Michele Girlanda

Toward an effective yet reliable many-body computation of magnetic couplings in bisnitronyl nitroxide biradicals

Configuration interaction calculations have been applied to the study of the magnetic coupling in a series of bisnitronyl nitroxide diradicals. Molecular orbitals obtained with different localization schemes have been considered in the generation of the configuration interaction space, with the aim of investigating the role played by the various fragments in the magnetic interaction. Polyene spacers are found significant, while fragments outside the magnetic-bridge-magnetic moiety can be neglected.

Operation of quantum cellular automaton cells with more than two electrons

We present evidence that operation of quantum cellular automaton (QCA) cells with four dots is possible with an occupancy of 4N+2 electrons per cell (N being an integer). We show that interaction between cells can be described in terms of a revised formula for cell polarization, which is based only on the difference between diagonal occupancies. We validate our conjectures with full quantum simulations of QCA cells for a number of electrons varying from 2 to 6, using the configuration–interaction method.

Analysis of polarization propagation along a semiconductor-based quantum cellular automaton chain

We present a simulation of a chain of three quantum cellular automaton (QCA) cells defined in a GaAs/AlGaAs heterostructure by means of depletion gates, focusing on the evaluation of the voltage unbalance that must be applied to the gates to enforce the correct polarization of the driver cell and propagate it through the other two cells. We use the configuration interaction method in each cell, including the electrostatic coupling between cells with an iterative self-consistent procedure. In particular, we investigate the issue, addressed in the recent literature, of chain malfunction due to the adverse effect on the driven cells of the electric field from the gates defining the driver cell. Our conclusion is that, as long as the gate voltage unbalance polarizing the driver cell is smaller than a threshold depending on geometric and material parameters, correct operation of the QCA wire can be obtained.

Simulation of a complete chain of QCA cells with realistic potentials

We present a numerical simulation of the operation of a chain of 3 Quantum Cellular Automanton (QCA) cells, with the inclusion of a realistic procedure to enforce, with an externally applied bias voltage unbalance, the polarization of the first cell. The polarization state is shown to propagate correctly, as long as the bias unbalance applied to the electrodes of the first cell is not so large as to directly perturb nearby cells. The addition of dummy cells is needed to balance the asymmetries existing at the ends of a chain: this is an indication of further difficulties, that may become relevant if fabrication of more complex arrays is attempted.

Problems and Perspectives in Quantum-Dot Based Computation

A review of recent modeling work on quantum cellular automata is presented. Detailed simulations at the single cell level, based on the Configuration-Interaction method, allow to analyze cell bistable behavior for different material systems and geometrical structures. In addition, they allow to evaluate the maximum allowed fabrication tolerances required for correct cell operation. Simulations at the circuit level, based on a simplified physical model of each cell, allow to analyze operation of single logic gates and more complex combinatorial logic networks. A simulation method based on simulated annealing is presented, which allows to compute the ground state and low-lying excited states of the QCA system without the need of exploring the whole configuration space.

Configuration-interaction based simulation of a quantum cellular automaton cell

We have investigated the behavior of a quantum cellular automaton cell made up of four quantum dots, in response to the electric field due to a nearby driver cell. We have implemented a simulation based on the single-shot configuration interaction technique, and we have studied the behavior of the cell for a number of electrons variable between 2 and 6. Our results support the conjecture that proper QCA operation can be obtained with a number of electrons per cell corresponding to 4N+2.