  Single atomic defects are prominent windows to look into host quantum states
because collective responses from the host states emerge as localized states
around the defects. Friedel oscillations and Kondo clouds in Fermi liquids are
quintessential examples. However, the situation is quite different for quantum
spin liquid (QSL), an exotic state of matter with fractionalized quasiparticles
and topological order arising from a profound impact of quantum entanglement.
Elucidating the underlying local electronic property has been challenging due
to the charge neutrality of fractionalized quasiparticles and the insulating
nature of QSLs. Here, using spectroscopic-imaging scanning tunneling
microscopy, we report atomically resolved images of monolayer
$\alpha$-RuCl$_3$, the most promising Kitaev QSL candidate, on metallic
substrates. We find quantum interference in the insulator manifesting as
incommensurate and decaying spatial oscillations of the local density of states
around defects with a characteristic bias dependence. The oscillation differs
from any known spatial structures in its nature and does not exist in other
Mott insulators, implying it is an exotic oscillation involved with excitations
unique to $\alpha$-RuCl$_3$. Numerical simulations can reproduce the observed
oscillation by assuming that itinerant Majorana fermions of Kitaev QSL are
scattered across the Majorana Fermi surface. The oscillation provides a new
approach to exploring Kitaev QSLs through the local response against defects
like Friedel oscillations in metals.
