Quantum Coherence and Decoherence in Magnetic Nanostructures

The prospect of developing magnetic qubits is discussed. The first part of the article makes suggestions on how to achieve the coherent quantum superposition of spin states in small ferromagnetic clusters, weakly uncompensated antiferromagnetic clusters, and magnetic molecules. The second part of the article deals with mechanisms of decoherence expected in magnetic systems. Main decohering effects are coming from nuclear spins and magnetic fields. They can be reduced by isotopic purification and superconducting shielding. In that case the time reversal symmetry of spin Hamiltonians makes spin-phonon coupling ineffective in destroying quantum coherence.