Majorana zero modes and topological superconductors

7 problems

• Chiral Majorana edge modes in anomalous Hall superconductor hybrids

  Open

  A quantum anomalous Hall film (a magnetic film with a quantized Hall current at zero field) covered by a superconductor was predicted to carry one-directional Majorana edge channels; a 2017 report of their signature was retracted, and they remain unobserved.

• Majorana protection scaling in quantum-dot Kitaev chains

  Partially resolved

  Chains of quantum dots joined by superconductors can be tuned to copy a model wire proposed in 2001 by Alexei Kitaev (a theoretical physicist) that has Majorana modes at its ends, and whether Majorana protection grows quickly with the number of dots decides their usefulness.

• Non-Abelian exchange or fusion of Majorana zero modes

  Open

  The decisive test of Majorana modes is that exchanging or combining them changes a quantum state in a way ordinary particles cannot, and no platform has shown this.

• Has a semiconductor nanowire reached the topological superconducting phase

  Contested

  Many experiments on superconductor-coated semiconductor wires saw signals expected from Majorana modes, but ordinary bound states caused by disorder give similar signals, so it is unproven that any wire has become topological.

• Origin of the parity signal in Majorana interferometer devices

  Contested

  Microsoft reported reading whether a nanowire holds an even or odd number of electrons through a flux-sensitive capacitance signal, and it is disputed whether this signal comes from Majorana modes or ordinary states.

• Are end states of magnetic atom chains on superconductors Majorana modes

  Contested

  Chains of iron atoms on superconducting lead show zero-energy states at their ends that were proposed as Majorana modes, but ordinary bound states can look similar, and the identification is unsettled.

• Are vortex-core zero modes in FeTeSe Majorana modes

  Contested

  Microscope measurements on the iron-based superconductor FeTe0.55Se0.45 find zero-energy states in some magnetic vortices but not all, and whether they are Majorana modes is debated.