746 research outputs found
Limits on Fundamental Limits to Computation
An indispensable part of our lives, computing has also become essential to
industries and governments. Steady improvements in computer hardware have been
supported by periodic doubling of transistor densities in integrated circuits
over the last fifty years. Such Moore scaling now requires increasingly heroic
efforts, stimulating research in alternative hardware and stirring controversy.
To help evaluate emerging technologies and enrich our understanding of
integrated-circuit scaling, we review fundamental limits to computation: in
manufacturing, energy, physical space, design and verification effort, and
algorithms. To outline what is achievable in principle and in practice, we
recall how some limits were circumvented, compare loose and tight limits. We
also point out that engineering difficulties encountered by emerging
technologies may indicate yet-unknown limits.Comment: 15 pages, 4 figures, 1 tabl
A Study of Optimal 4-bit Reversible Toffoli Circuits and Their Synthesis
Optimal synthesis of reversible functions is a non-trivial problem. One of
the major limiting factors in computing such circuits is the sheer number of
reversible functions. Even restricting synthesis to 4-bit reversible functions
results in a huge search space (16! {\approx} 2^{44} functions). The output of
such a search alone, counting only the space required to list Toffoli gates for
every function, would require over 100 terabytes of storage. In this paper, we
present two algorithms: one, that synthesizes an optimal circuit for any 4-bit
reversible specification, and another that synthesizes all optimal
implementations. We employ several techniques to make the problem tractable. We
report results from several experiments, including synthesis of all optimal
4-bit permutations, synthesis of random 4-bit permutations, optimal synthesis
of all 4-bit linear reversible circuits, synthesis of existing benchmark
functions; we compose a list of the hardest permutations to synthesize, and
show distribution of optimal circuits. We further illustrate that our proposed
approach may be extended to accommodate physical constraints via reporting
LNN-optimal reversible circuits. Our results have important implications in the
design and optimization of reversible and quantum circuits, testing circuit
synthesis heuristics, and performing experiments in the area of quantum
information processing.Comment: arXiv admin note: substantial text overlap with arXiv:1003.191
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