Quantum things I learned today!

ok one last post for today. I went to a very interesting talk by the new prof in our department Edo Waks. Here are some of the things I learned:
(btw I haven't checked if what i am saying here is right or not! Hopefully it's correct!!)

- a potential problem with quantum crypto is that instead of one photon being used to carry each information bit, multiple photons can be excited. Specifically, since the number of photons that are excited is a probabilistic process that follows a Poisson RV, it is highly likely that more than one photon gets excited. This leaves the problem that (can potentially break the original BB84 quantum key distribution protocol) that an eavesdropper can extract only one of the photons to get the info and leave the other photon to carry the same info to the receiver.
- So it is important to create single photon emitters. This is the work that Waks et al did based on quantum dots.
- The other problem that he looked at was that an optical communication channel with ideal emitters still has a decay factor which makes it need repeaters per 100km or so. In quantum world this problem also exists however repeaters cannot be used because of the no-cloning theory in Quantum physics. What they do is that instead they use entanglement and they induce the entanglements to have shared state by remote - essentially you have two particles, and you entangle them via some coherent source (so they do not have to be physically close). Then by manipulating one particle, the other particle is induces into similar state - this is the idea of why they can have entanglement teleportation as well as repeater technology.

one other really interesting thing was this idea of remote entanglement is where the EPR paradox came from.
From Wiki: "Although originally devised as a thought experiment that would demonstrate the incompleteness of quantum mechanics, actual experimental results refute the principle of locality, invalidating the EPR trio's original purpose. The "spooky action at a distance" that so disturbed the authors of EPR consistently occurs in numerous and widely replicated experiments. Einstein never really accepted quantum mechanics as a "real" and complete theory, struggling to the end of his life for an interpretation that could comply with his Relativity without implying "God playing dice", as he condensed his dissatisfaction with QM's intrinsic randomness and (still to be resolved) counter-intuitivity."

[BTW, here is an interesting introduction to Quantum Computing which was sent to me today]