A Clocking Strategy for Scalable and Fault-Tolerant QDCA Signal Distribution in Combinational and Sequential Devices - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 59

A Clocking Strategy for Scalable
and Fault-Tolerant QDCA Signal Distribution
in Combinational and Sequential Devices

Douglas Tougaw ( & )

Valparaiso University, Valparaiso, IN, USA
[e-mail]

Abstract. A signal distribution network (SDN) for Quantum-dot Cellular
Automata (QDCA) devices is described. This network allows the distribution
of a set of inputs to an arbitrary number of outputs in any desired order,
overcoming the challenges associated with wire crossings that have faced
QDCA systems in the past. The proposed signal distribution network requires
only four distinct clock signals, regardless of the number of inputs or outputs,
and those clock signals each repeat a very simple pattern. This network is
highly scalable, completing the distribution of N inputs to an arbitrary number
of distributed signals in 4N - 2 clock cycles. The operation of this device is
demonstrated by applying it to a two-input/one-output XOR gate and a three-
input/two-output

full adder. A modified SDN customized for use with

sequential devices is also shown.

Keywords: Quantum-dot Cellular Automata Field-Coupled Nanocomputing
Signal distribution network Wire crossing

1 Introduction to Quantum-dot Cellular Automata

Quantum-dot Cellular Automata (QDCA) is an emerging computing architecture
based on Field-Coupled Nanocomputing (FCN) that has demonstrated the ability to
use quantum mechanical interactions to implement both combinational and sequential
logic devices [ 1 - 9 ]. Devices implemented using QDCA cells show the potential to be
faster and smaller than conventional microelectronic devices. Perhaps even more
importantly, they are predicted to operate at a tiny fraction of the power required by
current devices.
QDCA devices are designed by carefully selecting the placement of cells and the
timing with which their tunneling barriers are raised and lowered. As shown in Fig. 1 ,
a QDCA cell is typically composed of four quantum dots located at the four corners of
a square. Tunneling is allowed between adjacent dots, and the cell occupancy is
controlled by a back plane voltage so that a total of two electrons occupy each four-
site cell. The electrons in each cell Coulombically repel each other, so the cell will
typically exhibit one of two states, either pointing diagonally left (a binary 0) or
diagonally right (a binary 1). Electrons in nearby cells will also interact with each
other Coulombically, causing adjacent cells along the same linear axis to align in the