  Inducing Cooper pairing in a thin ferromagnetic topological insulator in the
quantum anomalous Hall state (called quantum anomalous Hall insulator, QAHI) is
a promising way to realize topological superconductivity with associated chiral
Majorana edge states. However, finding evidence of superconducting proximity
effect in a QAHI has proven to be a considerable challenge due to inherent
experimental difficulties. Here we report the observation of crossed Andreev
reflection (CAR) across a narrow superconducting Nb electrode contacting the
chiral edge state of a QAHI, evinced by a negative nonlocal voltage measured
downstream from the grounded Nb electrode. This is an unambiguous signature of
induced superconducting pair correlation in the chiral edge state. Our
theoretical analysis demonstrates that CAR processes of the chiral edge are not
strongly dependent on the nature of the superconductivity that mediates them.
Nevertheless, the characteristic length of the CAR process is found to be much
longer than the superconducting correlation length in Nb, which suggests that
the CAR is in fact mediated by superconductivity induced on the QAHI surface.
The approach and results presented here provide a foundation for future studies
of topological superconductivity and Majorana physics, as well as for the
search for non-Abelian zero modes.
