A state that can appear at the ends of certain wires, or in the whirlpools of a superconductor, and that is its own antimatter twin. Two of them together make up one ordinary electron state split in half and kept in two separate places, so no disturbance at either place alone can read or destroy what it holds – which is why they are pursued for quantum computing.
Going deeper
A Majorana mode is a zero-energy state that is its own antiparticle, appearing at the ends of a topological superconductor. Two of them make up one ordinary state whose occupation is stored non-locally – which is why they are attractive for quantum computing, and why local measurements are poor evidence.
Half a state at each end
An ordinary electron state can be written formally as a combination of two Majorana components. Usually that decomposition is a mathematical exercise, because the two pieces sit on top of each other. In a superconductor they can separate: one at each end of a wire, or one in each vortex core, at exactly zero energy.
The pair then defines a single fermionic state that may be occupied or empty, with the information shared between two distant places. Nothing acting locally at one end can tell which it is, which is the protection. Exchanging such modes transforms the state in a way that depends on the order of the exchange – non-Abelian statistics – and this is what makes them a candidate for quantum operations that are protected by topology rather than by error correction alone.
Recipes
The ingredients are a superconductor, strong and broken . The canonical proposal puts an ordinary superconductor on the surface of a , where the plus induced pairing produce an effective spinless superconductor and vortices bind Majorana modes. The -wire version uses a nanowire with strong spin–orbit coupling in a magnetic field, proximitised by a superconductor; magnetic atom chains on superconductors are a third route.
materials are attractive here because all the ingredients exist as layers that can be stacked with clean interfaces: superconducting NbSe2, magnetic CrI3 or Fe3GeTe2, topological WTe2 and Bi2Se3. The promise is designing the recipe layer by layer instead of relying on one compound to have everything.
Why the evidence is hard
The usual signature is a peak in tunnelling conductance at exactly zero bias, ideally quantised at 2e2/h. But a zero-bias peak is not specific: , Andreev bound states, smooth potentials at the contact and the effect all produce one, and quantisation can be mimicked. Several prominent claims have been retracted or reinterpreted, and the field has become correspondingly careful.
Stronger evidence requires more than one probe: peaks at both ends that appear and disappear together, the expected dependence on magnetic field and chemical potential, a gap that closes and reopens at the transition, and ultimately non-local measurements or interference experiments that test the shared, non-local nature of the state. Demonstrating braiding – exchanging two modes and reading the result – remains the goal, and has not been achieved.
For specialists
A zero-energy state equal to its own conjugate, appearing at defects of a topological superconductor – wire ends, vortex cores, domain walls. A pair defines one non-local fermionic state, so exchanging them realises non-Abelian statistics and topologically protected operations. Recipes combine strong spin–orbit coupling, magnetism and superconductivity, which is what makes attractive; proximity-induced pairing on a topological surface is the canonical route, and a zero-bias conductance peak on its own is not proof.