🚀 go-pugleaf

leichter@zodiac.rutgers.edu writes:

>In article <1992Oct13.061724.25758@netcom.com>, rcain@netcom.com (Robert Cain)
>writes:
>> leichter@zodiac.rutgers.edu writes:
>| : 
>| : The attack against the *cryptosystem*, as opposed to the attack against QM,
>| : goes like this:  The eavesdropper clones the photons and SAVES them - he
>| 
>| How does he do this?  I know of no way to "save" a photon.  Somehow mirrors
>| would be involved and I think interaction with a mirror would alter the
>| polarization state.
>| 
>...
>You should be able to transmit these photons through a fiber (which
>contains them by total internal reflection) without losing the important
>information.  The fiber can be wrapped into a ring; you need an optical
>switch to get it into the ring at the start, and out at the end, but
>such things exist.

>Admitedly, keeping a photon going around a ring of fiber for, say, an
>hour is pretty unlikely.  But remember that you can keep amplifying the
>signal while retaining its information by using a laser - that was the whole
>basis by which we cloned the original photon!  In effect, you are coupling
>the original information to a laser pulse that you can keep feeding energy
>into to make up for the energy it loses.  I see no fundamental reason why
>you can't keep the information stable as long as you like this way.
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is not really true because a laser amplifier will always add
noise due to spontaneous emission. After some time, your signal to
noise ratio will start to degrade and at some point, you will lose
the signal totally.
From what I have seen with long distance fiber optic systems, you can
probably not do it over more than about 10000 or 20000 km of fiber,
which corresponds to a delay of about 0.1 seconds.

   J.-P. Weber