  A quantum spin liquid (QSL) is an exotic state in which electron spins are
highly entangled, yet keep fluctuating even at zero temperature. Experimental
realization of model QSLs has been challenging due to imperfections, such as
antisite disorder, strain, and extra or a lack of interactions in real
materials compared to the model Hamiltonian. Here we report the magnetic
susceptibility, thermodynamic, inelastic neutron scattering (INS), and
muon-spin relaxation studies on a polycrystalline sample of
PrZnAl$_{11}$O$_{19}$, where the Pr$^{3+}$ ions form an ideal two-dimensional
triangular lattice. Our results demonstrate that this system does not order nor
freeze, but keep fluctuating down to 50 mK despite large antiferromagnetic
couplings ($\sim$ -10 K). Furthermore, the INS and specific-heat data suggest
that PrZnAl$_{11}$O$_{19}$ is best described as a gapless QSL.
