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     Quantum cryptography can't be broken by factorisation. Could it one day replace standard cryptosystems?
 short distances it wouldn't be too hard to build a quantum key 
distribution network to encrypt data. Over longer distances, you would 
need quantum repeaters every 50 kilometres or so on the fibre-optic 
network, as it's difficult to maintain a quantum state over long 
distances. Even if they are cheap by then, it's a lot of investment.
=====end of quote=====

My fiber optic invention has a certain relevance here. A typical modern germania-doped-core (GeO2) silica optical fiber has a loss of about 0.19 decibels/kilometer (db/km).  Over 50 km, the loss is (50 km x 0.19 db/km) = 9.5 db, ignoring splice losses.  (A good splice has a loss of about 0.10 db.)  So, the quote above is indicating that above a loss of about 10 db, a quantum system is hard to maintain.  I have suggested in my patent application that isotope-modified fiber (where the Si-29 level is brought from nature's 4.67% (atom/atom) to 0.10 %, the loss might decrease by a factor of 10 to 20.  This means that the ultimate distance limit might increase to 50 x 10 = 500 km, to 50 x 20 = 1000 km.  That would be a major improvement if it works.
The reason that this new fiber would be necessary is this:  Ever since the invention of the EDFA (Erbium-doped fiber amplifier  http://en.wikipedia.org/wiki/Optical_amplifier ; ) in 1986, it has been used to amplify IR signals in the 1510-1560 nanometer band.  Using it and ordinary signals (not quantum signals) it is possible to go about 125 kilometers between amplifiers.  (In other words, that usage tolerates about 25 db of optical loss before an EDFA is necessary.)  However, apparently an EDFA cannot be used to amplify a quantum system.  http://www.nict.go.jp/en/press/2010/02/08-1.html    ; Or, at least, not directly.

      Jim Bell 
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