Magnetic Dipolar Ordering and Quantum Phase Transition in an Fe-8 Molecular Magnet.

We show that a crystal of mesoscopic Fe-8 single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class.