  One-dimensional topological superconductors host Majorana zero modes (MZMs),
the non-local property of which could be exploited for quantum computing
applications. Spin- polarized scanning tunneling microscopy measurements show
that MZMs realized in self- assembled Fe chains on the surface of Pb have a
spin polarization that exceeds that due to the magnetism of these chains. This
feature, captured by our model calculations, is a direct consequence of the
nonlocality of the Hilbert space of MZMs emerging from a topological band
structure. Our study establishes spin polarization measurements as a diagnostic
tool to uniquely distinguish topological MZMs from trivial in-gap states of a
superconductor.
