Counterfactual Quantum Thinkers

Damien Broderick (
Sun, 10 May 1998 16:36:55 +0000

Things have been a bit rough recently for I-kicked-the-damned-stone-OK?
Realists. I was enchanted and mildly gob-smacked today to read in the 2
May 1998 *New Scientist* of a new wrinkle on quantum computing. Alas, I
don't have the names to cite, because I read it in the library, and NS's
web site gives no details, just this screamer:

There's a weird world where computers give
you an answer just by being there. You don't
even have to switch them on

The bottom line is that some clever fellow has worked out how to get
answers out of a quantum preparation *without turning the machine on to run
the calculation*.

It's based on the bomb-testing idea I'm sure you're all familiar with from
Penrose's SHADOWS OF THE MIND. Elitzur and Vaidman showed that one time in
4 you can determine the state of a macro-system, *without observing it*, if
it is coupled to a quantum detector. All that's needed is for the
observation to be *possible* via that procedure, and quantum interference
can deliver you information about the real world without actually looking
at it. Some of the time, anyway. And since that's true, a sequence of
assays can multiply up your level of confidence to whatever degree you wish.

This has now been extended, in principle, to such tasks as intractable
computations. I'm not talking boring old-fashioned quantum computers here,
with their staid superposition of all possible calculations. Those dreary
things had to be turned on and powered up. This gadget doesn't need to be
activated until *after* it's done the job. (What's more, it's reported
that objects have now been `photographed' using this principle without
shining any light on them, simply because light *could have been* shone on

Penrose is evidently rubbing his hands with glee, because this sounds like
it might be the missing link in his theory that neural microtubules can
sustain quantal superpositions in something like a Bose condensate. The
brain might thus leap instantly and without frying itself to conclusions
that even the smartest Turing Jupiter brain would be agonising over to the
end of time...

I dunno. Sounds like witchcraft to me. Pretty logic, though.

Damien Broderick