Quantum dynamics of atomic magnets

After the first evidence for tunneling of the total angular momentum of Ho 3+ in YLiF 4 (insulating), where electronic and nuclear moments tunnel simultaneously (two‐body entanglement), we show that pairs of Ho 3+ ions also tunnel simultaneously (four‐body entanglement). Such co‐tunneling transitions, mediated by weak dipolar interactions, are discussed in terms of a four‐spin representation including the rare‐earth nuclear spin. This study of the entanglement of pairs of distant Ho 3+ ions constitutes a first step to elucidate many‐body quantum‐spin relaxation. Finally, implications on the feasibility of two‐spin qubits are discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)