THE QUANTUM INTELLIGENCER

A non-invasive method of reading out the ground-state parity of a Majorana system has been proposed. The authors pass a supercurrent through the low-energy modes of a topological system of magnetic adatoms on a superconductor, and report that changes in parity, appearing as zero-energy level crossings, register as discontinuities in the critical current, the threshold at which the superconducting link gives way. They extend the scheme to a configuration in which a single control adatom, holding a Yu-Shiba-Rusinov bound state, mediates the Majorana coupling, and find the effect holds at finite temperatures and across different tunnelling regimes. The method is offered as non-invasive, and therefore as a reading instrument for a system whose manipulation has so far been the greater part of the difficulty. It is a proposal; the authors describe no apparatus built and no measurement performed. Before the proposal can be judged, its terminology must be set in order. A Majorana zero mode is not a particle in the sense of an electron. It is a zero-energy excitation, a state that lives at a boundary in a superconducting system and is its own antiparticle: pair one with itself and the mathematics allow it to annihilate. The property that interests computation is that a pair of such modes stores information not in the position of a particle but in the parity of the system, that is, whether the number of electrons in the ground state is even or odd. Read the parity and you have read the bit. The difficulty has always been doing so without reaching into the system hard enough to destroy the thing being read. The route proposed here runs through adatoms: single atoms set down one at a time on a superconductor by a probe. Adatoms are a deliberate arrangement, and the authors note the technique as attractive because it permits precise atomic placement and some control over disorder, the disorder being the usual ruin of such schemes. Where two Majorana modes must be made to interact, the proposal places a single control adatom between them. That adatom hosts a Yu-Shiba-Rusinov state, named for the three theorists who predicted it: a bound state formed when a magnetic impurity sits on a superconductor, an impurity not absorbed into the lattice but intruding upon it well enough to hold a localised quantum state within the superconducting gap. The observable at the end of the arrangement is the critical current. A supercurrent is a current that flows without resistance; the critical current is the greatest current the system will sustain. Push past it and the superconducting link gives way. The authors' claim is that the system's parity announces itself at that threshold: when a zero-energy level crossing changes the ground state parity, the critical current registers a characteristic discontinuity. Read the jump and you have read the parity. —Inspector Grey Dispatch from The Prepared E0