  The Kitaev quantum spin liquid epitomizes an entangled topological state, for
which two flavors of fractionalized low-energy excitations are predicted: the
itinerant Majorana fermion and the Z2 gauge flux. Detection of these
excitations remains challenging, because of their fractional quantum numbers
and non-locality. It was proposed recently that fingerprints of fractional
excitations are encoded in the phonon spectra of Kitaev quantum spin liquids
through a novel fractional-excitation-phonon coupling. Here, we uncover this
effect in $\alpha$-RuCl3 using inelastic X-ray scattering with meV resolution.
At high temperature, we discover interlaced optical phonons intercepting a
transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature,
the optical phonons display a large intensity enhancement near the Kitaev
energy, JK~8 meV, that coincides with a giant acoustic phonon softening near
the Z2 gauge flux energy scale. This fractional excitation induced phonon
anomalies uncover the key ingredient of the quantum thermal Hall effect in
$\alpha$-RuCl3 and demonstrates a proof-of-principle method to detect
fractional excitations in topological quantum materials.
