Quantum Spin Liquid In antiferromagnets, the lowest-energy state is reached when neighboring spins on the underlying lattice point in opposite directions. Because of geometric constraints on some lattices (such as the two-dimensional triangular lattice), this magnetic ordering cannot be achieved even at temperatures close to absolute zero, and these compounds are predicted to be in a quantum spin liquid state. Yamashita et al. (p. 1246 ) measured the thermal conductivity of a recently identified candidate quantum spin liquid, the organic compound EtMe ₃ Sb[Pd(dmit) ₂ ] ₂ , and characterized its lowest-lying excitations. Two types of excitations were observed: ballistically propagating gapless excitations and excitations associated with a finite spin gap. These results contribute to our understanding of this unusual state of matter, which is potentially relevant to other two-dimensional quantum systems.