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January 26, 2011

Multibit error correction is a step towards practical reversible computing

Arxiv - Reversible Logic Based Concurrent Error Detection Methodology For Emerging Nanocircuits In a reversible logic gate, every output can be transformed back in to its input. If this is done perfectly then there would zero waste. Actually building such a logic gate is a difficult and has not been done.

Reversible computing seems to contradict one of the foundations of computer science: Rolf Landauer's principle that the erasure of a bit of information always dissipates a small amount of energy as heat. This is the basic reason that conventional chips get so hot. But this principle need not apply to reversible computing because if no bits are erased, no energy is dissipated. In fact, there is no known limit to the efficiency of reversible computing. If a perfectly reversible physical process can be found to carry and process the bits, then computing could become dissipation free.

New Scientist - Himanshu Thapliyal and Nagarajan Ranganathan, a pair of computer scientists at the University of South Florida have brought the technology one step closer by describing an error correcting scheme for reversible logic. Previous efforts could only detect single-bit errors, but their new method can deal with multi-bit errors, an essential requirement for practical reversible computing.



It works by reversing the computation on a series of outputs and comparing the result to the original inputs. If the two match, the calculation is guaranteed to be error-free. This method is actually better than the error-correction in current computers, which works by repeating the original calculation and comparing the result - a method which won't identify identical errors in both calculations. And since they use "garbageless" reversible logic gates, no energy is wasted.

New Scientist - the work is highly theoretical for now, but Thapliyal and Ranganathan demonstrate how it could be applied to a particular type of quantum computing known as quantum dot cellular automata.

MIT Technology Review - Reversible logic could cut the energy wasted by computers to zero. But significant challenges lie ahead.


6 page paper in Arxiv

To demonstrate the application of the proposed approach of concurrent error detection in emerging nanotechnologies, we choose quantum dot cellular automata (QCA) nanotechnology as an example since reversible logic has potential applications in QCA computing. Quantum dot cellular automata (QCA) is one of the emerging nanotechnologies in which it is possible to achieve circuit densities and clock frequencies much beyond the limit of existing CMOS technology. QCA has significant advantage in terms of power dissipation as it does not have to dissipate all its signal energy hence considered as one of the promising technologies to achieve the thermodynamic limit of computation. The basic QCA logic devices comprise the majority voter (MV), the inverter (INV), binary wire and the inverter chain.

QCA computing is based on majority voting, thus recently two new 3x3 (3 inputs: 3 outputs) reversible gates QCA1 and QCA2 suitable for majority based QCA computing are proposed.

The proposed methodology of concurrent error detection based on property of reversible logic is generic in nature, and will be applicable to any emerging nanotechnology, such as QCA, nano-CMOS designs, which may be susceptible to single or multiple transient and permanent faults. An application of the proposed approach for concurrent error detection in emerging technologies is illustrated for QCA nanotechnology.


Quantum dot cellular automata

Quantum dot cellular automata at wikipedia

Although QCA solves most of the limitations CMOS technology, it also brings its own. Optimistic assumptions suggest that intrinsic switching time of a QCA cell is in the order of terahertz, however, as mentioned earlier, switching speed is not limited by a cell’s intrinsic switching speed but by the proper quasi-adiabatic clock switching frequency setting. "Comparative analysis of circuit performance of QCA and CMOS against a representative computer task, suggests that realistic circuits of solid state QCA will have the maximum operating frequency of several megahertz. Small circuits of hypothetical molecular QCA might have the maximum operating frequency of several GHz, however, as the circuit size increases, capacitive loading effects will reduce the speed." Moreover, solid-state QCA devices cannot operate at room temperature. The only alternative to this temperature limitation is the recently proposed “Molecular QCA” which theoretically has an inter-dot distance of 2 nm and an inter-cell distance of 6 nm. Molecular QCA is also considered to be the only feasible implementation method for mass production of QCA devices. QCA technology resolves, in principle, the problems of current CMOS technology, and it is only limited by the availability of its practical fabrication methods.

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