Re: What are the limitations of superconducting microprocessors ?

Dear James

Superconducting microprocessors are still in their R/D phase and, wisely, 
industry will not use them until they are proved reliable in practice.

As regards high clock speeds, the wavelength at 1 GHz is c/f = 10 cm and 
at 10 GHz is 1 cm. To transmit a sharp transition signal (square wave) 
demands passage of higher harmonics up to say f times 10. All these 
signal component frequencies must arrive together - which demands a non-
dispersive transmission line (all signals travel at same speed).

The signal DELAY is crucial for in any practical setup the wires between 
different components vary in length. The delay per cm of wire is more 
than 100 picoseconds and, unless specially catered for we need all 
components to be in step without differential delays.
The principal advantage of superconducting computers will thus be their 
ability to house more components on the same chip - which currently is 
limited by thermal dissipation comcerns. This means shorter connecting 
wires. But the MORE you put on one chip the MORE reliable everything must 
be, as failure in any one thing renders the whole scrap.

Everyone keeps announcing they "almost have" rooom-temperature 
superconductors. This, if it ever happens, will help.

As always the main main limitations of computers will continue to be our 
ability to use them - to use to advantage the fact that they "think" 
entirely differently to us. There will be great strides in input/output 
devices, perhaps with "direct brain links" (via devices warn like 
spectacles or headphones. Simulated 3-d imagery will improve and the 
possibility of manufacturing 3-d objects (3-d "printers").
Presumably our entertainment and communication devices will be integrated 
(and compatible both with one another and ourselves) and new modes of 
interaction, enjoyment and practical use invented.

John