  The coherent transport of charge and spin is one key requirement of future
devices for quantum computing and communication. Scattering at defects or
impurities may seriously reduce the coherence of quantum-mechanical states,
thereby affecting device functionality. While numerous methods exist to
experimentally assess charge transport, the real-space detection of a
material's spin transport properties with nanometer resolution remains a
challenge. Here we report on a novel approach which utilizes a combination of
spin-polarized scanning tunneling microscopy (SP-STM) and the recently
introduced molecular nanoprobe (MONA) technique. It relies on the local
injection of spin-polarized charge carriers from a magnetic STM tip and their
detection by a single surface-deposited phthalocyanine molecule via reversible
electron-induced tautomerization events. Based on the particular electronic
structure of the Rashba alloy BiAg$_2$ which is governed by a
spin-momentum-locked surface state, we proof that the current direction
inverses as the tip magnetization is reversed by an external field. In a
proof-of-principle experiment we apply SP-MONA to investigate how a single Gd
cluster influences the spin-dependent charge transport of the Rashba surface
alloy.
