AntiFerromagnetic molecular rings for quantum computation.

Molecular magnets have been recently proposed as possible building blocks for a solid-state quantum computer. In order to substantiateand develop such a proposal, one needs to identify those molecules that are best suited for the qubit encoding and manipulation.Here, we focus on a heterometallic molecular ring, namely Cr7Ni, where the substitution of one Cr3+(S = 3/2) withNi2+(S = 1) provides an extra spin to the otherwise compensated molecule. We show that its ground state consists in an S = 1/2doublet, energetically well separated (D0/kB 13 K at zero magnetic field) from the first excited multiplet. This relatively large valueof D0, together with the reduced mixing of the subspaces corresponding to different values of the total spin S, enables a safe encodingof the |0æ and |1æ states with the ground-state doublet, and allows to coherently rotate the effective S = 1/2 spin, while keeping thepopulation loss to the excited states negligible. A further, intriguing challenge is represented by the implementation of the conditionaldynamics (two-qubit gates). We present here preliminary characterization of molecular ‘‘Cr7Ni-dimers’’, i.e., derivatives inwhich two Cr7Ni rings are linked with each other by means of delocalized aromatic amines. The resulting intercluster couplingsare estimated to be 61 K and are expected to be permanent, i.e., not tuneable during gating, as required by the standard approachto quantum computation. We discuss a computational scheme that allows in principle to overcome this limitation. The most relevantdecoherence mechanisms for Cr7Ni and possible ways to reduce their effects are discussed as well.