  We propose an experimental method utilizing a strongly spin-orbit coupled
metal to quantum magnet bilayer that will probe quantum magnets lacking long
range magnetic order, e.g., quantum spin liquids, via examination of the
voltage noise spectrum in the metal layer. The bilayer is held in thermal and
chemical equilibrium, and spin fluctuations arising across the single interface
are converted into voltage fluctuations in the metal as a result of the inverse
spin Hall effect. We elucidate the theoretical workings of the proposed bilayer
system, and provide precise predictions for the frequency characteristics of
the enhancement to the ac electrical resistance measured in the metal layer for
three candidate quantum spin liquid models. Application to the Heisenberg
spin-$1/2$ kagom{\'e} lattice model should allow for the extraction of any
spinon gap present. A quantum spin liquid consisting of fermionic spinons
coupled to a $U(1)$ gauge field should cause subdominant $\W^{4/3}$ scaling of
the resistance of the coupled metal. Finally, if the magnet is well-captured by
the Kitaev model in the gapless spin liquid phase, then the proposed bilayer
can extract the two-flux gap which arises in spite of the gapless spectrum of
the fermions. We therefore show that spectral analysis of the ac resistance in
the metal in a single interface, equilibrium bilayer can test the relevance of
a quantum spin liquid model to a given candidate material.
