AF molecular rings for quantum computation

Molecular magnets have been recently proposed as possible building blocks for a solid-state quantum computer. In order to substantiate and 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) with Ni²⁺(S = 1) provides an extra spin to the otherwise compensated molecule. We show that its ground state consists in an S = 1/2 doublet, energetically well separated (Δ₀/k_B ∼ 13 K at zero magnetic field) from the first excited multiplet. This relatively large value of Δ₀, together with the reduced mixing of the subspaces corresponding to different values of the total spin S, enables a safe encoding of the |0〉 and |1〉 states with the ground-state doublet, and allows to coherently rotate the effective S = 1/2 spin, while keeping the population loss to the excited states negligible. A further, intriguing challenge is represented by the implementation of the conditional dynamics (two-qubit gates). We present here preliminary characterization of molecular “Cr₇Ni-dimers”, i.e., derivatives in which two Cr₇Ni rings are linked with each other by means of delocalized aromatic amines. The resulting intercluster couplings are estimated to be ⩽1 K and are expected to be permanent, i.e., not tuneable during gating, as required by the standard approach to quantum computation. We discuss a computational scheme that allows in principle to overcome this limitation. The most relevant decoherence mechanisms for Cr₇Ni and possible ways to reduce their effects are discussed as well.