[8473] in cryptography@c2.net mail archive
Re: The Shining Cryptographers Net
daemon@ATHENA.MIT.EDU (John Denker)
Fri Jan 19 11:09:04 2001
Message-Id: <4.2.2.20010118183815.028bbd20@127.0.0.1>
Date: Thu, 18 Jan 2001 18:52:01 -0500
To: hal@finney.org, cryptography@c2.net
From: John Denker <jsd@research.att.com>
In-Reply-To: <200101182204.OAA23251@finney.org>
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At 02:04 PM 1/18/01 -0800, hal@finney.org wrote:
>the rotation stations could
>somehow count or limit the number of photons going through so that they
>would know when there were extra. I think this is possible in theory;
Right, it is. Here's a Gedankenexperiment: temporarily trap the signal in
a cylindrical waveguide resonator (organ pipe). The pressure on the
end-caps is proportional to photon number and independent of polarization
angle. From this we conclude we can measure number in a way that commutes
with polarization.
I went overboard when previously I said "any" attempt at integrity-checking
would mess up the signal. Still, integrity-checking of a single photon
would be hard.
> 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.
