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Proposal for Quantum-gate in permanently coupled AF spin rings, without need of local fields.

Filippo Troiani, M. Affronte, Stefano Carretta, Paolo Maria Santini, G. Amoretti

Published Jan 1, 2005• 1 citation
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Manipulating the persistent current in quantum rings

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•Jan 1, 2010•0 citations
We propose a scheme for the implementation of quantum gates which is based on the qubit encoding in antiferromagnetic molecular rings. We show that a proper engineering of the intercluster link would result in an effective coupling that vanishes as far as the system is kept in the computational space, while it is turned on by a selective excitation of specific auxiliary states. These are also shown to allow the performing of single-qubit and two-qubit gates without an individual addressing of the rings by means of local magnetic fields.
We propose an alternative concept for a scalable spin quantum computer that combines aspects of other proposals with the advantageous features of endohedral fullerenes. The key advantages are that electron spins instead of nuclear spins are used and that the manipulation of fullerene molecules is fairly easy. Qubits are set and read out via pulsed electron-spin resonance. Addressing is provided by local magnetic fields or field gradients (A(A(A gate). The qubit-qubit interaction is mediated by magnetic dipolar coupling and can be controlled via the direction of the magnetic field with respect to the distance vector of the qubits (J(J(J gate). Molecular as well as solid-state architectures are discussed.
We study the (de)localization phenomena of one-component lattice fermions in spin backgrounds. The O(3) classical spin variables on sites fluctuate thermally through the ordinary nearest-neighbor coupling. Their complex two-component (CP1\mathrm{CP}{}^{1}CP1-Schwinger boson) representation forms a composite U(1) gauge field on bond, which acts on fermions as a fluctuating hopping amplitude in a gauge invariant manner. For the case of antiferromagnetic (AF) spin coupling, the model has a close relationship with the $t\ensuremath{-}J$ model of strongly correlated electron systems. We measure the unfolded level spacing distribution of fermion energy eigenvalues and the participation ratio of energy eigenstates. The results for AF spin couplings suggest a possibility that, in two dimensions, all the energy eigenstates are localized. In three dimensions, we find that there exists a mobility edge, and we estimate the critical temperature ${T}_{\mathrm{L}\mathrm{D}}(\ensuremath{\delta})$ of the localization-delocalization transition at the fermion concentration $\ensuremath{\delta}$.