  Triangular lattice of rare-earth ions with interacting effective spin-$1/2$
local moments is an ideal platform to explore the physics of quantum spin
liquids (QSLs) in the presence of strong spin-orbit coupling, crystal electric
fields, and geometrical frustration. The Yb delafossites, NaYbCh$_2$ (Ch=O, S,
Se) with Yb ions forming a perfect triangular lattice, have been suggested to
be candidates for QSLs. Previous thermodynamics, nuclear magnetic resonance,
and muon spin rotation measurements on NaYbCh$_2$ have supported the suggestion
of the QSL ground states. The key signature of a QSL, the spin excitation
continuum, arising from the spin quantum number fractionalization, has not been
observed. Here we perform both elastic and inelastic neutron scattering
measurements as well as detailed thermodynamic measurements on high-quality
single-crystalline NaYbSe$_2$ samples to confirm the absence of long-range
magnetic order down to 40 mK, and further reveal a clear signature of magnetic
excitation continuum extending from 0.1 to 2.5 meV. The comparison between the
structure of the magnetic excitation spectra and the theoretical expectation
from the spinon continuum suggests that the ground state of NaYbSe$_2$ is a QSL
with a spinon Fermi surface.
