  Fractionalized excitations develop in many unusual many-body states such as
quantum spin liquids, disordered phases that cannot be described using any
local order parameter. Because these exotic excitations correspond to emergent
degrees of freedom, how to probe them and establish their existence is a
long-standing challenge. We present a general procedure to reveal the
fractionalized excitations using real-space entanglement entropy in critical
spin liquids that are particularly relevant to experiments. Moreover, we show
how to use the entanglement entropy to construct the corresponding spinon Fermi
surface. Our work defines a new pathway to establish and characterize exotic
excitations in novel quantum phases of matter.
