In article <WPVP3.7334$cPf.203393536 at news.telia.no>,
"Patrik Bagge" <pab at neramd.no> writes:
[about quantum correlation]
> ok, is there a way of expressing the nature of this entanglement
> (not necessary precise) to an amatour in the matters such as myself.
Sort of statistical. No, thats not right either. Let me use more words
to explain some properties of quantum physics that are needed:
- You can't measure the state of a system without influcencing it.
Example: Polarization of photons has some strange properties in QT (and yes,
you can demonstrate that easily). If you measure along one plane, you get
a value of either +1 or -1. Doing this, you have set the polarization plane!
That is: if you have a big number of photons, first measure polarization
along one plane, and then along the a plane at right angles to the first,
the 2nd measurement will always give equally distributed +1 and -1. The
first measurement has destroyed the polarization information along the
other plane.
This doesn't sound too strange, but has consequences if combined with the
next one:
- Modern quantum theory has sort of a globality property. That is:
If you create a combined system in a certain state, that state can't
necessarily be split into its components.
For example, if during a certain transition between energy levels of
an (molecule, atom, atom nucleus, particle --- choose one) a pair of
photons is emitted at the same time, they have a combined "spin" leading
to a combined polarization.
If you measure the polarization of one of the photons, you fix the
polarization plane of the other photon, too.
Now, this still isn't too strange, but...
- Modern quantum theory predicts that this happens even if said photons
travel off in opposite directions, and the measurement is done at
seperated places! (In fact, doing this kind of measurements can tell
us whether an alternate quantum physics with hidden variables, leading
to more causality und thus looking less strange, is possible; which is
one of the reasons why this experiment was done!)
The CERN people were able to demonstrate the coupling of polarizations at
a distance of 11 km, using measurements short enough to ensure that the
measurements could in some way have exchanged the information via another
channel.
Now, this can't be used to transmit information at faster-than-light, because
you can't transmit the combined state of the photons fast enough to both
ends... This is why somebody else in this thread of discussion said it is not
information but "information".
However, a practical use has been proposed already.
You can use this mechanism to create correlated keys at seperated places.
If somebody where to eavesdrop, he would influence the polarizations, so
you can detect this at either end. Eavesdropping made impossible by laws
of physics!
However, this leads to a possible denial of service attack. The bad guys can
measure the polarization, thus blocking your method of key exchange.
OTOH, if the bad guys have access to your glass fibre, they could simply
cut it instead and block your key exchange, too, so this isn't as bad as
it sounds in the first place.
Regards,
Ignatios
