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/kB ∼ 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.

Graphical abstractIn this work we show that heterometallic molecular rings with dominant antiferromagnetic exchange coupling between nearest neighbors are suitable electronic spin systems for the implementation of quantum algorithms. One of the main advantage in using molecular clusters is the size of the qubit that will help in the addressing single quantum object. We also underline a further resource of molecular clusters made by their excited states that in a proper computational scheme may constitute an important feature to consider.Figure optionsDownload full-size imageDownload as PowerPoint slide