TOPOLOGICAL QUANTUM COMPUTING

Phys. Rev. Lett., 2019, doi: 10.1103/PhysRevLett.122.147701

Majorana zero modes in the hybrid semiconductor-superconductor nanowire is one of the promising candidates for topological quantum computing. Recently, in nanowires with a superconducting island, the signature of Majorana zero modes can be revealed as a subgap state whose energy oscillates around zero in magnetic field. This oscillation was interpreted as overlapping Majoranas. However, the oscillation amplitude either dies away after an overshoot or decays, sharply opposite to the theoretically predicted enhanced oscillations for Majorana bound states, as the magnetic field increases. Several theoretical studies have tried to address this discrepancy, but are partially successful. This discrepancy has raised the concerns on the conclusive identification of Majorana bound states, and has even endangered the scheme of Majorana qubits based on the nanowires.

Recently, this problem has been solved by a group led by Professor Hai-Zhou Lu and Professor X. C. Xie. They consider a non-uniform spin-orbit coupling strength along the nanowire, e.g. with a steplike distribution. The steplike spin-orbit coupling is reasonable because the gates apply a nonuniform electrostatic potential and spin-orbit coupling depends on the electrostatic fields perpendicular to the nanowire. Moreover, the presence of the superconductor can also modify the electrostatic field in the nanowire due to screening effect and work-function mismatch between the superconductor and semiconductor. Thus the spin-orbit coupling is well expected to be nonuniform from the nanowire covered with superconductor to the part (tunnel barrier region) without the superconductor. Additionally, they found that these decaying oscillations caused by the steplike spin-orbit coupling are ubiquitous, also present for trivial Andreev bound states. Though many previous works have addressed non-uniform chemical potential/pairing in nanowires, this work is the first to point out the crucial role of non-uniform spin-orbit coupling and its direct link to the decaying subgap state oscillations.

Dong Pan (Institute of Semiconductors, CAS, Beijing, China)

doi: 10.1088/1674-4926/40/5/050203