Re: Is it time to stop research in Computer Architecture ?

On Oct 24, 6:31 pm, Bernd Paysan <bernd.pay...@xxxxxx> wrote:

One interesting property of quantum mechanics is that for irreversible
logic, there's a minimum amount of energy that is necessary to make it
happen.  Reversible logic does not have this drawback.  Therefore,
people investigate into reversible logic, even though the actual
components to get that benefit are not in sigh (not even carbon nanotube
switches have these properties, even though they are much closer to the
physical limits for irreversible logic).  Many people also forget that
quantum mechanics does not properly take changes in the system into
account, and that means that your reversible logic only works with the
predicted low power when the inputs are not changing any more - and this
is just the uninteresting case (the coherent one - changes in the system
lead to decoherence, and thereby to classical physics).

Let's see. Quantum mechanics properly applied takes account of
everything in the whole universe, which is, so far as I know, quantum
mechanical and reversible in it's entirety. If you could isolate
parts of the system, like your computing apparatus, then it would be
like a universe that is quantum mechanical and reversible in its
entirety. Such a device would have little use to us, because we could
neither give it new problems to work on nor read the results when it's
done.

In order to give the device a new problem, we must disturb it, but the
system can still retain enough coherence to function as a quantum
mechanical device. Only the entropy involved in the process of giving
the device input and reading the output has an irreducible cost in
energy that we must put on to the electric bill, as we will never get
it back, except as waste heat.

Thus, even though you can't do operations with *no* net cost in
energy, we can still build and operate devices that act as quantum
mechanical computers to an arbitrarily good approximation. Writing to
them and reading from them is always an irreversible process that, if
repeated often enough, will eventually lead to the device having no
useful quantum mechanical coherence left for us to exploit, as we have
destroyed it all through our reading and writing. In the interim, we
can do an awful lot of computation. Otherwise, "quantum computers"
would not be possible.

I'm having a hard time reconciling how I understand the problem with
what you just said, which seems too sweeping and too black and white.
Can you help me out?

Robert.
.

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