Quantum dynamics of atomic magnets

After the first evidence for tunneling of the total angular momentum of Ho³⁺ in YLiF₄ (insulating), where electronic and nuclear moments tunnel simultaneously (two-body entanglement), we show that pairs of Ho³⁺ 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 Ho³⁺ 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)