Architectures that would exploit the distinct characteristics of quantum-dot cellular automata (QCA) have been proposed for digital communication networks that connect advanced digital computing circuits. In comparison with networks of wires in conventional very-large-scale integrated (VLSI) circuitry, the networks according to the proposed architectures would be more compact. The proposed architectures would make it possible to implement complex interconnection schemes that are required for some advanced parallel-computing algorithms and that are difficult (and in many cases impractical) to implement in VLSI circuitry.

The difficulty of implementation in VLSI and the major potential advantage afforded by QCA were described previously in "Implementing Permutation Matrices by Use of Quantum Dots" (NPO-20801), NASA Tech Briefs, Vol. 25, No. 10 (October 2001), page 42. To recapitulate: Wherever two wires in a conventional VLSI circuit cross each other and are required not to be in electrical contact with each other, there must be a layer of electrical insulation between them. This, in turn, makes it necessary to resort to a noncoplanar and possibly a multilayer design, which can be complex, expensive, and even impractical. As a result, much of the cost of designing VLSI circuits is associated with minimization of data routing and assignment of layers to minimize crossing of wires. Heretofore, these considerations have impeded the development of VLSI circuitry to implement complex, advanced interconnection schemes.

On the other hand, with suitable design and under suitable operating conditions, QCA-based signal paths can be allowed to cross each other in the same plane without adverse effect. In principle, this characteristic could be exploited to design compact, coplanar, simple (relative to VLSI) QCA-based networks to implement complex, advanced interconnection schemes.

Figure 1. A QCA-Based Wire for Bidirectional Communication would be terminated in input and output branches at both ends.
The proposed architectures require two advances in QCA-based circuitry beyond basic QCA-based binary-signal wires described in the cited prior article. One of these advances would be the development of QCA-based wires capable of bidirectional transmission of signals. The other advance would be the development of QCA circuits capable of high-impedance state outputs. The high-impedance states would be utilized along with the 0- and 1-state outputs of QCA.

A QCA-based wire for bidirectional communication (see Figure 1) would be terminated in two branches at each end — one branch for input, the other for output. To enable binary signals to propagate both from the left input to the right output terminal and from the right input to the left output terminal, it would be necessary to apply suitably phased clock signals (bias voltages) to QCA subarrays at various positions along the main wire and the end branches. (For complex reasons that must be omitted from this article for lack of space, such clocking is needed in any event to prevent spurious outputs. Here, the clocking would be exploited for the additional purpose of bidirectional communication.)

One especially useful interconnection network is an N × N crossbar network. A QCA circuit capable of a high-impedance output state would be needed to implement a crosspoint switch in a crossbar network. This is because while all N input lines cross a given output line, only one input line is allowed to put a signal on that output line; in other words, the connections between the other input lines and the given output line are required to be of high impedance in order to block signals.

Figure 2. A Crossbar Network would be a coplanar assembly of QCA-based wires subdivided into QCA arrays excited by suitably timed clock signals.
Figure 2 depicts a proposed QCA-based crosspoint switch and a 3 × 3 crossbar network. The crosspoint switch would contain several branched QCA subarrays excited by suitably phased clock signals, and one of the quantum cellular automatons would serve as a control switch. The input signal Ii would propagate toward the output line along one branch and, by suitable clocking and coupling, would be converted to another signal, If, propagating toward the output line along another branch. The application of a "0" signal to the control switch would cause Ii and If to be of the same state (both 0 or both 1), thereby causing the signal Ii to be coupled onto the output line; in effect, the crosspoint switch would be in a low-impedance state. On the other hand, the application of a "1" signal to the control switch would cause If to be the opposite of Ii, thereby preventing coupling of either Ii or If onto the output line; in effect, the crosspoint switch would be in a high-impedance state.

This work was done by Amir Fijany, Nikzad Toomarian, Katayoon Modarress, and Matthew Spotnitz of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Computers/Electronics category.

This Brief includes a Technical Support Package (TSP).
Compact Interconnection Networks Based on Quantum Dots

(reference NPO-20855) is currently available for download from the TSP library.

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This article first appeared in the January, 2003 issue of NASA Tech Briefs Magazine.

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