  Unlike conventional magnets where the magnetic moments are partially or
completely static in the ground state, in a quantum spin liquid they remain in
collective motion down to the lowest temperatures. The importance of this state
is that it is coherent and highly entangled without breaking local symmetries.
Such phenomena is usually sought in simple lattices where antiferromagnetic
interactions and/or anisotropies that favor specific alignments of the magnetic
moments are "frustrated" by lattice geometries incompatible with such order
e.g. triangular structures. Despite an extensive search among such compounds,
experimental realizations remain very few. Here we describe the investigation
of a novel, unexplored magnetic system consisting of strong ferromagnetic and
weaker antiferromagnetic isotropic interactions as realized by the compound
Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both
experimentally and theoretically that it displays all the features expected of
a quantum spin liquid including coherent spin dynamics in the ground state and
the complete absence of static magnetism.
