From: Dan.Oelke@rdxsunhost.aud.alcatel.com (Daniel R. Oelke)
Date: Tue, 18 Mar 1997 08:22:28 -0800 (PST)
To: rah@shipwright.com
Subject: Re: Dark Fiber Redux
Message-ID: <199703181622.KAA01573@spirit.aud.alcatel.com>
MIME-Version: 1.0
Content-Type: text/plain



> 
> The implications for cryptography are rather cool. I don't *think*  these
> doped optical amplifiers interfere with so-called quantum cryptography (not
> to be confused with quantum computing, of course). If I remember my SciAm
> back issues, quantum crypto is cool, because if anyone touches the signal
> from Alice to Bob they're detected immediately. Or, is it that the signal
> drops? Can't remember which. Anyway, you need uninterrupted fiber to do QC,
> and that's what you have with optical non-switched amplification, whether
> you're disturbing the photons is another story. Anyone here know for sure?
> I remember discussion about this, but I don't remember the answer.
> 

I am not an expert in the *exact* physics of quantum crypto or of 
optical amplifiers, but I do have a least a college "modern physics"
level of understanding of both.  (and write software for a lot of
fiber optics hardware)

Quantum Crypto depends on the fact that you can't observe
the photons without disturbing them, and that this disturbance
would be detectable at the receiver.  
(i.e. Heisenberg's uncertainty principle at work)

Optical Amplifiers work by receiving a relatively small 
number of photons, and passing them through a device 
where each photon creates in effect a cascade of photons
to be generated thereby amplifying the signal.

Now, this cascade mechanism in itself might be enough "measurement"
to cause problems - but I don't know - and won't persue this
avenue further.

The problem is that while the waveform will be propagated, the
orignal photons won't necessarily make it from one end to the
other.  Without these original photons making the entire 
journey I don't see how the quantum crypto would work.

There are also pratical limitations on the number of optical
amplifiers you can have in a system before having an
electrical regenerator.  Typical numbers are around 5 
amplifiers, eaching going about 80km.  This doesn't 
allow for around the world pure glass communications. ;-(

Dan
--------------------------------------------------------------------------
Dan Oelke - droelke@aud.alcatel.com        Alcatel Telecom, Richardson, TX

There's a Wocket in my Pocket, And a Findow in my Window, 
And a Nook Gase in my Book Case