The particle that is its own enemy
In 1937, the brilliant and reclusive Italian physicist Ettore Majorana proposed a strange new class of particle: a fermion that acts as its own antiparticle. Unlike an electron, which is annihilated in a burst of energy when it meets its antimatter twin, the positron, a Majorana fermion would be its own doppelgänger. For decades this remained a theoretical curiosity. Particle accelerators searched for them without success. Then, in 2012, a team at the Kavli Institute of Nanoscience at Delft University of Technology (TU Delft) announced they had found the signature of Majorana particles not as free-roaming entities, but as quasiparticles (emergent phenomena) at the ends of a tiny, specially constructed wire.
The experiment, led by Professor Leo Kouwenhoven, did not find a fundamental particle. Instead, it created the specific conditions under which the collective behavior of electrons mimics the properties of a Majorana fermion. The team fabricated a device from an indium antimonide (InSb) semiconductor nanowire. InSb is a material with strong spin-orbit coupling, a property required for the experiment. The nanowire was connected to a normal electrical contact (gold) at one end and a superconductor (niobium titanium nitride) at the other. When the device was cooled to cryogenic temperatures just above absolute zero and subjected to a magnetic field of around 100 millitesla, the team observed a peculiar electrical signal.
A peak at zero
The signal that caused the excitement was a zero-bias conductance peak. This means that a sharp spike in electrical conductance appeared exactly at zero voltage. Under normal circumstances, voltage is required to make a current flow, but the presence of a pair of Majorana "zero modes" at either end of the nanowire provides a path for electrons to travel with no energy cost. This zero-bias peak remained stubbornly locked at zero voltage even as the researchers varied the magnetic field and gate voltages. The results, published in the journal Science, were hailed as the first strong evidence for the existence of Majorana bound states.
Subsequent research has shown that other physical effects, such as quantum dots or material disorder, can sometimes mimic this zero-bias peak, making definitive proof a complex challenge. A later, more sensational claim of a quantized Majorana signal from the same lab in 2018 was retracted in 2021 due to inconsistencies in the data analysis. The original 2012 experiment is a foundational piece of evidence in the ongoing global effort to create and control Majorana zero modes. These exotic quasiparticles are of intense interest because their physical properties could make them ideal building blocks for fault-tolerant quantum computers, which are exceptionally stable and resistant to environmental noise.