Clock Topologies for Molecular Quantum-Dot Cellular Automata

Abstract

Quantum-dot cellular automata (QCA) is a low-power, non-von-Neumann, general-purpose paradigm for classical computing using transistor-free logic. Here, classical bits are encoded on the charge configuration of individual computing primitives known as “cells.” A cell is a system of quantum dots with a few mobile charges. Device switching occurs through quantum mechanical inter-dot charge tunneling, and devices are interconnected via the electrostatic field. QCA devices are implemented using arrays of QCA cells. A molecular implementation of QCA may support THz-scale clocking or better at room temperature. Molecular QCA may be clocked using an applied electric field, known as a clocking field. A time-varying clocking field may be established using an array of conductors. The clocking field determines the flow of data and calculations. Various arrangements of clocking conductors are laid out, and the resulting electric field is simulated. It is shown that that control of molecular QCA can enable feedback loops, memories, planar circuit crossings, and versatile circuit grids that support feedback and memory, as well as data flow in any of the ordinal grid directions. Logic, interconnect and memory now become indistinguishable, and the von Neumann bottleneck is avoided.

Implementing Logical Circuits with Four Phase Quantum Dot Cellular Clock using QCA Designer

“…The major role of clocking [23] is to control the flow of data and calculations. In [17], it is shown that the molecular QCA control results into enabling the feedback loops, versatile circuit grid and the support of feedback and memories.…”

“…The clocking fields can be used for clocking the molecular QCA. It can be implemented using an array of conductors [9] which is further used to determine the flow of data and calculations. Another advantage of using QCA clocks is to provide power gain for strengthening weak signals, enabling latching and permits adiabatic operations [10][11].…”

Clocked Quantum-dot Cellular Automata Circuits Tolerate Unwanted External Electric Fields

“…This will enable electric-field clocking of QCA cells using the z-component of an applied electric field. [24][25][26] A clocking field E z ẑ with sufficiently strong E z < 0 can over come the affinity of the mobile electron for the neutralizing charge on dot N , driving the cell from the "Null" state to the active state biased by neighbor interactions. As long as an adequate activating field is applied, the bit is latched.…”

Limits of adiabatic clocking in quantum-dot cellular automata

“…Each QCA cell occupies only a few nm 2 in area, potentially offering high device densities of 10 14 cm −2 . Significant challenges must be solved for any realistic QCA implementation, such as limiting device power at high density using reversible gates 12,13 , designing robust wire crossings 14 and clocking networks 15,16 , and interfacing with the existing CMOS architecture.…”

“…In 2-state QCA, the two polarization states are defined by occupation of the antipodal sites and the clocking field is interpreted as a modulation of the inter-dot tunneling barriers 12 . In 3-state QCA, we introduce additional dots associated with a non-interacting or inactive null state 16 . By applying a relative voltage to the null dots we control the energy of the null state and can activate/deactivate cells in a clock zone.…”