From: jim bell <jimbell@pacifier.com>
Date: Wed, 21 Feb 1996 08:45:55 +0800
To: cypherpunks@toad.com
Subject: Re: Optical repeaters
Message-ID: <m0toxMH-00090qC@pacifier.com>
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At 08:04 AM 2/20/96 -0800, jamesd@echeque.com wrote:
>At 07:35 PM 2/19/96 -0800, Timothy C. May wrote:
>>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. 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.
>
>An amplification mechanism will usually couple the signal to the vacuum, 
>and introduce vacuum noise.   Another way of thinking of vacuum noise 
>is that amplification mixes the state of the signal, with the (unknown) 
>phase of the universe at infinity.

I would think that the action of a optical fiber amplifier is, in a sense, 
"negative loss," or the inverse of the kind of loss normally found in a 
fiber cable.  Thus, if quantum crypto can be done through a long fiber at 
all, the fiber amplifier shouldn't negatively affect it.  However, if 
polarization is important (and I know that "polarization maintaining" fibers 
are available) then I assume that the amplifier would probably also have to be 
designed to ensure that polarization would also be maintained through the 
EDFA (Erbium-Doped Fiber Amplifier.)

Jim Bell
jimbell@pacifier.com
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