  We report a new kagome quantum spin liquid candidate Cu$_3$Zn(OH)$_6$FBr,
which does not experience any phase transition down to 50 mK, more than three
orders lower than the antiferromagnetic Curie-Weiss temperature ($\sim$ 200 K).
A clear gap opening at low temperature is observed in the uniform spin
susceptibility obtained from $^{19}$F nuclear magnetic resonance measurements.
We observe the characteristic magnetic field dependence of the gap as expected
for fractionalized spin-1/2 spinon excitations. Our experimental results
provide firm evidence for spin fractionalization in a topologically ordered
spin system, resembling charge fractionalization in the fractional quantum Hall
state.
