  The isolated susceptibility $\chi_{\rm I}$ may be defined as a
(non-thermodynamic) average over the canonical ensemble, but while it has often
been discussed in the literature, it has not been clearly measured. Here, we
demonstrate an unambiguous measurement of $\chi_{\rm I}$ at avoided
nuclear-electronic level crossings in a dilute spin ice system, containing
well-separated holmium ions. We show that $\chi_{\rm I}$ quantifies the
superposition of quasi-classical spin states at these points, and is a direct
measure of state concurrence and populations.

Quantum Physics;Other Condensed Matter;Strongly Correlated Electrons