  Correlation is a fundamental statistical measure of order in interacting
quantum systems. In solids, electron correlations govern a diverse array of
material classes and phenomena such as heavy fermion compounds, Hunds metals,
high-Tc superconductors, and the Kondo effect. Spin-spin correlations, notably
investigated by Kaufman and Onsager in the 1940s 6, are at the foundation of
numerous theoretical models but are challenging to measure experimentally.
Reciprocal space methods can map correlations, but at the single atom limit new
experimental probes are needed. Here, we determine the correlations between a
strongly hybridized spin impurity and its electron bath by varying the coupling
to a second magnetic impurity in the junction of a scanning tunneling
microscope. Electronic transport through these coupled spins reveals an
asymmetry in the differential conductance reminiscent of spin-polarized
transport in a magnetic field. We show that at zero field, this asymmetry can
be controlled by the coupling strength and is directly related to either
ferromagnetic (FM) or antiferromagnetic (AFM) spin-spin correlations.
