[8477] in cryptography@c2.net mail archive
Re: The Shining Cryptographers Net
daemon@ATHENA.MIT.EDU (hal@finney.org)
Fri Jan 19 11:09:34 2001
From: hal@finney.org
Date: Thu, 18 Jan 2001 20:13:38 -0800
Message-Id: <200101190413.UAA24210@finney.org>
To: cryptography@c2.net, hal@finney.org, jsd@research.att.com
John Denker wrote:
> At 02:04 PM 1/18/01 -0800, hal@finney.org wrote:
> > I don't think she could learn much with a single photon,
>
> I'm not so sure about that. Remember, photon counters (which measure
> A_dagger A) are not the only measuring devices in the world. There are
> also voltmeters (which measure A_dagger plus A). For low-amplitude analog
> signals, the voltmeter is vastly more informative. I have not yet cobbled
> up a believable apparatus for measuring the polarization angle of a single
> photon, but I don't think it would be terribly hard to do so.
I'd like to hear more about how this is possible in theory, even without
regard to issues of practicality. I know you mentioned EPR entangled
photons but I don't see how you could use them. (Actually I did come
up with way, but it would also have allowed FTL communications so I'm
pretty sure it wouldn't have worked...)
With the voltage meter, I think what you're measuring is essentially
energy, which is a continuous Hilbert space, so one measurement can
provide a lot of information. But photon polarization is a two basis
state system so I don't see how one measurement can give you much.
If Eve really can accurately determine the rotation angle by passing
just a single photon through the system that really shoots down the SC
Net idea.
Hal
