Magnetic qubits as hardware for quantum computers

We propose two potential realizations for quantum bits based on nanometre-scale magnetic particles of large spin S and high-anisotropy molecular clusters. In case (1) the bit-value basis states |0⟩ and |1⟩ are the ground and first excited spin states S_z = S and S-1, separated by an energy gap given by the ferromagnetic resonance frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, |0⟩, and antisymmetric, |1⟩, combinations of the twofold degenerate ground state S_z = ±S. In each case the temperature of operation must be low compared to the energy gap, Δ, between the states |0⟩ and |1⟩. The gap Δ in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware.