconductor, and that would produce a jump in the current.
This current spike is what Kouwenhoven's team found. When the researchers then removed any one of the conditions needed to induce Majorana fermions—such as the magnetic field, or replacing the superconducting electrode with another metal electrode—the current spike at the second electrode vanished.
The results don't provide a direct detection of Majorana fermions. But the Dutch team did a "very compelling" job of eliminating all other possible explanations, says Jason Alicea, a theoretical physicist at the University of California, Irvine. However, the study doesn't completely nail the case for the presence of Majorana fermions, he cautions. The current spike is only 5% of what theory predicts. But that may be because the equipment used to chill the experimental setup must be improved to get closer to absolute zero, where the signal for Majoranas should be the strongest.