Ring-shaped nanomagnets: from quantum effects to spin-cluster qubits
Prof. Filippo Troiani
Istituto Nanoscienze-CNR, S3, Modena (Italy)
Molecular nanomagnets represent a wide class of spin cluster, whose structural and magnetic properties can be widely engineered by chemical synthesis . Historically, much attention has been devoted to high-spin molecules (the so-called single-molecule magnets) with anisotropy barriers, where the magnetic memory is ultimately limited by the quantum tunneling of magnetization. The present talk will be concerned with a different class of molecules, namely antiferromagnetic spin rings, that have recently attracted a widespread interest also in view of their possible use in quantum-information processing. In this perspective, three different aspects will be considered. Firstly, recent progress in the control on intermolecular coupling between ring-shaped nanomagnets will be discussed . Such capability represent a first, crucial requirement on the way of growing a scalable hardware based on the nanomagnet as a building block, and has enabled the demonstration of (equilibrium-state) entanglement between pairs of rings . Secondly, the electron-spin decoherence in single nanomagnets and coupled rings will be considered. In both cases, decoherence results from the dynamics of the nuclear bath, and specifically from the build-up of quantum correlations between electron and nuclear spins. Based on a microscopic model of the molecules, we show how the chemical elements and the physical processes that drive decoherence can drastically depend on the specific linear superposition of interest : as an illustrative example, we compare the cases of singlet-triplet superpositions in ring dimers with that of the Bell states. The final part of the talk will be devoted to a novel approach to the use of antiferromagnetic spin rings in quantum information processing, based on the use of electric fields as a means for the manipulation of the quantum state . Here, the computational degree of freedom is not the spin projection – as in most spin-based approaches – but spin chirality. Magnetically frustrated systems – such as homometallic odd-numbered rings – with antisymmetric exchange (Dzyaloshinskii-Moriya) interaction present ground-state multiplets with well defined chirality. The general conditions for the existence of such spin-electric coupling in spin rings will be discussed, and preliminary results will be presented on hyperfine-induced decoherence of spin chirality.
 D. Gatteschi, R. Sessoli, and J. Villain, Molecular nanomagnets (Oxford University Press, Oxford, 2007).
 G. Timco, S. Carretta, F. Troiani, F. Tuna, R. J. Pritchard et al., Nat. Nanotechnol. 4, 173 (2009).
 A. Candini, G. Lorusso, F. Troiani , A. Ghirri, S. Carretta et al., Phys. Rev. Lett. 104, 037203 (2010).
 A. Szallas and F. Troiani, Phys. Rev. B 82, 224409 (2010).
 M. Trif, F. Troiani, D. Stepanenko, and D. Loss, Phys. Rev. Lett. 101, 217201 (2008).