The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide attractive targets for the study of quantum effects at the meso-scopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field 2–4. There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits— 'qubits', the basic building blocks of a quantum computer—based on molecular magnets 5–8. But strong decoherence 9 must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave 10) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic V IV 15 clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferro-magnetically coupled S 5 1/2 spins, leading to an S 5 1/2 collective ground state 11–13. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 ms (at liquid helium temperatures) are a realistic prospect