  Water exists as two nuclear-spin isomers, para and ortho, determined by the
overall spin of its two hydrogen nuclei. For isolated water molecules the
conversion between these isomers is forbidden and they act as different
molecular species. Yet, these species are not readily separable, and little is
known about their specific physical and chemical properties, conversion
mechanisms, or interactions. Here we demonstrate the production of isolated
samples of both spin isomers in pure beams of para and ortho water, with both
species in their respective absolute ground state. These single-quantum-state
samples are ideal targets for unraveling spin-conversion mechanisms, for
precision spectroscopy and fundamental-symmetry-breaking studies, and for
spin-enhanced applications, e. g., laboratory astrophysics and -chemistry or
hypersensitized NMR experiments.
