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From: howard@cpocd2.UUCP (Howard A. Landman)
Newsgroups: comp.arch,comp.lsi
Subject: Re: Josephson Junction computers
Message-ID: <504@cpocd2.UUCP>
Date: Mon, 16-Mar-87 14:21:31 EST
Article-I.D.: cpocd2.504
Posted: Mon Mar 16 14:21:31 1987
Date-Received: Wed, 18-Mar-87 06:38:55 EST
References: <8702240351.AA00239@angband.s1.gov> <7718@utzoo.UUCP> <492@cpocd2.UUCP> <1486@vice.TEK.COM>
Reply-To: howard@cpocd2.UUCP (Howard A. Landman)
Organization: Intel Corp. ASIC Services Organization, Chandler AZ
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Xref: mnetor comp.arch:594 comp.lsi:63

In article <1486@vice.TEK.COM> keithl@vice.TEK.COM (Keith Lofstrom) writes:
>Advantages:
>   JJ's prime advantage is their very low power,  allowing them to be placed
>closely together.  With proper design, you can design JJ gates with switch
>energies of just a few kT.  JJ gates can be made to match transmission lines
> - hard to do at low power with semiconductor circuits requiring >100mV
>logic swings.

One problem I failed to mention in my previous posting is data storage.  While
JJ gates are very low power, the standard way to store a bit in a JJ circuit
is to put a current into a superconducting loop.  There's a really interesting
feature to this current, which is that it is equivalent to some "trapped"
magnetic flux through the loop.  Now, this flux is governed by the simple
Bohr-Sommerfeld quantization that most of you first heard about in the Bohr
model of the hydrogen atom: there are discrete energy levels that can be
trapped.  Thus the minimum you can switch is a single flux quantum; and there
exist JJ circuits that do just that!

However, the quantization levels depend on the size of the loop, and increase
as the loop gets smaller (remember "particle in a box"?).  This is all
freshman physics, but the implication is that the amount of energy required
to store one bit of information goes *UP* as JJ circuits get smaller, and
hence the power consumption goes up, and the power density (power per area)
goes *WAY* up.  Hence JJ cicuits will not scale gracefully to smaller sizes.
They are already "quantum limited".

This same effect, taken the other way to large loops (say 1 meter across),
is what makes JJ flux detectors so extremely sensitive.

>Imagine what superconducting transmission lines can do for you here (Note:
>superconducting wires are still dispersive - at a Terahertz or so).

As I mentioned before, not much.  It's easy to get transmission speeds of
.5 to .8 the speed of light using non-superconducting wires.  Try teflon
twin-ax driven by ECL, for example, as used in large Amdahl and Fujitsu
machines.  Not even superconductors can get > 1.0.
-- 

	Howard A. Landman
	...!intelca!mipos3!cpocd2!howard