From: "Phil G. Fraering" <pgf@srl01.cacs.usl.edu>
Date: Tue, 20 Feb 1996 15:27:44 +0800
To: tcmay@got.net (Timothy C. May)
Subject: Re: Optical repeaters
Message-ID: <199602200444.AA01625@srl03.cacs.usl.edu>
MIME-Version: 1.0
Content-Type: text/plain


   From: tcmay@got.net (Timothy C. May)
   Newsgroups: hks.lists.cypherpunks

\Given that a quantum cryptography system depends on *single photons* to
/work, I'm not sure that talk of amplifification makes sense. Between source
\and receiver, a photon either makes it or doesn't. If it makes it, it makes
/it will its full "quanta" of energy, of course. If it doesn't make it, due
\to tunnel-penetrating the walls of the fiber or scattering off an impurity
/in the fiber, then it just doesn't make it, so no
\amplification/regeneration is possible. (Regenerate _what_?)

/But the quantum measurement issue, aside from the above, is an
\interesting one.  We have to be careful here (and I'm including
/myself, not just using the royal "we"). It isn't clear to me that the
\amplification/regeneration process counts as making a measurement,
/from some recent work I've read (sorry, don't recall the references,
\but could be a recent issue of "Scientific American"). In interference
/measurements, the wave function collapses if individual photons are
\counted and recorded (whatever "recorded" really means...) and the
/interference pattern vanishes. If the photons are not counted and/or
\recorded, the pattern reappears.

/By analogy, it is not clear to me that a simple regeneration mechanism,
\with no local observer or recording apparatus, will collapse the wave
/function.

I'm sorry I quoted so much, but let's think for a second: what is
a regeneration mechanism going to do?

Idea number one: It could act on an individual photon. This photon
could have its energy per photon increaced. Except for doppler effects
or possibly some wierd non-conservative electric/magnetic fields (and
I'm just starting again, and can't remember if there are some obscure
non-conservative electric fields), I don't think there are any methods
that don't involve <<measurement>> of the photon. Please note that the
known ways of "pumping" a photon's frequency, such as interaction with
electrons in the orbitals of atoms in a crystal lattice, involve
<<measurement>>.

Please note I'm using the term <<measurement>> in brackets so that the
physics-challenged here will realize that what is being discussed is
the Q-M concept of measurement and not the sending out of some guy
with a yardstick to look at photons.

Another idea that occured to me is that the number of photons are
being manipulated: one photon spawns many. As a practical example,
check any image-intensifier tube.

\Seems to me an experiment may have already been done along these
/lines: separate fibers producing an interference pattern and then these
\inline amps added...if the interference pattern remains, as I would expect,
/then the amps/regenerators did not constitute a "measurement" in QM terms.

I'm getting the impression of an optical-loop interferometer that
relies on the timing of a photon's arrival as the first-order
effect in the generation of an interference pattern. I'm not sure if
it'll work, but I'm willing to bet there are some useful optical-loop
interferometer experiments that don't rely on preserving all the
states of the photons in question. Don't ring gyroscopes rely on
measuring path length?

Other than the two examples I mentioned above, the only things I could
think of involved modifying quantum parameters themselves, which is
what's supposed to be avoided, right?

BTW, since I'm not on cypherpunks and am reading this sporadically
thanks to news.hks.net (thanks guys!), could y'all email any responses
to me?

I think I need that refresher course in electrodynamics and Q-M pretty
bad. 


\--Tim May

/Boycott espionage-enabled software!
\We got computers, we're tapping phone lines, we know that that ain't allowed.
/---------:---------:---------:---------:---------:---------:---------:----
\Timothy C. May              | Crypto Anarchy: encryption, digital money,
/tcmay@got.net  408-728-0152 | anonymous networks, digital pseudonyms, zero
\W.A.S.T.E.: Corralitos, CA  | knowledge, reputations, information markets,
/Higher Power: 2^756839 - 1  | black markets, collapse of governments.
\"National borders aren't even speed bumps on the information superhighway."

I'd delete that but it's a work of art... 

Getting back to more real-world applications, anyone out there have
any ideas why netscape's stock is still so high when all the stuff
they came out with that was allegedly secure basically wasn't?

And more immediate than that, I was thinking of ordering some
textbooks using a borrowed credit card from a bookstore in Oregon.
Is it safe now? ;-)

If it turns out to not have been safe, can I sue Netscape for false
advertizing, even if I knew it was false?

Phil

Phil Fraering           "Nice shark...
pgf@srl03.cacs.usl.edu   _pretty_ shark..."
========================================================================
is it just me, or is everyone else having bsd flashbacks too?