Unconventional computing, also known as alternative computing, refers to computing with unusual methods. An unusual method may be e.g. an unusual theoretical model or an unusual physical basis. The term "unconventional computing" was coined in 1998.
Permacomputing is interested in expanding the lowest layers of the technological possibility space, especially in order to develop computer technology that better integrates with natural processes. This, along with the strive for a greater technological diversity, makes unconventional computing techniques interesting from the permacomputing point of view.
Fluidics, using liquids or gases in place of electricity, is probably the most mature alternative technological basis for computing. A fully working fluidic digital computer, Flodac, was already built in 1964. Its performance class was similar to relay computers (tens of cycles per second), but it was mentioned in the paper that clock speeds up to 250 Hz could be reached with similar but more compact circuitry.
Flodac-like logic gates are based on how fluids move within static structures. So, unlike mechanical and relay computers, Flodac had no moving parts. These structures could probably be printed with rather rudimantary etching techniques or 3D printing, unlike semiconductor manufacturing that requires extreme purity of the material and the etching process.
Optical computing is also quite mature. It has often been envisioned as a way to stick to the Moore's law after the limits of silicon microchips have been reached. The material constraints are different to those of semiconductors, which may also make it an interesting option from the DIY point of view.
Many unconventional computing technologies such as DNA computing are still at a very early stage of development (as in "the addition of two small numbers was succesfully demonstrated"). And even those who aren't are often unsuitable for conventional digital computers.
Some unconventional computing techniques use living organisms. The use of Physarum slime molds has been studied for a long time, and they can e.g. solve shortest-path problems.
Quantum computing is probably the most hyped type of unconventional computing because of the ability of a quantum computer to do an operation "in millions of parallel universes at a time".
Today, integrated circuits are so dominant that even historically important component technologies can be considered "unconventional". These include fully mechanical parts (like those in mechanical calculators or the Zuse Z1), relays (Z3), electron tubes (most 1950s computers), discrete transistors (most early-to-mid-1960s computers), parametrons (some Japanese computers mostly from the 1950s) and Symmag (the French computer CAB500 from 1957).
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