Modeling and manufacturability assessment of bistable quantum-dot cells

TL;DR

This paper models bistable quantum-dot cells for quantum cellular automata, analyzing their robustness and manufacturability, and proposes a design with adjustable bias voltages to mitigate fabrication asymmetries.

Contribution

It introduces a 2-D configuration-interaction model for quantum-dot cells and proposes a gate-defined cell layout with bias adjustments to improve manufacturability.

Findings

Simple hole-array based QCA cells are highly sensitive to fabrication tolerances.

Gate-defined cells with adjustable biases can compensate for asymmetries.

The proposed layout enables experimental demonstration of QCA chains.

Abstract

We have investigated the behavior of bistable cells made up of four quantum dots and occupied by two electrons, in the presence of realistic confinement potentials produced by depletion gates on top of a GaAs/AlGaAs heterostructure. Such a cell represents the basic building block for logic architectures based on the concept of Quantum Cellular Automata (QCA) and of ground state computation, which have been proposed as an alternative to traditional transistor-based logic circuits. We have focused on the robustness of the operation of such cells with respect to asymmetries deriving from fabrication tolerances. We have developed a 2-D model for the calculation of the electron density in a driven cell in response to the polarization state of a driver cell. Our method is based on the one-shot Configuration-Interaction technique, adapted from molecular chemistry. From the results of our simulations, we conclude that an implementation of QCA logic based on simple ``hole-arrays'' is not feasible, because of the extreme sensitivity to fabrication tolerances. As an alternative, we propose cells defined by multiple gates, where geometrical asymmetries can be compensated for by adjusting the bias voltages. Even though not immediately applicable to the implementation of logic gates and not suitable for large scale integration, the proposed cell layout should allow an experimental demonstration of a chain of QCA cells.