Quantum spin liquids are new states of matter that are characterized by long-range entanglement and support exotic excitations. Unlike traditional magnetic ground states, the spin liquid state does not break conventional symmetries. After 40 years since the original proposal for such a state by Anderson, these systems have only recently been realized in experiments. The compound Herbertsmithite is a leading candidate for having a quantum spin liquid ground state. Inelastic neutron scattering measurements on single crystals reveal that the spin excitations are fractionalized, a remarkable first. More recent studies of the low-energy excitations with NMR and neutron techniques reveal evidence for a spin-gap consistent with theoretical calculations, providing the strongest proof yet of the existence of this unusual ground state.