  A new family of supramolecular, antiferromagnetically exchange-coupled dimers
of single-molecule magnets (SMMs) has recently been reported [W. Wernsdorfer,
N. Aliaga-Alcalde, D.N. Hendrickson, and G. Christou, Nature 416, 406 (2002)].
Each SMM acts as a bias on its neighbor, shifting the quantum tunneling
resonances of the individual SMMs. Hysteresis loop measurements on a single
crystal of SMM-dimers have now established quantum tunneling of the
magnetization via entangled states of the dimer. This shows that the dimer
really does behave as a quantum-mechanically coupled dimer. The transitions are
well separated, suggesting long coherence times compared to the time scale of
the energy splitting. This result is of great importance if such systems are to
be used for quantum computing. It also allows the measurement of the
longitudinal and transverse superexchange coupling constants.
